Accepted: 23 July 1999

M. Siegemund ´ J. van Bommel ´ C. InceDepartment of Anesthesiology,Academic Medical Center,University of Amsterdam, The Netherlands

M. Siegemund ())Laboratory of Experimental Anesthesiology,Academic Medical Center, University of Amsterdam,Meibergdreef 9, 1105 AZ Amsterdam, The Netherlandsemail: m.siegemund@amc.uva.nlTel.: + 31 (20) 5664820, Fax: + 31 (20) 6979004

Introduction

Measurement of tissue oxygenation and haemoglobinsaturation is of major importance in the intensive carepatient, since conventional monitoring techniques failto provide information about areas of tissue where oxy-gen supply is insufficient to sustain mitochondrial respi-ration (tissue dysoxia) [1]. Regional dysoxia is consid-ered to be a major factor contributing to the develop-ment and maintenance of multiple organ failure.Whether shock-related tissue distress is caused by a de-crease in intracellular concentration of oxygen, result-ing in a decline in aerobically produced adenosine triph-osphate (ATP), or whether disturbances in cellular met-abolic pathways are responsible, is a matter of ongoingdebate [2, 3]. Global and regional, neuronal and humor-al factors, as well as intrinsic metabolic and vascularcontrol pathways ensure adequate delivery of oxygenat-ed blood to the tissues [4±6]. During shock and hypoxa-emia, where oxygen availability becomes restricted, re-gional redistribution of blood occurs and the metabolicproperties of tissue cells become altered. Lack of knowl-edge of basic mechanisms controlling both oxygentransport and utilisation in the microcirculation, as wellas the inadequacy of clinical techniques for assessment

of the adequacy of tissue oxygenation, are reasons forthe uncertainty surrounding the potential benefits ofvarious therapeutic strategies in different types ofshock.

Maintenance of adequate tissue oxygen transport canbe considered a primary objective in intensive care man-agement. For this reason, assessment of tissue oxygen-ation is essential. An ideal technique for measurementof tissue oxygenation should provide quantitative, accu-rate and reproducible real-time information about oxy-gen supply and utilisation in specific tissue beds. In addi-tion, it should clearly distinguish which compartment issensed, i. e. arterial, venous, microcirculatory or tissuecompartments [7, 8]. Heterogeneity of oxygen supplyand demand exists between organs as well as at the levelof each organ; this heterogeneity increases further dur-ing states of shock and sepsis. This implies that regionalmeasurement should be made at the organ which is themost sensitive to the condition causing the critical ill-ness, as the heterogeneity in microvascular flow and ox-ygenation, as well as the differing metabolic needs of thevarious tissues, make a common critical partial pressureof oxygen (PO2) unlikely [9]. For clinical application,such a device should be safe, non-invasive and easy toapply.

Techniques for the measurement of tissue oxygen-ation can be grossly subdivided into two groups (Ta-ble 1). The first utilises the electrochemical propertiesof noble metals to measure the oxygen content of tis-sues. Clark-type membrane covered oxygen electrodesare available as tissue penetrating needle electrodes, im-plantable catheter electrodes, or non-invasive surfaceelectrodes. The second group of techniques utilises theoptical properties of haemoglobin and indicator dyes tomeasure haemoglobin saturation, PO2, partial pressureof carbondioxide (PCO2) or pH. These optical tech-niques are either invasive or non-invasive and have thepotential to sense a variety of regional and cellular vari-ables reflecting the adequacy of tissue oxygenation. The

M. SiegemundJ. van BommelC. Ince

Assessment of regional tissue oxygenation

Intensive Care Med (1999) 25: 1044±1060Ó Springer-Verlag 1999 REVIEW

aim of this paper is to review currently available tech-niques for measurement of tissue oxygenation and hae-moglobin saturation and to discuss (patho)physiologicaland therapeutic information these techniques can pro-vide.

Current clinical techniques for assessment of regionaltissue oxygenation

Lactate levels have been previously regarded as an indi-cator of anaerobic metabolism due to the unavailabilityof oxygen. Lactate is generated from pyruvate in a re-versible reaction by the cytosolic enzyme lactate dehy-drogenase; it increases when the rate of pyruvate pro-duction in the cell exceeds its utilisation in the mito-chondria [10]. Lactate determination is a widely usedmethod to assess prognosis or response to a specifictherapy in critically ill patients; levels above 2 mEq/lare used as a marker of tissue dysoxia [11, 12]. Besideglobal (shock, hypoxia) or regional (small bowel infarc-

tion) causes of anaerobic lactate production, non-hy-poxic causes of increased lactate production also exist.Decreased lactate clearance, accelerated aerobic glycol-ysis (e. g. by sympathomimetic drugs), or a dysfunctionof the pyruvate-dehydrogenase enzyme complex whichbrings pyruvate into the mitochondria and converts itto acetyl CoA as substrate for the citric acid cycle [13],may decrease the prognostic value of lactate measure-ment [14]. Furthermore, intact metabolic capabilities ofthe liver make a significant increase of lactate unlikely,so any individual organ failure may pass undetected[15]. For these reasons, measurement of lactate levelsprovide a relatively late and uncertain marker of the ad-equacy of tissue oxygenation [9].

Mixed venous oxygen saturation (SvO2) can bereadily measured with intermittent blood gas analysesfrom a pulmonary artery catheter (PAC) or by continu-ous measurement with a fibreoptic PAC. The SvO2 isan average of the venous effluents from all perfused vas-cular beds. High flow, low extraction vascular beds (e. g.kidney, gut) have a larger influence than organs with a

1045

Table 1 Measurement techniques for determinants of tissue oxygenation (+/� = minimally invasive; [+] not manufactured)

Technique Clinically: Invasive Method Variables Penetration depth Reference

Available Underdevelop-ment

Oxygen electrode + + + Polarographicelectrode(needle, catheterand multiwiresurface)

PO2 Needle: 15±20 mmSurface: 20 mm;Half spherical

[54, 62±64]

Optodes + + + Optical deter-mination ofpH, PCO2 andPO2 by fluores-cence

pH, PCO2 andPO2

Unknown [72, 73, 79, 81]

Near-infraredspectroscopy

+ + � Absorbanceanalysis of near-infrared lightspectra

Blood volume,[Hb]/[HbO2],cytochrome aa3

Different [83, 87, 90, 92, 100,104]

Reflectancespectrophotometry

[+] [+] +/� Spectrum analysisof reflected light

Regional Hbsaturation

250 mm [115, 116, 124]

NADH fluores-cence

� + � Fluorescenceimaging

Mitochondrialenergy state

200 mm [125, 127±129, 135]

Pd-porphyrinphosphorescence

� � � Quenching ofPd-porphyrinphosphorescence

PO2 500 mm [70, 139, 140, 146,151]

Tonometry + + +/� PCO2 measure-ment in an air- orsaline-filled tono-metric balloon

Intestinal PCO2 Uncertain [35, 43, 48, 49]

Cytoscan imaging � + +/� Direct visualisa-tion of the micro-circulation

Microvasculararchitecture;red blood cellvelocity

500 mm [152]

high oxygen extraction such as the heart. Furthermore,a critical value of SvO2 that defines an inadequate re-gional oxygen delivery (DO2) is difficult to determine.Therapies aimed at restoring SvO2 to normal or even su-pranormal levels have failed to show an increased sur-vival rate [16]. The relationship of whole body oxygendemand to cardiac output, and hence DO2, determinesthe SvO2 under conditions of stable arterial oxygenationvariables, thereby making SvO2 of limited value for theassessment of the adequacy of tissue oxygenation [9,17]. In sepsis, microcirculatory shunting mechanismscan cause persisting high global as well as regional ve-nous oxygen saturations, while regional tissue dysoxiais present [7].

In principle, regional venous oxygen saturation canbe measured from every organ of interest. Since the in-troduction of fibreoptic oximetry catheters, hepatic andjugular venous haemoglobin saturations (SjvO2) havebeen measured in both experimental and clinical stud-ies. Dahn and colleagues compared central venous withhepatic venous saturations (ShvO2) in 22 intensive carepatients and found a significant difference in septic pati-ents but not in trauma victims [18]. ShvO2 measure-ments during liver transplantation and cardiac surgeryshowed that this technique could be utilised to monitoreffects of therapeutic interventions [19, 20]. The dura-tion and severity of low ShvO2 values were also predic-tive for liver dysfunction and mortality after hepatecto-my [21]. Continuous monitoring of ShvO2 has beenused to indicate the therapeutic efficacy of inhaled nitricoxide in a patient with right ventricular failure [22].

Jugular bulb venous haemoglobin saturation hasbeen introduced into clinical practice to estimate the ad-equacy of cerebral perfusion under stable cerebral met-abolic conditions [23]. The preferable site of catheter in-sertion is still under debate but is currently consideredto be most appropriate on the side of dominant venousdrainage (usually the right) [24±26]. SjvO2 measurementhas been used following severe head injury to monitorthe influence on cerebral oxygenation of hyperventila-tion [27] and changes in cerebral perfusion pressure[23, 26, 28±30]. Gopinath et al. found that jugular ve-nous desaturations during the first 24 h after traumawere strongly associated with a poor neurological out-come [31]. Although the technique could be used to de-fine an optimal cerebral perfusion pressure during cere-bral aneurysm surgery [32], the application of the tech-nique during cardiac surgery raises more problems, pos-sibly due to acute changes in cerebral metabolic rate af-ter temperature fluctuations associated with cardiopul-monary bypass [23, 33]. By contrast, a recent case reportdescribed a significant improvement of SjvO2 after initi-ation of intra-aortic balloon pump counterpulsation fortherapy of a low postoperative cardiac output [34].

Intestinal regional capnography

Regional capnography relies on the principle that CO2generated in tissues diffuses freely across tissue and cellmembranes. The classic intestinal tonometry techniquedescribed by Fiddian-Green and Baker [35] uses a naso-gastric tube with a saline-filled silicone balloon for theintraluminal determination of PCO2 in the gastric mu-cosa. Measurement of arterial bicarbonate (HCO3

± )and use of the Henderson-Hasselbalch equation allows,with certain assumptions, the calculation of intestinalmucosal pH (pHi). After 30 to 90 min of equilibration,the PCO2 of stomach or gut interstitial fluid and the sa-line approximate each other. At 30 min, the equilibra-tion is only 77% complete, so that PCO2 values deter-mined in saline by a conventional blood gas analyserneed to be multiplied by 1.24 to compensate for this ef-fect [36].

The original assumption that arterial HCO3- is in

equilibrium with gut mucosal bicarbonate is not alwayscorrect. Decreased gastric mucosal blood flow in statesof shock or following administration of sodium bicar-bonate may lead to a significant difference between in-testinal tissue and systemic HCO3

-, and would thereforeoverestimate the gastric pHi [37]. Measures of gastricpHi, obtained by sampling gastric juice directly, havebeen shown by Mohsenifar et al. to predict weaning out-come in 29 ventilated patients [38]. Patients who couldnot be weaned from mechanical ventilation had sub-stantially reduced gastric intramucosal pHi values. Thefact that those successfully weaned had normal pHimeasurements indicates that monitoring of the gastricmicrocirculation may reveal covert cardiovascular in-sufficiency and inappropriate timing for weaning frommechanical ventilation. Tang et al. were able to show aprominent increase in gastric intramural PCO2 duringhaemorrhagic and anaphylactic shock with miniaturisedion-selective field-effect transistor sensors [39]. By mea-suring gastric pH with a miniature glass electrode direct-ly applied to the mucosa, they were able to show thatcalculated intestinal pH, which is based on the assump-tion that HCO3

± concentration in the stomach wall andarterial blood are the same, underestimates the localpHi.

The question as to whether increasing intestinalPCO2 (piCO2) reflects anaerobic metabolism with hy-drogen ion production, or simply reduced CO2 off-loadby a decrease in splanchnic blood flow, was evaluatedby Schlichtig and Bowles [37]. Their study in healthydogs suggested that the intestinal±arterial PCO2 differ-ence increases from 4±6 mmHg under normal condi-tions to 25±35 mmHg during stagnant gastric bloodflow and that levels above this indicate anaerobic gener-ation of CO2. Microcirculatory shunting (e. g. during en-dotoxaemia and sepsis) complicates the clear distinctionbetween impaired perfusion, altered cellular energy me-

1046

tabolism [40] and anaerobic energy generation, since ar-eas with reduced perfusion and CO2 off-load are next tohypoxic regions [41, 42]. The systemic arterial PCO2,which is of obvious importance in critically ill patientswith pathological respiratory changes, also influencesthe piCO2 and thus the pHi. The intestinal±arterialPCO2 gap has thus been propagated as a more accuratevalue describing the correlation between regional tissueblood flow and oxygenation, as it corrects for systemicabnormalities altering intestinal PCO2 [43].

Intestinal capnography has been extended to analysePCO2 from an air-filled balloon in a conventional cap-nometer [44, 45]. After measurement, the air is redirect-ed back to the balloon for automated on-line assessmentat 10±15 min intervals. The in vitro bias, precision andreproducibility of air tonometry are consistent with aclinically reliable device [46]. In a study in mechanicallyventilated, septic patients, the accuracy of this methodwas close to that of conventional saline tonometry; val-ues measured at 10-min intervals showed a short re-sponse time and correlated well with conventionaltonometry [45]. Despite the value of intestinal tonome-try in the prediction of mortality and morbidity [41,47±49], its usefulness for guiding therapy is still underdebate [50, 51].

Oxygen electrodes

Oxygen electrodes for the measurement in tissue and mi-crocirculation are conventionally based on the classicalClark electrode [52]. These types of electrodes consist ofa noble metal (e. g. silver, gold and platinum) which re-duces oxygen due to a negative polarising voltage. Thechange in voltage between the reference electrode (an-ode) and the measuring electrode (cathode) is propor-tional to the amount of oxygen molecules being reducedon the cathode [53]. In the Clark electrode, both anodeand cathode are placed behind an oxygen permeableand electrically insulating membrane. Transport of oxy-gen occurs due to diffusion from the surrounding tissuewith a high PO2, to the area behind the membrane of theelectrode where the oxygen pressure is near zero. To ob-tain reliable measurements, the O2 consumption of theelectrode should be small compared to the local O2 levelwithin the tissue [54], otherwise the electrodes have thepotential to alter their own environment. The sensitivityof the electrode is determined by the diffusion constantof the electrode membrane, while the surface of the elec-trode defines the amount of oxygen generating the cur-rent after combining to hydrogen. Calibration of theseoxygen electrodes in a solution of known oxygen tensionis needed prior to each measurement.

As needle tip oxygen electrodes measure PO2 at asingle point, the heterogeneous oxygen distribution inthe microcirculation is not represented. Lübbers and

co-workers overcame this problem by using a devicewhich punctures the tissue in adjustable steps; theywere thus able to show the expected heterogeneity anddistribution of quantitatively measured PO2 [55]. Basedon the idea that electrode penetration may cause PO2disturbance by tissue damage, a needle electrode with aresponse time under 500 ms was developed. The needleprobe is moved through the tissue in quick forward andbackward steps, which is expected to revive the tissuefrom the previous compression caused by the forwardmotion of the needle. The variance of PO2 measure-ments is depicted by means of PO2 histograms showinga distribution picture of the values [54, 56]. Alternativeuses of Clark-type electrodes for measurement of tissueoxygenation have made use of flexible catheter elec-trodes that are placed in tissue. Such PO2 catheters arewell suited for long-term use and have been used for sur-gical implantation [57].

Oxygen electrodes can also be applied on the surfaceof the tissue to avoid tissue damage by electrode pene-tration. Such surface devices have been developed to in-clude an array of PO2 electrodes (so-called multiwireelectrodes). The Mehrdraht Dortmund Oberflächen(MDO) tissue oxygen electrode is an array of eight indi-vidual polarographic electrodes and a silver referenceelectrode encased in glass and covered with cellophaneand Teflon membranes, both 12-mm thick, which ensurea stable electrode output even when pressure is appliedto the electrode surface. The eight measuring pointseach register PO2 values from non-overlapping half-spherical volumes with a radius of approximately20 mm. It is important to note that the distribution ofPO2 values measured is a combination of the spatial dis-tribution under the electrode and the temporal distribu-tion during the measurement [40]. Surface electrodes[36, 40, 58±61] and needle electrodes [62±64] have beenapplied to muscle and intestine in human and animalstudies. Two recent studies in porcine models of haem-orrhage and endotoxaemia compared a surface elec-trode with reflectance spectrophotometry (describedbelow) of jejunal mucosa [65, 66]. During shock, the mu-cosal PO2 and oxygen saturation, measured by electrodeand spectrophotometry, respectively, dropped signifi-cantly in both models. After resuscitation from haemor-rhage with shed blood and fluid, the spectrophotometri-cally determined haemoglobin saturation returned tobaseline, whereas the PO2 values measured by the sur-face electrode indicated a progressive deterioration oftissue PO2. Resuscitation from endotoxic shock with flu-id and either dopamine or dopexamine increased jejunaltissue PO2 and haemoglobin saturation in a similar man-ner. Reasons for these discrepancies may be the unequalpenetration depth of the two techniques (Table 1) re-stricting the measurement of the electrode strictly tothe villus tip and also the different microvascular bloodflow distribution in the two kinds of shock.

1047

The area of tissue which is effectively measured byoxygen electrodes (the so-called catchment area) hasbeen estimated to be about 15±20 mm deep [7]. Thissmall penetration depth is a major limitation in the in-terpretation of oxygen electrode derived data [67, 68].Mechanical forces produced by the tip and shaft of nee-dle electrodes, or the high pressure applied by surfaceelectrodes, may cause tissue damage as well as altera-tions in microvascular flow which lead to irregular histo-grams and low PO2 readings [54, 69]. A further limita-tion of oxygen electrodes is, paradoxically, their sensi-tivity to oxygen. Vessels carrying high PO2 blood in thecatchment volume of the electrode will bias the elec-trode value despite the surrounding tissue remaining hy-poxic; these tissue oxygen electrodes are thus sensitiveto changes in arterial PO2 [70].

Optode sensors

Optodes are used to measure concentrations of sub-stances by photochemical reactions creating changes inoptical properties of indicator compounds [54, 71].Changes in the concentration of H+ ions or CO2 produc-es altered fluorescence or absorbance in indicators. pH-sensitive, photochemical dyes absorb light of a certainwavelength from a reference light source [72] or alterthe intensity of fluorescence proportional to the concen-

tration of hydrogen ions. CO2 is measured using the al-tered optical properties of a pH indicator behind anion-impermeable membrane. Oxygen-dependentquenching of fluorescence of indicator substances (e. g.ruthenium) has been used for optical sensing of PO2and has been incorporated into catheters [72, 73]. At-tenuation of fluorescence (quenching) occurs in propor-tion to the molecular O2 concentration surrounding thesensor. The emitted, fluorescent light is transmittedthrough an optical fibre to a microprocessor to quantifychanges compared to pre-insertion calibration data [71].

Such optical sensors have been included in intravas-cular catheters (e. g. Paratrend 7, Diametrics Medical,High Wycombe, UK) for the on-line measurement ofPCO2 and pH. To provide continuous monitoring of ox-ygenation, an additional miniaturised Clark electrodefor PO2 measurement and a thermocouple for the deter-mination of local temperature is included. Their use asalternatives to conventional ex vivo arterial blood gashas been shown in patients during long-term application[74, 75].

These thin, multiparameter catheters with either aClark-type electrode or an optical PO2 sensor (Neuro-trend, Diametrics Medical, High Wycombe, UK) havebeen used to measure local gas tensions in tissues(Fig. 1). During neurosurgical operations and after se-vere head trauma, the measurement of regional intrace-rebral tissue oxygenation may be of clinical importancefor both management and prognostication. Several re-cent studies investigated the possibilities of this catheterto measure brain tissue oxygenation under differentpathological conditions [57, 76, 77] or for the detectionof regional ischaemia during cerebral bypass surgery[78, 79]. During cerebral circulatory compromise, thebrain tissue PO2 and pH decreased and tissue PCO2 in-creased compared to controls with normal cerebral per-fusion. They identified a critical range of cerebral oxy-genation below 15±20 mmHg. Valadka and colleaguescompared a multiparameter catheter with a convention-al PO2 catheter electrode (Licox, GMS, Kiel-Mielken-dorf, Germany) in patients after severe head injury[57]. Neither sensor had adverse effects and both pro-vided reliable oxygen measurements over a period ofseveral days with a mean normal cerebral tissue oxygentension between 30 and 35 mmHg [80].

1048

Fig.1 Diagram of a catheter tip for the optical measurement ofPO2, PCO2, pH and temperature in either tissue or blood (Para-trend/Neurotrend, Diametrics Medical, High Wycombe, UK).The membrane of the catheter consists of a permeable hydropho-bic polyethylene matrix surrounding optical fibres with microporesof about 0.2 mm. These pores are filled with hydrophilic polyacryla-mide gel. In the pH sensor, green light is transmitted along the op-tical fibre through a pH indicator dye immobilised in the gel andreflected back by a mirror. The intensity of back reflected light isa function of the pH. In the CO2 sensor the indicator is dissolvedin a sodium bicarbonate solution and sealed behind a gas perme-able (but H+ impermeable) polyethylene membrane. The changeof light intensity due to this indicator dye is proportional to thePCO2. The oxygen cell consists of ruthenium dye bound to selec-tively gas-permeable silicone rubber. The measurement of PO2 isbased upon the quenching of fluorescence of the ruthenium dye.Temperature is measured by a thermocouple

Optode technology has also been used for real-timemeasurement of intestinal PCO2 by Knichwitz and col-leagues who inserted the fibreoptic catheter into thegut of experimental animals [81, 82]. Changes in ventila-tion pattern of the experimental animals were quicklyand continuously reflected by changes in measured localintestinal PCO2 values [81]. Comparable optic sensorsimplanted in a hindlimb muscle in dogs in haemorrhagicshock have been used to measure PO2, PCO2 and pHover several hours [72]. Rapid normalisation of gas con-centrations was seen during the resuscitation periodwhile the pH recovery was protracted. The possibilityof measuring all three variables in a single catheterseems to be a useful addition to the on-line evaluationof tissue oxygenation and acid±base status. Althoughpreliminary results are promising, the influence of therelatively small catchment area of these optical sensorsin relation to the whole organ remains to be determined.

Near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a non-invasive,optical technique applying the principles of light trans-mission and absorption for the measurement of oxygen-ated and deoxygenated haemoglobin (Fig.2) or cyto-chrome aa3 (cyt aa3) in tissues [83]. The relative trans-parency of tissues to near-infrared light enables mea-surement of absorption peaks in this part of the spec-trum. Analysis of the absorption spectra allows evalua-tion of the oxygen-dependent optical properties of hae-moglobin and the redox state of cyt aa3, the terminal cy-tochrome of the respiratory chain. Oxygenation of hae-moglobin results in less red light and more infrared ab-sorption than deoxygenated haemoglobin (Fig. 2). Cytaa3, has a different absorption band than the other cyto-chromes arising from its central copper atom in thenear-infrared region between 800 and 900 nm. Althoughthe redox state of cyt aa3 is primarily thought to be de-termined by available oxygen, it can also be influencedby other intra-mitochondrial factors including mem-brane potential, turnover and pH [84]. Anoxia quicklyresults in a complete reduction of this enzyme but doesnot always reflect cellular ATP content [85]. Monitoringthe redox state of cytochrome aa3 in this way could po-tentially be of particular importance for the assessmentof cellular oxidative metabolism and tissue dysoxia.

The light absorption of substances at a specific wave-length detected by NIRS theoretically allows quantifica-tion of their concentration by the Beer±Lambert law.This relates the concentration of a solute to the intensityof light transmitted through the solution by an exponen-tial function. However, such calculations require knowl-edge of the effective length of the optical path traversedby photons through the tissue to measure variations ofconcentration from an arbitrary baseline. Unlike a

known substance dissolved in a clear solvent in a cuvetteof known dimensions, the intensity and the pathlengthsof the transmitted light in human tissue is altered bythe effects of light scattering and unspecific light absorp-tion. Measurement of reflected light intensity preventsthe excessive absorption of transmitted light due tolarge organ dimensions (e. g. newborn vs adult crani-um). Since light does not directly traverse organs, the ef-fective optical pathlength is significantly longer than thedistance between light source and detector, and everyphoton will have traversed a different path. One way ofovercoming this uncertainty about the effective opticalpathway to obtain quantitative data is to measure thesaturation ratio of oxygenated to deoxygenated haemo-globin. However, a spectrometer based on this principlewas unable to detect decreased cerebral blood flow andoffered no advantage over a conventional NIRS instru-ment [86]. Since water is relatively well maintained intissue and its concentration is known, the effective opti-cal pathlength of the tissue can be measured by thestrong absorption of light by water at 740, 820 and970 nm [87±89]. The transcranial optical length of thehead is about four times its physical dimension. Pico-second laser pulses of light to measure the time photonstake to traverse the tissue offer a different approach toimprove the accuracy of NIRS [90]. Scattering of lightin tissue causes a variable arrival time of the photons tothe detector. This principle allows the construction of

1049

Fig.2 Basic components of near-infrared spectroscopy systems tomeasure [Hb] and [HbO2] by light absorbance of either reflectedor transilluminated light. Optical pathlength and intensity of thelight detected by a photon-counter as well as the brain area moni-tored are influenced by the placement of light source and receiver

spectral images in relation to a fixed fraction of early-ar-riving transmitted photons. This time-intensity profileof photon scattering can be used to estimate the absorp-tion and scattering coefficients of the illuminated tissuefor the quantification of substances in units of concen-tration [91]. In summary, the different determinationmethods of the optical pathlength mentioned above, aswell as continuous refinement of both devices and math-ematical algorithms used to calculate changes in con-centration from optical tissue properties complicate theinterpretation of investigations with different NIRS de-vices. Wahr and colleagues have provided a thoroughreview of these problems [92].

Although NIRS can be used in virtually any organ, ithas mainly been applied to studies of cerebral oxygen-ation [92]. Early clinical experience with transmissionNIRS was obtained in newborns and infants [91,93±95]. Breathing abnormalities resulted in a significantfall in oxygenated cerebral haemoglobin, while the sub-sequent bradycardia caused further deterioration in ce-rebral oxygenation. An increase in the amount of totalhaemoglobin in the brain after bradycardia seemed toreflect reactive hyperaemia [96]. Aldrich and co-work-ers applied NIRS to the fetal scalp during different stag-es of labour. They found a significant positive correla-tion between umbilical vein oxygen pressure and pHwith mean cerebral oxygen saturation [97] as well as arise in fetal cerebral oxygenation with maternal oxygenadministration [98]. In contrast, late fetal heart rate de-celeration in the first stage of labour was associatedwith a decrease in cerebral oxygenation [99].

The use of NIRS in adults during coronary artery by-pass surgery, as well as in patients with acute brain dis-ease, has indicated conventional SjvO2 to be a bettermonitor of cerebral oxygenation than NIRS [100±102].Nollert et al. found that the saturation in the jugularvein increased during cardiopulmonary bypass, whilehaemoglobin saturation and oxidised cyt aa3 measuredwith NIRS fell [103]. The reason for this lack of correla-tion between the two measurements is uncertain, since70% of the haemoglobin in the brain at any given timeis believed to be in the venous compartment [92] andthus NIRS derived oxygenation values should reflectmore venous than microcirculatory haemoglobin satu-ration. Pollard et al. showed that changing venous intra-cerebral blood volume by body positioning had no influ-ence on the NIRS measurements, in contrast to an in-crease in blood volume induced by hypercapnia [104].It thus seems uncertain which physiological compart-ment is sensed by NIRS. Studies in volunteers showeda good correlation between NIRS and either cerebralblood volume [105] or a calculated brain haemoglobinoxygen saturation [106]. Blood flow in extracranial tis-sues had no significant influence on either cerebral hae-moglobin concentration or saturation measurements[102, 107, 108]. Bearing the difficulties of the technique

in mind, NIRS does seem to offer new information ontissue oxygenation, not only of the brain but also of oth-er organs [109±113]. In general, NIRS measurements ofhaemoglobin saturation and concentration appearmore accurate than estimation of intracerebral bloodflow and volume [114].

Reflectance spectrophotometry

Light absorption of reflected visible light on the tissuesurface can also provide information about haemoglo-bin saturation, although its catchment area will be re-stricted to the tissue surface. Reflectance spectropho-tometry can either measure discrete wavelengths orwhole reflectance spectra. The reflected spectra can befitted with known spectra of fully oxygenated and fullydeoxygenated haemoglobin to determine the relativesaturation of microvascular haemoglobin. The signal in-tensity at the isobestic points of haemoglobin allows anevaluation of capillary haemoglobin concentration rep-resenting the tissue blood volume [115]. The ErlangenMicrolightguide Spectrophotometer (BodenseewerkGerätetechnik, Überlingen, Germany) is such a system,consisting of a flexible light guide to transmit light froma light source to the tissue. Six surrounding optical fibrescollect and transmit reflected light to a photomultiplierafter passage through a rotating interference bandpassfilter [116]. This procedure allows complete reflectancespectra to be measured between 502 and 628 nm, wherethe optical properties of haemoglobin/oxyhaemoglobin(Hb/HbO2 )dominate the reflected spectra. This instru-ment enables measurements of Hb, HbO2 and bloodvolume originating from small surface catchment areasof moving organs such as the heart and gut and allowsthe resolution of spatial heterogeneity [117].

In open-heart surgery this technique has shown im-proved microcirculatory oxygenation after completedrevascularisation [117]. Application to the fetal scalpduring delivery [118] showed development of criticallylow haemoglobin saturations, indicating that local oxy-gen reserves are almost exhausted. Reflectance spec-troscopy has also been applied during neurosurgical ar-teriovenous malformation operations [119], in plasticsurgery [120, 121] and for the evaluation of skin haemo-globin oxygenation in patients with chronic venous in-sufficiency and peripheral arterial occlusive disease[122, 123]. In transplantation of tissue flaps, reflectancespectroscopy provided useful information about oxy-genation of the different transplant regions [121],whereas evaluation of tissue blood flow by hydrogenclearance was not as accurate [120]. Sato et al. firstused reflectance spectrophotometry to measure intesti-nal haemoglobin saturation by introducing the flexiblelight guide through the working channel of a gastro-scope [124]. This elegant set-up was recently applied to

1050

septic patients and compared to gut tonometry in a con-trolled, prospective randomised study [115]. In hyperdy-namic sepsis, decreased oxygen and haemoglobin con-centrations appeared in discrete, mucosal areas. Low-dose dopexamine partially improved mucosal oxygen-ation to higher saturation values, but did not alter thehigh regional PCO2 measured by tonometry. Patients inhyperdynamic sepsis showed significantly lower intesti-nal mucosal haemoglobin saturations compared to con-trol patients.

NADH fluorescence

Metabolic pathways such as pyruvate oxygenation, fatoxidation and the citric acid cycle produce reducedequivalents of nicotinamide adenine dinucleotide(NADH) and H+ to maintain oxidative phosphorylationby reducing molecular oxygen for the production ofATP. NADH/NAD+ is the main source of energy trans-fer from the citric acid cycle to the respiratory chain inthe mitochondria [125, 126]. During tissue dysoxia,NADH accumulates as less NADH is oxidised toNAD+. Because the redox state of mitochondrialNADH/NAD+ directly reflects the mitochondrial ener-gy state, it can be considered as a direct indicator of dys-oxia in mammalian cells.

NADH has two absorption maxima at the ultravioletend of the light spectrum (at 250 and 360 nm). NAD+

has only one and hardly absorbs light at 360 nm (Fig. 3).Thus, like cytochrome aa3, the absorption properties of

NAD is dependent on its redox state. NADH, however,differs from the cytochromes in that the absorbency at360 nm causes fluorescence at 460 nm. Photomultipliersor sensitive video cameras can detect this blue fluores-cence and provide information about the tissue bioener-getics by use of endogenous NADH in vivo [7]. The abil-ity to image the fluorescence further allows the study ofregional heterogeneity of tissue dysoxia in organ surfacesin vitro and in vivo [127±129]. Duboc et al. applied a fi-breoptic version of the technique in humans during heartcatheterisation and in skeletal muscle [130, 131]. Afterintroducing a thin optical fibre via a conventional heartcatheter to the endoyocardium, a transient increase inNADH fluorescence was observed after contrast medi-um injection into a stenotic coronary artery [130]. TheNADH fluorescence technique has been incorporatedinto intravital microscopes for microscopic studies ofthe bioenergetics of cells in vivo [132, 133] and in fluores-cence microscopic studies of microsections of freeze-clamped biopsy specimens from hearts with myocardialischaemia [134]. Applied to an early haemorrhagic shockmodel in healthy dogs, NADH fluorescence of myocar-dial sections showed a marked heterogeneity betweencontiguous small regions with much less transmural het-erogeneity [135]. Administration of a nitric oxide syn-thase (NOS) inhibitor significantly worsened myocardialischaemia by decreasing coronary flow. This observationis consistent with findings in a NADH video fluorimeterstudy of tyrode-perfused septic rat hearts which showedthe appearance of focal dysoxic areas when treated withNOS inhibitors [129, 136]. These findings suggest thatthe myocardium has naturally occurring ªweakº mi-crovascular units which are prone to dysoxia duringstates of hypotension and shock [7].

The feasibility of NADH video fluorimetry for clini-cal use is illustrated in NADH video images we havemade in a pig model of ischaemia/reperfusion of heartand gut. Figure 4 shows NADH images before, duringand after a coronary occlusion period lasting 3 min.The effect of temporary ligation of the superior mesen-

1051

Fig.3 Experimental Langendorff set-up for the measurement ofNADH fluorescence in isolated perfused hearts. The mercury arclamp in combination with a 360-nm bandpass filter BP providethe excitation light for NADH. The dichroic mirror DM separatesthe excitation light from the emitted fluorescence. The fluorescentlight is detected by a charge-coupled device video camera afterpassing a 460-nm BP filter. The fluorescence images are capturedon videotape and computer analysed

teric artery is shown in Fig. 5. To extend these measure-ments for clinical use, a dual wavelength video-imagingsystem would have to be applied to account for distur-bances of scatter and non-specific absorption of theemitted fluorescence [127]. The main limitation of theNADH fluorescence technique for studying tissue ener-gy state is the current inability to quantify NADH levelsfrom measured fluorescence [137].

Palladium (Pd)-porphyrin phosphorescence

Oxygen dependent quenching of Pd-porphyrin phos-phorescence to measure oxygen pressures in tissueswas first introduced by Wilson et al. and Vanderkooiet al. [138, 139]. The technique is based upon the princi-ple that a Pd-porphyrin molecule, when excited by apulse of light, can either release this absorbed energy

1052

Fig.4 NADH video fluorimetry of a pigheart in vivo during clamping and reperfu-sion of the left coronary artery. A NADHfluorescence image from the epicardium ofa normoxic left ventricle in a mechanicallyventilated pig (O2/N2O: 30%/70%) showslittle fluorescence. Due to absorption oflight by blood, the epicardial vessels appearas dark structures. B 30 s after total occlu-sion of the left main coronary artery an in-crease in fluorescence is observed. Thiscontinues to increase in intensity, as can beseen 3 min after clamping C. The left ven-tricle 20 s after reperfusion shows a resto-ration of NADH fluorescence to baselinevalues D

Fig.5 NADH video fluorimetry of a pig il-eal loop in vivo during clamping of the su-perior mesenteric artery for 45 min. ANADH fluorescence of a small intestinalloop in a ventilated pig (fractional inspiredoxygen 0.30). The absorption of light byblood allows the observation of intestinalvessels. B An increase in fluorescence isobserved following 30 min of mesentericartery occlusion. C After reperfusion, theNADH fluorescence of the gut segmentreturns to baseline. In this phase the ob-scuring of intestinal blood vessels due toreactive hyperaemia can be observed

as light (phosphorescence) or transfer it to molecularoxygen. After porphyrin is excited by light, the decayin phosphorescence is dependent on the amount of oxy-gen present. Since less oxygen results in longer decaytimes, the quenching of phosphorescence is a sensitivemeans of detecting hypoxia [137]. The Stern±Volmerequation gives the relationship between decay time andoxygen tension. Calibration constants associated withthe Stern±Volmer equation allow calculation of oxygentensions from measured decay times [140]. These cali-bration constants are a property of the suitably pre-pared compound and thus the technique, unlike oxygenelectrodes, requires no calibration prior to each mea-surement [140].

The advantage of this water-soluble compound isthat it can be injected intravenously to measure oxygenconcentrations predominantly of the microvascularcompartment [138, 141]. This is accomplished by bind-ing Pd-porphyrin to a large molecule such as albuminto confine the dye mainly to the vascular compartment.The phosphorimeter measures oxygen dependent phos-phorescence decay following excitation by a pulse oflight [133] and can be attached to a microscope (Fig.6)to measure the PO2 in single blood vessels of Pd-por-phyrin infused animals [137, 141±144]. Such devices

need to be carefully validated prior to use [140]. Invivo, Pd-porphyrin phosphorescence has been used toassess the arteriolar, venular and capillary PO2 in ham-ster and mouse skinfold models [142, 145±150] and inskeletal muscle [141, 143] and intestine of rats [133,137, 144]. Helmlinger and colleagues investigated therelationship between oxygenation and pH in a mousemodel of human colon cancer using an intravital micro-scope equipped with the ability to measure quenchingof phosphorescence and fluorescence of a pH-sensitivedye [151]. They found heterogeneous pH and PO2 pro-files, and a discordant relationship between local pHand corresponding PO2 values.

Optical fibres attached to phosphorimeters allowmeasurement of PO2 in areas not easily accessible tomicroscopes, as well as in more clinically relevant ani-mal models (Fig. 6). The optical fibres lead the excita-tion light and the phosphorescence signal to and fromthe tissue for measurement of microvascular oxygen-ation (mPO2) from organ surfaces. Because the decaytime of the phosphorescence is measured, no directcontact with the tissue is needed and the fibres canbe applied to moving organs such as the heart [152,153]. Such measurements of mPO2 incorporate bloodvessels under the optic fibre over an area of approxi-mately 1 cm2 to a penetration depth of about 0.5 mm[137, 138]. Our research group compared the Pd-por-phyrin phosphorescence fibre technique with a micro-scopic phosphorimeter to validate the microvascularcompartment being measured by fibre phosphorime-try. The PO2 in the gut microcirculation was measured

1053

Fig.6 Schematic diagram of a microscopic and a fibre phospho-rimeter set-up placed over the tissue and microcirculation of an ex-perimental animal infused with albumin-bound Pd-porphyrin.Flash lamps provide green excitation light, which is guided eitherthrough an intravital microscope or through an optical fibre to thetissue and back. A photomultiplier tube PMT detects the oxygen-dependent Pd-porphyrin phosphorescence after passage througha dichroic mirror DM. The output of the PMT is amplified and dig-itised and a computer calculates the decay time of phosphores-cence as well as the associated PO2 value

Fig.7 Microvascular oxygen pressure mPO2 of the gut in a ratmodel of severe, normovolaemic haemodilution (n = 7). Bloodwas withdrawn from anaesthetised and ventilated rats after infu-sion of Pd-porphyrin and replaced by a plasma protein solution.Heart rate and blood pressure remained constant. Measurementof intestinal mPO2 remained unaffected over a wide range of hae-modilutions despite a gradual decrease in mesenteric venous PO2PmvO2 and an increased oxygen extraction ratio O2ER. mPO2 ismaintained until a haematocrit Ht of 14%, after which it falls be-low the venous PO2 indicating the presence of functional shuntingin the intestinal microcirculation. Values are mean � SD. *p < 0.05 PmvO2 vs mPO2

in first order arterioles, venules and capillaries in theileum of rats at three different fractional inspired oxy-gen levels. Simultaneous measurements of PO2 withthe fibreoptic technique showed excellent correlationwith microscopically measured PO2 in capillaries andfirst-order venule vessels, but not with arteriolar andvenous values of PO2 [144]. The values given by the fi-bre phosphorimeter thus represent predominantly cap-illary and venous PO2 and we thus refer to this vari-able as the microvascular PO2 (mPO2). We further de-veloped a preparation method for the compound,which makes the Stern±Volmer calibration constantsinsensitive to pH [140]. The pH independence of thedecay time is favourable, especially in the microcircu-lation where changes in pH are likely to occur underpathological conditions and may then influence themeasurements.

Measurement of mPO2 by fibre phosphorimetry inshock models revealed lower serosal mPO2 than mesen-teric venous values [70]. The presence of this PO2 differ-ence (PO2 gap) is a direct indication for shunting in themicrocirculation [7]. Treatment with a very small

amount of a haemoglobin-based oxygen carrier abol-ished this difference in mesenteric venous and serosalmPO2 values [154]. In contrast, a comparison of mesen-teric venous PO2 and ileal mPO2 in a rat model of severeisovolaemic haemodilution showed a stable relationshipbetween the two values provided the mesenteric flowwas able to compensate for the low haematocrit(Fig. 7). For simultaneous measurement of mPO2 at dif-ferent locations we have developed a multifibre phos-phorimeter and applied it to measurements of mPO2 invarious organs in large animal models of shock and sep-sis [70, 137, 153, 154]. Figure 8 shows a typical measure-ment of mPO2 in intestinal mucosa and serosa as well asin the renal subcapsular cortex during endotoxaemicshock in a pig. The examples show an increased differ-ence between the venous outflow of the gut and the se-rosal mPO2 measured by Pd-porphyrin phosphores-cence. Resuscitation of the endotoxaemic pig with fluidand dopexamine restored tissue oxygenation and equal-ised the two PO2 measurements. This tissue±venousPO2 gap may be an example of microvasculature shunt-ing in states of shock [7].

1054

Fig.8 Simultaneous and continuous mea-surement of microvascular oxygenation inthe kidney cortex and small intestine of apig during endotoxaemia achieved by infu-sion of 1 mg/kg per h of lipopolysaccharide.After a fall in mean arterial pressure below60 mmHg and a 30-min shock phase, treat-ment with dopexamine was started. A Se-rosal microvascular PO2 mpO2 and mesen-teric venous PO2 v. mesent. pO2 show a PO2gap during the shock phase which is cor-rected following dopexamine therapy.B The mPO2 of the kidney cortex showsonly minor changes during the course ofthe experiment, while the mucosal mPO2 ofthe ileum shows restoration from low val-ues seen in the untreated shock phase

Imaging of the microcirculation

Inadequate microvascular flow together with impairedtissue oxygenation contribute to tissue damage occur-ring in sepsis and shock leading to multiple organ fail-ure. In addition to measurement of tissue oxygenation,a method for direct visualisation of the microcirculationin humans would be expected to give sensitive insightsinto the pathogenesis of various acute and chronic dis-eases. Such a method could also elucidate the influenceof vascular tone, oxygenation, cytokines and local hu-moral factors on the distribution of microcirculatoryblood flow.

A newly developed optical device using orthogonalpolarisation spectral imaging (Cytoscan, Cytometrics,Philadelphia, Penna., USA) allows on-line microscopicobservation of the microcirculation by use of an imageguide placed on tissue surfaces. The instrument consistsof a small endoscopic-like light guide attached to a lightsource with filters and provides polarised green light.The light is projected through a beam splitter into a se-ries of lenses through the light guide placed on the tissuesurface. A charge-coupled device video camera detectsthe reflected image from beneath the surface of the illu-minated area. An analyser is located in the reflectedlight path between the object and the camera. Becausethe chosen wavelength of incident light is particularlyabsorbed by haemoglobin, the red blood cells can beclearly observed as they flow through the microcircula-tion. The video images have a remarkably high resolu-tion and can be readily computer analysed. Using com-mercially available software to process images obtainedfrom intravital microscopic observations, red blood cellflux can be measured [8, 155, 156].

We applied the device to the pig gut serosa in thestudy of haemorrhagic shock and ischaemia-reperfu-sion. During haemorrhage, a decline in the number ofperfused capillaries in conjunction with a sluggish bloodflow could be clearly observed. In the reperfusion phasefollowing 45 min of aortic cross-clamping, microcircula-

tory units with blood flow were seen next to units withno flow [152]. This pathological heterogeneity in micro-circulatory flow patterns is consistent with the idea thatshunting pathways exist associated with the ªno-reflowphenomenaº occurring during ischaemia and reperfu-sion.

Until now, information regarding the dynamics of mi-crocirculatory blood flow during different pathologicalconditions has only been available from intravital mi-croscopic studies in animal models. In humans sufferingfrom peripheral vascular disease, the measurement ofnailfold microcirculation by direct capillaroscopy hasbeen the only available clinical technique to study mor-phological and dynamic changes in the microcirculation.The particular advantage of the Cytoscan technique isthat it can be readily applied to humans. Figure 9 showsan example of a recording of the human microcircula-tion under the tongue of the author where single eryth-rocytes can be seen flowing through the capillaries andvenous vessels. It is expected that this new technologywill have applications in several areas of medicinewhere microcirculatory dysfunction is thought to under-lie the disease progression.

Conclusions

The purpose of this paper was to describe and compareavailable techniques for measuring tissue and microcir-culatory oxygenation. Although many promising tech-nologies are available, there is still no truly satisfactorytechnique. Reasons for this include uncertainties as towhich oxygen transport pathways are crucial in thepathogenesis of sepsis and shock, as well as the clinicalavailability of techniques to monitor these pathwaysand guide therapy. A conclusion which arises from thisreview is that, in order to make an impact, clinical mon-itoring techniques will have to sense oxygen transportpathways at the microcirculatory level. It is expectedthat the optical spectroscopic methods described in this

1055

Fig.9 Images of the sublingualmicrocirculation using orthogo-nal polarisation spectral imag-ing (Cytoscan). A The lightguide was gently placed underthe tongue to observe the ana-tomical structure of the sublin-gual microcirculation. B Singleblood cells arrows can clearlybe seen flowing through thecapillaries

paper will contribute to the development of clinicallyapplicable techniques providing information about oxy-gen availability and utilisation in tissue upon which ther-apy can be based.

Acknowledgements The authors thank Dr. O. Eerbeek for hishelp with the NADH-fluorescence imaging and K. Mathura forpreparing the tongue images.

1056

References

1. Robin ED (1980) Of men and mito-chondria: coping with hypoxic dysoxia.Am Rev Respir Dis 122: 517±531

2. Gutierrez G (1991) Cellular energymetabolism during hypoxia. Crit CareMed 19: 619±626

3. Fink MP (1998) Cytopathic hypoxia:mitochondrial dysfunction as a poten-tial mechanism contributing to organfailure in sepsis. In: Sibbald WJ, Mess-mer K, Fink MP (eds) Tissue oxygen-ation in acute medicine. (Update in in-tensive care and emergency medicine33) Springer, Berlin Heidelberg NewYork, pp 128±137

4. Johnson PC (1986) Autoregulation ofblood flow. Circ Res 59: 483±495

5. Johnson BA, Weil MH (1991) Rede-fining ischemia due to circulatory fail-ure as dual defects of oxygen deficitsand or carbon dioxide excesses. CritCare Med 19: 1432±1438

6. Vallet B (1998) Vascular reactivity andtissue oxygenation. Intensive CareMed 24: 3±11

7. Ince C, Sinaasappel M (1999) Micro-circulatory oxygenation and shuntingin sepsis and shock. Crit Care Med 27:

8. Ellis C (1998) Microcirculatory flows,microcirculatory responsiveness, mi-crocirculatory and regional (arteri-olar/venular) O2 saturations. In: Sib-bald WJ, Messmer K, Fink MP (eds)Tissue oxygenation in acute medicine.(Update in Intensive Care Medicineand Emergency Medicine 33) Sprin-ger, Berlin Heidelberg New York To-kyo, pp 204±225

9. Dantzker DR (1993) Adequacy of tis-sue oxygenation. Crit Care Med 21:S40±S43

10. Vincent J-L (1998) The available clini-cal tools ± oxygen-derived variables,lactate and pHi. In: Sibbald WJ, Mess-mer K, Fink MP (eds) Tissue oxygen-ation in acute medicine. (Update in in-tensive care and emergency medicine33) Springer, Berlin Heidelberg NewYork, pp 193±203

11. Bakker J, Gris P, Coffernils M, KahnRJ, Vincent JL (1996) Serial blood lac-tate levels can predict the develop-ment of multiple organ failure follow-ing septic shock. Am J Surg 171:221±226

12. Bakker J, Coffernils M, Leon M, GrisP, Vincent JL (1991) Blood lactate lev-els are superior to oxygen-derivedvariables in predicting outcome in hu-man septic shock. Chest 99: 956±962

13. Hotchkiss RS, Karl IE (1992) Reeval-uation of the role of cellular hypoxiaand bioenergetic failure in sepsis.JAMA 267: 1503±1510

14. Vincent J-L (1996) End-points of re-suscitation: arterial blood pressure, ox-ygen delivery, blood lactate, or. ..? In-tensive Care Med 22: 3±5

15. Kreisberg RA (1980) Lactate homeo-stasis and lactic acidosis. Ann InternMed 92: 227±237

16. Gattinoni L, Brazzi L, Pelosi P, LatiniR, Tognoni G, Pesenti A, Fumagalli R(1995) A trial of goal-oriented hemo-dynamic therapy in critically ill pa-tients. N Engl J Med 333: 1025±1032

17. Ruokonen E, Takala J, Uusaro A(1991) Effect of vasoactive treatmenton the relationship between mixed ve-nous and regional oxygen saturation.Crit Care Med 19: 1365±1369

18. Dahn MS, Lange MP, Jacobs LA(1988) Central mixed and splanchnicvenous oxygen saturation monitoring.Intensive Care Med 14: 373±378

19. Kaiser U, Langrehr JM, Haack M,Mohnhaupt A, Neuhaus P, Rossaint R(1995) Hepatic venous catheterizationin patients undergoing positive end-expiratory ventilation after OLT: tech-nique and clinical impact. Clin Trans-plant 9: 301±306

20. Christiansen CL, Ahlburg P, JakobsenC-J, Andresen EB, Paulsen PK (1998)The influence of propofol and midazo-lam/halothane anesthesia on hepaticSvO2 and gastric mucosal pH duringcardiopulmonary bypass. J Cardiotho-rac Vasc Anesth 12: 418±421

21. Kainuma M, Nakashima K, Sakuma I,Kawase M, Komatsu T, Shimada Y,Nimura Y, Nonamin T (1992) Hepaticvenous hemoglobin oxygen saturationpredicts liver dysfunction after hepat-ectomy. Anesthesiology 76: 379±386

22. Gatecel C, Mebazza A, Kong R, Gui-nard N, Kermarrec N, MatØo J, PayenD (1995) Inhaled nitric oxide improveshepatic tissue oxygenation in rightventricular failure: value of hepatic ve-nous oxygen saturation monitoring.Anesthesiology 82: 588±590

23. De Deyne C, Van Aken J, Decruye-naere J, Struys M, Colardyn F (1998)Jugular bulb oximetry: review on a ce-rebral monitoring technique. ActaAnaesthesiol Belg 49: 21±31

24. Stocchetti N, Paparella A, Bridelli F,Bacchi M, Piazza P, Zuccoli P (1994)Cerebral venous oxygen saturationstudied with bilateral samples in theinternal jugular veins. Neurosurgery34: 38

25. Beartds SC, Yule S, Kassner A, Jack-son A (1998) Anatomical variation ofcerebral venous drainage: the theoreti-cal effect on jugular bulb blood sam-ples. Anaesthesia 53: 627±633

26. Metz C, Holzschuh M, Bein T, Woert-gen C, Rothoerl R, Kallenbach B, Tae-ger K, Brawanski A (1998) Monitoringof cerebral oxygen metabolism in thejugular bulb: reliability of unilateralmeasurements in severe head injury. JCereb Blood Flow Metab 18: 332±343

27. Thigarajan A, Goverdhan PD, Chari P,Somasunderam K (1998) The effect ofhyperventilation and hyperoxia on ce-rebral venous oxygen saturation in pa-tients with traumatic brain injury. An-esth Analg 87: 850±853

28. Chan K-H, Miller JD, Dearden NM,Andrews PJD, Midgley S (1992) Theeffect of changes in cerebral perfusionpressure upon middle cerebral arteryblood flow velocity and jugular bulbvenous oxygen saturation after severebrain injury. J Neurosurg 77: 55±61

29. Coplin WM, O'Keefe GE, Grady MS,Grant GA, March KS, Winn HR,Lam AM (1998) Accuracy of continu-ous jugular bulb oximetry in the inten-sive care unit. Neurosurgery 42:533±540

30. ViguØ B, Ract C, Benayed M, ZlotineN, Leblanc PE, Samii K, BissonnetteB (1999) Early SjvO2 monitoring in pa-tients with severe brain trauma. Inten-sive Care Med 25: 445±451

1057

31. Gopinath SP, Robertson CS, ContantCF, Hayes C, Feldman Z, NarayanRK, Grossman RG (1994) Jugular ve-nous desaturation and outcome afterhead injury. J Neurol NeurosurgPsychatry 57: 717±723

32. Moss E, Dearden NM, Berridge JC(1995) Effects of changes in mean arte-rial pressure on SjO2 during cerebralaneurysm surgery. Br J Anaesth 75:527±530

33. Trubiano P, Heyer EJ, Adams DC,McMahon DJ, Christiansen I, RoseEA, Delphin E (1996) Jugular venousbulb oxyhemoglobin saturation duringcardiac surgery: accuracy and reliabili-ty using a continuous monitor. AnesthAnalg 82: 964±968

34. Loick HM, Möllhoff T, Engers G, Dei-wick M, Weyand M (1997) Improve-ment of jugular bulb oxygen tensionafter hemodynamic support by intra-aortic balloon counterpulsation. J Car-diothorac Vasc Anesth 11: 83±85

35. Fiddian-Green RG, Baker S (1987)Predictive value of the stomach wallpH for complications after cardiac op-erations. Comparison with other moni-toring. Crit Care Med 15: 153±136

36. Fink MP, Cohn SM, Lee PC, Roth-schild HR, Deniz YF, Wang H, Fid-dian-Green RG (1989) Effect of li-popolysaccharide on intestinal intra-mucosal hydrogen ion concentrationin pigs: evidence of gut ischemia in anormodynamic model of septic shock.Crit Care Med 17: 641±646

37. Schlichtig R, Bowles SA (1994) Distin-guishing between aerobic and anaero-bic appearance of dissolved CO2 in in-testine during low flow. J Appl Physiol76: 2443±2451

38. Mohsenifar Z, Hay A, Hay J, LewisMI, Koerner SK (1993) Gastric intra-mural pH as a predictor of success orfailure in weaning patients from me-chanical ventilation. Ann Intern Med119: 794±798

39. Tang W, Weil MH, Sun S, Noc M, Gaz-muri RJ, Bisera J (1994) Gastric intra-mural pCO2 as monitor of perfusionfailure during hemorrhagic and ana-phylactic shock. J Appl Physiol 76:572±577

40. van der Meer JT, Wang H, Fink MP(1995) Endotoxemia causes ileal mu-cosal acidosis in the absence of mucos-al hypoxia in a normodynamic porcinemodel of septic shock. Crit Care Med23: 1217±1226

41. Tùnnessen TI (1997) Biological basisfor pCO2 as a detector of ischemia.Acta Anaesthesiol Scand 41: 659±669

42. Vallet B, Ince C (1999) Noninvasiveassessment of tissue oxygenation. Se-min Respir Crit Care Med 20: 3±10

43. Russel JA (1997) Gastric tonometry:does it work? Intensive Care Med 23:3±6

44. Guzman J, Kruse JA (1997) Continu-ous assessment of gastric intramucosalpCO2 and pH in hemorrhagic shockusing capnometric recirculating gastonometry. Crit Care Med 25: 533±537

45. Heinonen PO, Jousela IT, BlomqvistKA, Olkkola KT, Takkunen OS(1997) Validation of air tonometricmeasurement of gastric regional con-centrations of CO2 in critically ill sep-tic patients. Intensive Care Med 23:524±529

46. Kolkman JJ, Zwaarekant LJ, Boshui-zen K, Groeneveld AB, MeuwissenSG (1997) In vitro evaluation of intra-gastric pCO2 measurement by airtonometry. J Clin Monit 13: 115±119

47. Taylor DE, Gutierrez G (1999) Gas-trointestinal tonometry: basic princi-ples and recent advances in monitor-ing regional CO2 metabolism. SeminRespir Crit Care Med 20: 17±27

48. Brinkmann A, Calzia E, Träger K,Radermacher P (1998) Monitoringthe hepato-splanchnic region in thecritically ill patient ± measurementtechniques and clinical relevance. In-tensive Care Med 24: 542±556

49. Groeneveld ABJ, Kolkmann JJ (1994)Splanchnic tonometry: a review ofphysiology, methodology, and clinicalapplications. J Crit Care 9: 198±210

50. Pargger H, Hampl KF, Christen P,Staender S, Scheidegger D (1998) Gas-tric intramucosal pH-guided therapyin patients after elective repair of in-frarenal abdominal aneurysms: is itbeneficial? Intensive Care Med 24:769±776

51. Uusaro A (1998) Gastric tonometry,tissue hypoxia and MSOF ± is there alink? Intensive Care Med 24: 753±755

52. Clark LC (1956) Monitor and controlof blood and tissue oxygen tension.Trans Am Soc Artif Intern Org 2:41±46

53. Winlove CP, Sair M, Evans TW (1998)Monitoring the systemic circulation insepsis with microelectrodes. In: Sib-bald WJ, Messmer K, Fink MP (eds)Tissue oxygenation in acute medicine.(Update in intensive care and emer-gency medicine 33) Springer, BerlinHeidelberg New York, pp 226±240

54. Lübbers DW (1996) Oxygen elec-trodes and optodes and their applica-tion in vivo. Adv Exp Med Biol 388:13±33

55. Lübbers DW, Baumgartl H, ZimelkaW (1994) Heterogeneity and stabilityof local pO2 distribution within thebrain tissue. Adv Exp Med Biol 345:567±574

56. Fleckenstein W, Weiss CH (1984) Acomparison of pO2-histograms fromrabbit hindlimb muscles obtained bysimultaneous measurements with hy-podermic needle electrodes and withsurface electrodes. Adv Exp Med Biol169: 447±453

57. Valadka AB, Gopinath SP, ContantCF, Uzura M, Robertson CS (1998)Relationship of brain tissue PO2 tooutcome after severe head injury. CritCare Med 26: 1576±1581

58. Hasjackl M, Luz G, Sparr H, GermannR, Salak N, Friesenecker B, Deusch E,Meusburger S, Hasibeder W (1997)The effects of progressive anemia onjejunal mucosal and serosal tissue oxy-genation in pigs. Anesth Analg 84:538±544

59. Vallet B, Lund N, Curtis SE, Kelly D,Cain SM (1994) Gut and muscle tissuePO2 in endotoxemic dogs during shockand resuscitation. J Appl Physiol 76:793±800

60. Armbruster K, Nöldge-SchomburgGFE, Dressler IMJ, Fittkau AJ, Ha-berstroh J, Geiger K (1997) The ef-fects of desflurane on splanchnic he-modynamics and oxygenation in theanesthetized pig. Anesth Analg 84:271±277

61. Joachimsson P-O, Sjöberg F, ForsmanM, Johansson M, Casimir Ahn H, Rut-berg H (1996) Adverse effects of hy-peroxemia during cardiopulmonarybypass. J Thorac Cardiovasc Surg 112:812±819

62. Boekstegers P, Weidenhofer S, Kaps-ner T, Werdan K (1994) Skeletal mus-cle partial pressure of oxygen in pa-tients with sepsis. Crit Care Med 22:640±650

63. Stein JC, Ellis CG, Ellsworth ML(1993) Relationship between capillaryand systemic venous PO2 during non-hypoxic and hypoxic ventilation. AmJ Physiol 265: H537±H542

64. Sair M, Etherington PJ, Curzen NP,Winlove CP, Evans TW (1996) Tissueoxygenation and perfusion in endotox-emia. Am J Physiol 271: H1620-H1625

65. Germann R, Haisjackl M, Schwarz B,Salake N, Deusch E, Pajk W, WolfHJ, Riedmann B, Hasibeder W (1997)Dopamine and intestinal mucosal tis-sue oxygenation in a porcine model ofhaemorrhage. Br J Anaesth 79:357±362

1058

66. Germann R, Haisjackl M, Schwarz B,Deusch E, Meusburger S, Gruber E,Pajk W, Hausdorfer H, Bonatti J, Furt-ner B, Ulmer H, Hasibeder W (1997)Inotropic treatment and intestinal mu-cosal tissue oxygenation in a model ofporcine endotoxemia. Crit Care Med25: 1191±1197

67. Gutierrez G, Lund N, Acero AL, Ma-rini C (1989) Relationship of venouspO2 to muscle pO2 during hypoxemia.Am J Physiol 67: 1093±1099

68. Hasibeder W, Germann R, Wolf HJ,Haisjackl M, Hausdorfer H, Ried-mann B, Bonatii J, Gruber E, SchwarzB, Waldenberger P, Friesenecker B,Furtner B (1996) Effects of short-termendotoxemia and dopamine on mucos-al oxygenation in porcine jejunum.Am J Physiol 270: G667±G675

69. Wagner K, Bossen W, Schramm U(1992) Tissue alterations by the pene-tration of a pO2 sensing needle probe.Adv Exp Med Biol 317: 639±644

70. Sinaasappel M, van Iterson M, Ince C(1999) Microvascular oxygen pressurein the pig intestine during haemor-rhagic shock and resuscitation. J Phys-iol 514: 245±253

71. Shapiro BA (1995) Intra-arterial andextra-arterial pH, pCO2, and pO2monitors. Acta Anaesthesiol Scand39: 69±74

72. McKinley BA, Parmley CL, ButlerBD (1998) Skeletal muscle pO2,pCO2, and pH in hemorrhage, shock,and resuscitation in dogs. J Trauma44: 119±127

73. Lübbers DW (1995) Optical sensorsfor clinical monitoring. Acta Anaes-thesiol Scand 39 [Suppl 104]: 37±54

74. Pappert D, Rossaint R, LewandowskiK, Kuhlen R, Gerlach H, Falke KJ(1995) Preliminary evaluation of anew continuous intra-arterial bloodgas monitoring device. Acta Anaes-thesiol Scand 39: 67±70

75. Zollinger A, Spahn DR, Singer T,Zalunardo MP, Stoehr S, Weder W,Pasch T (1997) Accuracy and clinicalperformance of a continuous intra-ar-terial blood gas monitoring systemduring thoracoscopic surgery. Br JAnaesth 79: 47±52

76. Hoffman WE, Charbel FT, Edelman Get al (1997) Brain tissue gases and pHduring arteriovenous malformation re-section. Neurosurgery 40: 294±301

77. Hoffman WE, Charbel FT, Edelman G(1996) Brain tissue oxygenation, car-bon dioxide, and pH in neurosurgicalpatients at risk for ischemia. AnesthAnalg 82: 582±586

78. Hoffman WE, Charbel FT, Abood C,Ausman JL (1997) Regional ischemiaduring cerebral bypass surgery. SurgNeurol 47: 455±459

79. Hoffman WE, Charbel FT, EdelmanG, Misra M, Ausman JI (1998) Com-parison of the effect of etomidate anddesflurane on brain tissue gases andpH during prolonged middle cerebralartery occlusion. Anesthesiology 88:1188±1194

80. Meixensberger J, Dings J, Kuhnigk H,Roosen K (1993) Studies of tissue pO2in normal and pathological brain cor-tex. Acta Neurochir 59 [Suppl]: 58±63

81. Knichwitz G, Rötker J, Brüssel T,Kuhmann M, Mertes N, Möllhoff T(1996) A new method for continuousintramucosal pCO2 measurement inthe gastrointestinal tract. Anesth An-alg 83: 6±11

82. Knichwitz G, Rötker J, Möllhoff T,Richter K-D, Brüssel T (1998) Contin-uous intramucosal pCO2 measurementallows early detection of intestinalmalperfusion. Crit Care Med 26:1550±1557

83. Jöbsis FF (1977) Noninvasive infraredmonitoring of cerebral and myocardialoxygen sufficiency and circulatory pa-rameters. Science 198: 1264±1267

84. Connett RJ, Honig CR, Gayeski TEJ,Brooks GA (1990) Defining hypoxia:a systems view of VO2, glycolysis, en-ergetics, and intracellular PO2. J ApplPhysiol 68: 833±842

85. Matsumoto H, Oda T, Hossain MA,Yoshimura N (1996) Does the redoxstate of cytochrome aa3 reflect brainenergy level during hypoxia? Simulta-neous measurements by near infraredspectrophotometry and 31P nuclearmagnetic resonance spectroscopy. An-esth Analg 83: 513±518

86. Colier WNJM, van Haaren NJCW,Oeseburg B (1995) A comparativestudy of two near infrared spectropho-tometers for the assessment of cere-bral haemodynamics. Acta Anaesthe-siol Scand 39 [Suppl 107]: 101±105

87. Wray S, Cope M, Delpy DT, Wyatt JS,Reynolds EOR (1988) Characteriza-tion of the near infrared absorptionspectra of cytochrome aa3 and hemo-globin for non-invasive monitoring ofcerebral oxygenation. Biochim Bio-phys Acta 933: 184±192

88. Cope M, Delpy DT, Wray S, Wyatt JS,Reynolds EOR (1988) A CCD spec-trophotometer to quantitate the con-centration of chromophores in livingtissue utilizing the absorption peak at975 nm. Adv Exp Med Biol 248: 33±40

89. Matcher SJ, Cope M, Delpy DT (1994)Use of water absorption spectrum toquantify tissue chromophore concen-tration changes in near infrared spec-troscopy. Phys Med Biol 39: 177±169

90. Benaron DA, Stevenson DK (1993)Time-of-flight and absorbance imag-ing of biologic media. Science 259:1463±1466

91. Rolfe P (1995) Experience with inva-sive and non-invasive sensors for ana-esthesia. Acta Anesthesiol Scand 39:S61±S68

92. Wahr JA, Temper KK, Samra S, DelpyDT (1996) Near-infrared spectrosco-py: theory and applications. J Cardio-thorac Vasc Anesth 10: 406±418

93. Wyatt JS, Cope M, Delpy DT, Wray S,Reynolds EOR (1986) Quantificationof cerebral oxygenation and haemody-namics in sick newborn infants by nearinfrared spectrophotometry. Lancet :1063±1066

94. Ince C, Bruining HA (1991) Opticalspectroscopy for measurement of tis-sue hypoxia. In: Vincent J-L (ed) Year-book of intensive care and emergencymedicine. Springer, Berlin HeidelbergNew York, pp 161±171

95. Kurth CD, Steven JM, Nicolson SC(1995) Cerebral oxygenation duringpediatric cardiac surgery using deephypothermic circulatory arrest. Anes-thesiology 82: 74±82

96. Wickramasinghe YABD, Livera LN,Spencer SA, Rolfe P, Thorniley M(1992) Plethysmographic validation ofnear infrared spectroscopic monitor-ing of cerebral blood volume. ArchDis Child 67: 407±411

97. Aldrich CJ, D'Atona D, Wyatt JS,Spencer JA, Peebles DM, ReynoldsEO (1994) Fetal cerebral oxygenationmeasured by near-infrared spectrosco-py shortly before birth and acid-basestate at birth. Obstet Gynecol 84:861±866

98. Aldrich CJ, Wyatt JS, Spencer JA,Reynolds EO, Delpy DT (1994) Theeffect of maternal oxygen administra-tion on human fetal cerebral oxygen-ation measured during labour by nearinfrared spectroscopy. Br J Obstet Gy-naecol 101: 509±513

99. Aldrich CJ, D'Atona D, Spencer JA,Wyatt JS, Peebles DM, Delpy DT,Reynolds EO (1995) Late fetal heartdecelerations and changes in cerebraloxygenation during first stage of la-bour. Br J Obstet Gynaecol 102: 9±13

1059

100. Tateishi A (1995) Qualitative compari-son of carbon dioxide-induced changein near-infrared spectroscopy versusjugular venous oxygen saturation inadults with acute brain disease. CritCare Med 23: 1734±1738

101. Sapire KJ, Gopinath SP, Farhat G,Thakar DR, Gabrielli A, Jones JW,Robertson CS, Chance B (1997) Cere-bral oxygenation during warming aftercardiopulmonary bypass. Crit CareMed 25: 1655±1662

102. Grubhofer G, Lassnigg A, Manlik F,Marx E, Trubel W, Hiesmayer M(1997) The contribution of extracrani-al blood oxygenation on near-infraredspectroscopy during carotid throm-boendarterectomy. Anaesthesia 52:116±120

103. Nollert G, Möhnle P, Tassani-Prell P,Reichart B (1995) Determinants of ce-rebral oxygenation during cardiac sur-gery. Circulation 92 [Suppl II]:II327±II333

104. Pollard V, Prough DS, DeMelo AE,Deyo DJ, Uchida T, Widman R(1982) The influence of carbon dioxideand body position on near-infraredspectroscopic assessment of cerebralhemoglobin oxygen saturation. An-esth Analg 82: 278±287

105. Gupta AK, Menon DK, Czosnyka M,Smielewski P, Kirkpatrick PJ, JonesJG (1997) Non-invasive measurementof cerebral blood volume in volun-teers. Br J Anaesth 78: 39±43

106. Pollard V, Prough DS, DeMelo AE,Deyo DJ, Uchida T, Stoddart HF(1996) Validation in volunteers of anear-infrared spectroscope for moni-toring brain oxygenation in vivo. An-esth Analg 82: 269±277

107. Harris DNF, Bailey SM (1993) Nearinfrared spectroscopy in adults. An-aesthesia 48: 694±696

108. Kirkpatrick PJ, Smielewski P, Whit-field PC, Czosnyka M, Menon D, Pi-ckard JD (1995) An observationalstudy of near-infrared spectroscopyduring carotid endarterectomy. J Neu-rosurg 82: 756±763

109. Vollmar B, Ruttinger D, Menger MD(1997) Monitoring of microvascularhemoglobin oxygenation in liver andskeletal muscle tissue of endotocin ex-posed rats using reflection spectropho-tometry. Adv Exp Med Biol 428:397±402

110. Vollmar B, Menger MD (1997) As-sessment of microvascular oxygen sup-ply and tissue oxygenation in hepaticischemia/reperfusion. Adv Exp MedBiol 428: 403±408

111. Cooper PG, Wilson GJ, Hardman DT,Kawaguchi O, Huang YF, Martinez-Coll A, Carrington RA, Puchert E,Crameri R, Horamand C, Hunyor SN(1997) Blood oxygen desaturation het-erogeneity during muscle contractionrecorded by near infrared spectrosco-py. Adv Exp Med Biol 428: 285±292

112. De Blasi RA, Ferrari M, Antonelli M,Conti G, Amenrader N, Gasparetto A(1996) O2 consumption±O2 deliveryrelationship and arteriolar resistancein the forearm of critically ill patientsmeasured by near infrared spectrosco-py. Shock 6: 319±325

113. Noriyuki T, Ohdan H, Yoshioka S,Miyata Y, Asahara T, Dohi K (1997)Near-infrared spectroscopic methodof assessing the tissue oxygenationstate of living lung. Am J Respir CritCare Med 156: 1656±1661

114. Newton CRJC, Wilson DA, GunnoeE, Wagner B, Cope M, Traystman RJ(1997) Measurement of cerebral bloodflow in dogs with near infrared spec-troscopy in the reflectance mode is in-valid. J Cereb Blood Flow Metab 17:695±703

115. Temmesfeld-Wollbrück B, Szalay A,Mayer K, Olschewski H, Seeger W,Grimminger F (1998) Abnormalitiesof gastric mucosal oxygenation in sep-tic shock: partial responsivness to dop-examine. Am J Respir Crit Care Med157: 1586±1592

116. Frank KH, Kessler M, Appelbaum K,Dümmler W (1989) The Erlangen mi-cro-lightguide spectrophotometerEMPHO 1. Phys Med Biol 34:1883±1900

117. Frank K, Kessler M, Appelbaum K,Zündorf J, H-P. A, Siebenhaar G(1990) In situ monitoring of organs.In: Vincent J-L (ed) Update in inten-sive care and emergency medicine, vol13. Springer, Berlin Heidelberg NewYork, pp 145±159

118. Hoper J, Kessler M, Frank K, Tau-schek D, Zuendorf J, Lang N, MauchE (1991) Monitoring of intracapillaryHbO2 in foetal scalp during delivery.Adv Exp Med Biol 251: 485±489

119. Meyer B, Schaller C, Frenkel C,Schramm J (1998) Physiological stealaround AVMs of the brain is notequivalent to cortical ischemia. NeurolRes 20 [Suppl 1]: S14±S17

120. Machens HG, Pallua N, Mailänder P,Pasel J, Frank KH, Reimer R, BergerA (1995) Measurements of tissueblood flow by the hydrogen clearancetechnique (HCT): a comparative studyincluding laser Doppler flowmetry(LDF) and the Erlangen micro-light-guide spectrophotometer (EMPHO).Microsurgery 16: 808±817

121. Wolff KD, Marks C, Ükermann B,Specht M, Frank KH (1996) Monitor-ing of flaps by measurement of intra-capillary haemoglobin oxygenationwith EMPHO II: experimental andclinical study. Br J Oral MaxillofacSurg 34: 524±529

122. Thum J, Caspary L, Creutzig A, Alex-ander K (1996) A new method for theassessment of tissue hemoglobin oxy-genation in patients with chronic ve-nous insufficiency. Microvasc Res 51:317±326

123. Thum J, Caspary L, Creutzig A, Stap-pler T, Alexander K (1997) Non-inva-sive determination of dermal hemo-globin oxygenation and concentrationin patients with peripheral arterial oc-culsive disease. Vasa 26: 11±17

124. Sato N, Kamada T, Shichiri M, Kawa-nao S, Abe H, Hagihara B (1989) Mea-surement of hemoperfusion and oxy-gen sufficiency in humans. Gastroen-terology 76: 814±819

125. Chance B (1976) Pyridine nucleotideas an indicator of the oxygen require-ments for energy-linked functions ofmitochondria. Circ Res 38: 31±38

126. Ashruf JF, Coremans JM, BruiningHA, Ince C (1995) Increase of cardiacwork is associated with decrease of mi-tochondrial NADH. Am J Physiol 269:H856±H862

127. Coremans JMMC, Ince C, BruiningHA, Puppels G (1997) (Semi-)quanti-tative analysis of NADH fluorescenceimages in blood-perfused heart. Bio-phys J 72: 1849±1860

128. Ince C, Ashruf JF, Avontuur JA, Wie-ringa PA, Spaan JA, Bruining HA(1993) Heterogeneity of the hypoxicstate in rat heart is determined at cap-illary level. Am J Physiol 264:H294±H301

129. Avontuur JA, Bruining HA, Ince C(1995) Inhibition of nitric oxide syn-thesis causes myocardial ischemia inendotoxemic rats. Circ Res 76:418±425

130. Duboc D, Toussaint M, Donsez D,Weber S, Guerin F, Degeorges M, Re-nault G, Polianski J, Pocidalo JJ(1986) Detection of regional myocar-dial ischaemia by NADH laser fluo-rimetry during human left heart cathe-terisation. Lancet II: 522

1060

131. Duboc D, Renault G, Polianski J,Muffat-Joly M, Toussaint M, GuerinF, Pocidalo J, Fardeau M (1986)NADH measured by laser fluometryin skeletal muscle in McArdle's dis-ease. N Engl J Med 316: 1664±1665

132. Toth A, Pal M (1996) Are there oxy-gen-deficient regions in resting skele-tal muscle? Am J Physiol 270:H1933±H1939

133. Sinaasappel M, van Iterson M, Ince C(1996) In vivo application of Pd-por-phine for measurement of oxygen con-centration in the gut. In: Kohen E,Hirschberg JG (eds) Analytical use offluorescent probes in oncology. Ple-num Press, New York, pp 91±96

134. Vetterlein F, Prange M, Lubrich D, Pe-dina J, Neckel M, Schmidt G (1995)Capillary perfusion pattern and mi-crovascular geometry in heteroge-neous hypoxic areas of hypoperfusedrat myocardium. Am J Physiol 268:H183±H194

135. Miyazaki K, Hori S, Inoue S, AdachiT, Bessho M, Kuwahira I, Mori H,Nakazawa H, Aikawa N, Ogawa S(1997) Characterization of energy me-tabolism and blood flow distributionin myocardial ischemia in hemorrhagicshock. Am J Physiol 273: H600±H607

136. Avontuur JAM, Bruining HA, Ince C(1997) Nitric oxide causes dysfunctionof coronary autoregulation in endo-toxemic rats. Cardiovasc Res 35:368±376

137. Sinaasappel M (1997) MicrovascularpO2 in models of shock. PhD thesis.University of Amsterdam, Amster-dam, p 133

138. Wilson DF, Pastuszko A, DiGiacomoJE, Pawlowski M, Schneiderman R,Delivoria-Papadopoulos M (1991) Ef-fect of hyperventilation on oxygen-ation of the brain cortex of newbornpiglets. J Appl Physiol 70: 2691±2696

139. Vanderkooi JM, Maniara G, GreenTJ, Wilson DF (1987) An opticalmethod for measurement of dioxygenconcentration based upon quenchingof phosphorescence. J Biol Chem 262:5476±5482

140. Sinaasappel M, Ince C (1996) Calibra-tion of Pd-porphyrin phosphorescencefor oxygen concentration measure-ments in vivo. J Appl Physiol 81:2297±2303

141. Shonat RD, Johnson PC (1997) Oxy-gen tension gradients and heterogene-ity in venous microcirculation: a phos-phorescence quenching study. Am JPhysiol 272: H2233±H2240

142. Kerger H, Saltzman DJ, Menger MD,Messmer K, Intaglietta M (1996) Sys-temic and subcutaneous microvascularpO2 dissociation during 4-h hemor-rhagic shock in conscious hamsters.Am J Physiol 270: H827±H836

143. Zheng L, Golub AS, Pittman RN(1996) Determination of PO2 and itsheterogeneity in single capillaries.Am J Physiol 271: H365±H372

144. Sinaasapple M, Donkersloot C, vanBommel J, Ince C (1999) PO2 mea-surements in the rat intestinal micro-circulation. Am J Physiol 276:G1515±G1520

145. Torres Filho IP, Kerger H, IntagliettaM (1996) pO2 measurements in arteri-olar networks. Microvasc Res 51:202±212

146. Torres Filho IP, Intaglietta M (1993)Microvessel PO2 measurements byphosphorescence decay method. Am JPhysiol 265: H1434±H1438

147. Intaglietta M, Johnson PC, WinslowRM (1996) Microvascular and tissueoxygen distribution. Cardiovasc Res32: 632±643

148. Kerger H, Torres Filho IP, Rivas M,Winslow RM, Intaglietta M (1995)Systemic and subcutaneous microvas-cular oxygen tension in conscious Syri-an golden hamsters. Am J Physiol 268:H802±H810

149. Kerger H, Tsai AG, Saltzman DJ,Winslow RM, Intaglietta M (1997)Fluid resuscitation with O2 vs. non-O2carriers after 2 h of hemorrhagic shockin conscious hamsters. Am J Physiol272: H525±H537

150. Kerger H, Saltzman DJ, Gonzales A,Tsai AG, van Ackern K, WinslowRM, Intaglietta M (1997) Microvascu-lar oxygen delivery and interstitial ox-ygenation during sodium pentobarbit-al anesthesia. Anesthesiology 86:372±386

151. Helmlinger G, Yuan F, Dellian M, JainRK (1997) Interstitial pH and pO2 gra-dients in solid tumors in vivo: high-res-olution measurements reveal a lack ofcorrelation. Nature Med 3: 177±182

152. Siegemund M, van Iterson M, Ince C(1998) Oxygen electrodes, optode mic-rosensing, near infrared spectroscopyand spectrophotometry. In: SibbaldWJ, Messmer K, Fink MP (eds) Tissueoxygenation in acute medicine. (Up-date in intensive care and emergencymedicine 33) Springer, Berlin Heidel-berg New York, pp 241±259

153. van Iterson M, Siegemund M, Dries J,Trouwborst A, Ince C (1998) Mi-crovascular oxygenation of the heartand gut during haemorrhage in pigs(abstract). Intensive Care Med 24[Suppl 1]: 190

154. van Iterson M, Sinaasappel M, BurhopK, Trouwborst A, Ince C (1998) Low-volume resuscitation with hemoglo-bin-based oxygen carrier after hemor-rhage improves gut microvascular oxy-genation in swine. J Lab Clin Med 132:421±431

155. Klyscz T, Juenger M, Jung F, Zeintl H(1997) Cap image ± a newly-developedcomputer-aided videoframe analysissystem for dynamic capillaroscopy.Biomed Tech 42: 168±175

156. Ellis CG, Ellsworth ML, Pittman RN,Burgess WL (1992) Application of im-age analysis for evaluation of redblood cell dynamics in capillaries. Mi-crovasc Res 44: 214±25