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Basics in Arterial Blood
Gas Interpretation
Crisbert I. Cualteros, M.D.
Obtaining Blood Gas Obtaining Blood Gas SamplesSamples
Radial artery- best siteRadial artery- best site located superficially, easy to palpate located superficially, easy to palpate
& stabilize& stabilize excellent collateral circulation via excellent collateral circulation via
ulnar arteryulnar artery not adjacent to large veinsnot adjacent to large veins probing needle relatively pain-free if probing needle relatively pain-free if
periosteum is avoidedperiosteum is avoided
TechniqueTechnique for Radial Artery for Radial Artery PuncturePuncture
Explain process to patient. Examine skin, Explain process to patient. Examine skin, palpate radial & ulnar arteries. Perform palpate radial & ulnar arteries. Perform modified Allen Test.modified Allen Test.
The Allen TestThe Allen Test
have the patient have the patient clench his/her fistclench his/her fist
press on both press on both radial and ulnar radial and ulnar arteriesarteries
have the patient have the patient unclench fistunclench fist
test for good test for good collateral flow.collateral flow.
Technique for Radial Artery Technique for Radial Artery PuncturePuncture
Position patient- hyperextend wrist. Clean Position patient- hyperextend wrist. Clean site with 70% isopropyl alcohol.site with 70% isopropyl alcohol.
Use latex gloves while doing procedure.Use latex gloves while doing procedure. Local anesthesia may be used.Local anesthesia may be used. Use G20 or G21 needle. Flush syringe with Use G20 or G21 needle. Flush syringe with
sodium heparin (10 mg/ml or 1,000 sodium heparin (10 mg/ml or 1,000 units/ml) & empty. units/ml) & empty. 0.15-0.25 ml of heparin will 0.15-0.25 ml of heparin will anticoagulate 2-4 ml of blood.anticoagulate 2-4 ml of blood.
Technique for Radial Artery Technique for Radial Artery PuncturePuncture
Palpate artery with one hand while holding Palpate artery with one hand while holding properly prepared syringe & needle with otherproperly prepared syringe & needle with otherhand. Hold syringe like a pencil & enter skin at hand. Hold syringe like a pencil & enter skin at 4545o. Advance needle slowly.
Never redirect needle without first withdrawing to Never redirect needle without first withdrawing to subcutaneous tissue.subcutaneous tissue.
Obtain 2-4 ml blood. If possible don’t aspirate.Obtain 2-4 ml blood. If possible don’t aspirate. Remove air bubbles from syringe. Immediately Remove air bubbles from syringe. Immediately
seal syringe with cap.seal syringe with cap. Place sample in ice slush. Analyze blood sample Place sample in ice slush. Analyze blood sample
within 10 minutes.within 10 minutes. Apply pressure to site until bleeding has stopped.Apply pressure to site until bleeding has stopped.
Potential ComplicationsPotential Complications
PainPain
Hematoma, hemorrhageHematoma, hemorrhage
Trauma to vesselTrauma to vessel
ArteriospasmArteriospasm
Air or clotted-blood Air or clotted-blood
emboliemboli
Vasovagal responseVasovagal response
Arterial occlusionArterial occlusion
InfectionInfection
Indications for ABGIndications for ABG
Assess ventilation & acid-base Assess ventilation & acid-base balancebalance
Assess oxygenation statusAssess oxygenation status
Ventilatory/ Ventilatory/ Acid-Base StatusAcid-Base Status
Henderson-Hasselbach Parameters & their Henderson-Hasselbach Parameters & their normal laboratory rangesnormal laboratory ranges
pH= pH= [HCO[HCO33]p]p
PPC02C02
pHpH PCO2PCO2
(mmHg)(mmHg)[HCO3]p[HCO3]p
(mmol/L)(mmol/L)
NormalNormal 7.35-7.457.35-7.45 35-4535-45 22-2622-26
AcidoticAcidotic < 7.35< 7.35 > 45> 45 < 22< 22
AlkaloticAlkalotic > 7.45> 7.45 < 35< 35 > 26> 26
Traditional Metabolic Acid-Base NomenclatureTraditional Metabolic Acid-Base Nomenclature
NomenclatureNomenclature pHpH PCOPCO22[HCO3][HCO3]pp
BEBE
Metabolic acidosisMetabolic acidosisUncompensated (acute)Uncompensated (acute) NN (-)(-)Partly compensated Partly compensated (subacute)(subacute)
(-)(-)
Compensated (chronic)Compensated (chronic) NN (-)(-)
Metabolic alkalosisMetabolic alkalosisUncompensated (acute)Uncompensated (acute) NN (+)(+)
Partly compensated Partly compensated (subacute)(subacute)
(+)(+)
Compensated (chronic)Compensated (chronic) NN (+)(+)
Traditional Respiratory Acid-Base NomenclatureTraditional Respiratory Acid-Base NomenclatureNomenclatureNomenclature pHpH PCOPCO
22[HCO3][HCO3]pp
BEBE
Respiratory Respiratory acidosisacidosisUncompensated (acute)Uncompensated (acute) NN NNPartly compensated Partly compensated (subacute)(subacute)
Compensated (chronic)Compensated (chronic) NN Respiratory Respiratory alkalosisalkalosisUncompensated (acute)Uncompensated (acute) NN NNPartly compensated Partly compensated (subacute)(subacute)
Compensated (chronic)Compensated (chronic) NN
Base Excess/ DeficitBase Excess/ Deficit Blood with large buffering capacity:Blood with large buffering capacity:
significant changes in acid content with little change in free Hsignificant changes in acid content with little change in free H++ concentrations (pH)concentrations (pH)
Acidemia or alkalemia: Acidemia or alkalemia: buffering capacity, > potential for pH buffering capacity, > potential for pH change from any given change in Hchange from any given change in H++ content content
Buffering capacity depends on:Buffering capacity depends on:[HCO[HCO33
--]]RBC massRBC massother factors other factors
Base excess/deficit= (measured pH – predicted pH) x 100 x 2/3Base excess/deficit= (measured pH – predicted pH) x 100 x 2/3Normal metabolic acid-base status:Normal metabolic acid-base status: ++ 3 mmol/L 3 mmol/LRelatively balanced metabolic acid-base status:Relatively balanced metabolic acid-base status: ++ 5 5
mmol/Lmmol/LClinically significant imbalance:Clinically significant imbalance: ++ 10 mmol/L 10 mmol/L
Nomenclature & Criteria for Clinical InterpretationNomenclature & Criteria for Clinical Interpretation
Clinical TerminologyClinical Terminology CriteriaCriteria
Ventilatory failureVentilatory failure (respiratory acidosis) (respiratory acidosis) PPaaCOCO22 > 45 mm > 45 mm HgHg
Acute ventilatory failureAcute ventilatory failure (respiratory acidosis) (respiratory acidosis) PPaaCOCO22 > 45 > 45 mmHgmmHg pH < 7.35pH < 7.35
Chronic ventilatory failureChronic ventilatory failure (respiratory acidosis) (respiratory acidosis) PPaaCOCO2 2 > 45 > 45 mmHgmmHgpH 7.36- 7.44pH 7.36- 7.44
Alveolar hyperventilationAlveolar hyperventilation (respiratory alkalosis) (respiratory alkalosis) PPaaCOCO22 < 35 < 35 mmHgmmHg
Acute alveolar hyperventilationAcute alveolar hyperventilation (respiratory (respiratory PPaaCOCO22 < 35 mmHg < 35 mmHgalkalosis)alkalosis) pH > 7.45pH > 7.45
Chronic alveolar hyperventilationChronic alveolar hyperventilation (respiratory (respiratory PPaaCOCO2 2 < 35 < 35 mmHgmmHg alkalosis)alkalosis) pH 7.36-7.44pH 7.36-7.44
Nomenclature & Criteria for Clinical InterpretationNomenclature & Criteria for Clinical Interpretation
Clinical TerminologyClinical Terminology CriteriaCriteria
AcidemiaAcidemia pH < 7.35pH < 7.35AlkalemiaAlkalemia pH > 7.45pH > 7.45AcidosisAcidosis HCOHCO33
- - < 22 mmol/L< 22 mmol/LBD > 5 mmol/LBD > 5 mmol/L
AlkalosisAlkalosis HCOHCO33-- > 26 mmol/L > 26 mmol/L
BE > 5 mmol/LBE > 5 mmol/LCombinedCombined Respiratory Acidosis & Metabolic AcidosisRespiratory Acidosis & Metabolic Acidosis
Respiratory Alkalosis & Metabolic AlkalosisRespiratory Alkalosis & Metabolic AlkalosisMixedMixed Respiratory Acidosis & Metabolic AlkalosisRespiratory Acidosis & Metabolic Alkalosis
Respiratory Alkalosis & Metabolic AcidosisRespiratory Alkalosis & Metabolic Acidosis
Respiratory Respiratory AcidosisAcidosis
AcuteAcute
pH = pH = 0.08 x (PCO0.08 x (PCO22 – 40) – 40)
1010
ex. PCOex. PCO2 2 = 60= 60
pH = pH = 0.08 x (60 - 40)0.08 x (60 - 40) = 0.16 = 0.16
1010
expected pH = 7.40 – 0.16 = 7.24expected pH = 7.40 – 0.16 = 7.24
HCOHCO33-- increases 0.1 – 1 meq/L per 10 mmHg PCO increases 0.1 – 1 meq/L per 10 mmHg PCO2 2 increaseincrease
Compensation: cellular buffering: HCO3
renal adaptation: H+ secretion, Cl-
reabsorption, net acid excretion
Respiratory acidosisRespiratory acidosis
ChronicChronic
pH = pH = 0.03 x (PCO0.03 x (PCO22 – 40) – 40)
1010
ex. PCOex. PCO22 = 60 = 60
pH = pH = 0.03 x (60 – 40) 0.03 x (60 – 40) = 0.06= 0.06
1010
expected pH = 7.40 – 0.06 = 7.34expected pH = 7.40 – 0.06 = 7.34
HCOHCO33-- increases 1-3.5 meq/L per 10 mmHg PCO increases 1-3.5 meq/L per 10 mmHg PCO22
increaseincrease
Respiratory Respiratory AcidosisAcidosis
COPDCOPD OO22 excess in COPD excess in COPD DrugsDrugs
• BarbituratesBarbiturates• AnestheticsAnesthetics• NarcoticsNarcotics• SedativesSedatives
Extreme ventilation-Extreme ventilation-perfusion mismatchperfusion mismatch
Exhaustion Exhaustion Inadequate MVInadequate MV Neurologic disordersNeurologic disorders
Neuromuscular diseaseNeuromuscular disease• PoliomyelitisPoliomyelitis• ALLALL• G-B syndromeG-B syndrome• Electrolyte deficiencies Electrolyte deficiencies
(K(K++, PO, PO44--))
• Myasthenia gravisMyasthenia gravis Excessive COExcessive CO22
productionproduction• TPNTPN• SepsisSepsis• Severe burnsSevere burns• NaHCONaHCO33 administration administration
Respiratory Respiratory AlkalosisAlkalosis
AcuteAcute
pH = pH = 0.08 x (40 – PCO0.08 x (40 – PCO22))
1010
ex. PCOex. PCO22 = 20 = 20
pH = pH = 0.08 x (40 – 20)0.08 x (40 – 20) = 0.16 = 0.16
1010
expected pH = 7.40 + 0.16 = 7.56expected pH = 7.40 + 0.16 = 7.56
HCOHCO33-- decreases 0-2 meq/L per 10 mmHg PCO decreases 0-2 meq/L per 10 mmHg PCO22
decreasedecrease
Compensation: cellular buffering
renal response: retention of endogenous acids, excretion of HCO3
Respiratory AlkalosisRespiratory Alkalosis
ChronicChronic
pH = pH = 0.03 x (40 – PCO0.03 x (40 – PCO22))
1010
ex. PCOex. PCO22 = 20 = 20
pH = pH = 0.03 x (40 – 20)0.03 x (40 – 20) = 0.06 = 0.06
1010
expected pH = 7.40 + 0.06 = 7.46expected pH = 7.40 + 0.06 = 7.46
HCOHCO33-- decreases 2-5 meq/L per 10 mmHg PCO decreases 2-5 meq/L per 10 mmHg PCO22
decreasedecrease
Respiratory AlkalosisRespiratory Alkalosis
Primary central Primary central disordersdisorders
Hyperventilation Hyperventilation syndrome, anxietysyndrome, anxiety
Cerebrovascular diseaseCerebrovascular disease Meningitis, encephalitisMeningitis, encephalitisPulmonary diseasePulmonary disease Interstitial fibrosisInterstitial fibrosis PneumoniaPneumonia Pulmonary embolismPulmonary embolism Pulmonary edema Pulmonary edema
(some patients)(some patients)
HypoxiaHypoxiaSepticemia, Septicemia,
hypotensionhypotensionHepatic failureHepatic failureDrugsDrugs SalicylatesSalicylates NicotineNicotine XanthinesXanthines Progestational Progestational
hormoneshormonesHigh altitudeHigh altitudeMechanical ventilatorsMechanical ventilators
Metabolic AcidosisMetabolic Acidosis
Anion GapAnion Gap artificial disparity between major plasma cations artificial disparity between major plasma cations
& anions that are routinely measured& anions that are routinely measured major plasma cationsmajor plasma cations – – major plasma anionsmajor plasma anions [Na[Na++]] – – ([Cl([Cl--] + [HCO3] + [HCO3--])]) 12 12 ++ 2 (normal) 2 (normal) Minor cations: KMinor cations: K++, Ca, Ca++++
Minor anions: phosphates, sulfates, organic Minor anions: phosphates, sulfates, organic anionsanions
Metabolic Metabolic AcidosisAcidosis
Anion gap acidosisAnion gap acidosis
~ process increases “minor anions”~ process increases “minor anions”
~ ex. lactatemia, ketonemia, renal failure, excessive ~ ex. lactatemia, ketonemia, renal failure, excessive
organic salt treatment, dehydration, ingestion organic salt treatment, dehydration, ingestion
(salicylates, methanol, ethylene glycol, (salicylates, methanol, ethylene glycol, paraldehyde)paraldehyde)
~ process which decreases “minor cations” rare!~ process which decreases “minor cations” rare! Non-anion gap acidosisNon-anion gap acidosis
~ associated with increased plasma Cl~ associated with increased plasma Cl-- that has replaced that has replaced
HCOHCO33--
~ ex. GI loss of HCO~ ex. GI loss of HCO33-- (diarrhea), renal wasting of HCO (diarrhea), renal wasting of HCO33
--
(RTA), ingestion of acids, parenteral (RTA), ingestion of acids, parenteral hyperalimentation, carbonic anhydrase inhibitors hyperalimentation, carbonic anhydrase inhibitors
Metabolic Metabolic AcidosisAcidosis
Abnormalities:Abnormalities: Overproduction of acidsOverproduction of acids Loss of buffer storesLoss of buffer stores Underexcretion of acidsUnderexcretion of acids
Metabolic AcidosisMetabolic Acidosis
Expected PCOExpected PCO22 = ( [HCO = ( [HCO33--] x 1.5) + 8 ] x 1.5) + 8 ++ 2 2
ex. [HCOex. [HCO33--] = 11] = 11
expected PCOexpected PCO22 = (11 x 1.5) + 8 = (11 x 1.5) + 8 ++ 2 = 22.5- 26.5 2 = 22.5- 26.5
PCOPCO22 decreases 1- 1.5 mmHg per 1 meq/L HCO decreases 1- 1.5 mmHg per 1 meq/L HCO33- -
decreasedecrease
Metabolic Metabolic AcidosisAcidosis
CompensationCompensation pCOpCO22 (hyperventilation) (hyperventilation) Pathway:Pathway:
pCO2
HCO3
ratio H+ conc
Acidification of ECF ECF pH
Stimulation of brainstem RR pCO2
Normalization of pH
HCO3
Metabolic Metabolic AcidosisAcidosis
CompensationCompensation Ionic shiftIonic shift
• KK++ moves extracellularly for H moves extracellularly for H++
• HCOHCO33-- generation, H generation, H++ excretion excretion
Corrected [HCOCorrected [HCO33--] for Anion Gap ] for Anion Gap
Metabolic AcidosisMetabolic Acidosis
Measured serum [HCOMeasured serum [HCO33--] + (anion gap – ] + (anion gap –
12)12)
Metabolic AlkalosisMetabolic Alkalosis
Expected PCOExpected PCO22 = ( [HCO = ( [HCO33--] x 0.75 ) + 20 ] x 0.75 ) + 20 ++ 5 5
ex. [HCOex. [HCO33--] = 34] = 34
expected PCOexpected PCO2 2 = (34 x 0.75) + 20 = (34 x 0.75) + 20 ++ 5 = 40.5- 5 = 40.5- 50.550.5
PCOPCO22 increases 0.5- 1 mmHg per 1 meq/L HCO increases 0.5- 1 mmHg per 1 meq/L HCO33--
increaseincrease
Metabolic Metabolic AlkalosisAlkalosis
Pathway Pathway
HCO3PaCO2
HCO3
ratio H+ conc
Alkalinization of ECF PaCO2 with mild hypoxemia
Normalization of pH
Causes of Metabolic Alkalosis
Hypokalemia*Ingestion of large amounts of alkali or licoriceGastric fluid loss: Vomiting, NG suctioning*Hyperaldosteronism 20 to nonadrenal factors Bartter’s syndrome Inadequate renal perfusion diuretics (inhibiting NaCl reabsorption)*Bicarbonate administration Sodium bicarbonate overcorrection Blood transfusionAdrenocortical hypersecretion (e.g tumor)Steroids*Eucapnic ventilation posthypercapnia
* Common in the ICU
Limits of CompensationLimits of CompensationImbalance [HCO3
-] meq/L PCO2 mmHg
Respiratory AcidosisRespiratory Acidosis
Acute Acute 0.1- 1/ 10 mmHg PCO0.1- 1/ 10 mmHg PCO22
ChronicChronic 1- 3.5/ 10 mmHg1- 3.5/ 10 mmHg
PCOPCO22Respiratory AlkalosisRespiratory Alkalosis
AcuteAcute 0- 2/ 10 mmHg PCO0- 2/ 10 mmHg PCO22ChronicChronic 2- 5/ 10 mmHg PCO2- 5/ 10 mmHg PCO22
Metabolic AcidosisMetabolic Acidosis 1- 1.5/ 1 meq/L 1- 1.5/ 1 meq/L
[HCO[HCO33--] ]
Metabolic AlkalosisMetabolic Alkalosis 0.5- 1/ 1 meq/L 0.5- 1/ 1 meq/L
[HCO[HCO33--] ]
Steps for Analyzing Acid- Base Steps for Analyzing Acid- Base DisturbancesDisturbances
Is patient acidemic or alkalotic? Is patient acidemic or alkalotic? pHpH Is disturbance primarily respiratory or metabolic? Is disturbance primarily respiratory or metabolic?
PCOPCO22, [HCO, [HCO33--]]
If disturbance respiratory, is it acute or chronic?If disturbance respiratory, is it acute or chronic? If disturbance metabolic, is anion gap normal or If disturbance metabolic, is anion gap normal or
abnormal? abnormal? If disturbance metabolic, is the respiratory If disturbance metabolic, is the respiratory
system compensating adequately?system compensating adequately? If disturbance is anion gap metabolic acidosis, are If disturbance is anion gap metabolic acidosis, are
there any other metabolic disturbances present?there any other metabolic disturbances present?
Oxygenation StatusOxygenation Status
Normal ValuesNormal Values
Seated PO2 = 104.2 – 0.27 (age in years)Seated PO2 = 104.2 – 0.27 (age in years)
Supine PO2 = 103.5 – 0.42 (age in years)Supine PO2 = 103.5 – 0.42 (age in years)
Patients Patients << 60 y. o. 60 y. o.
PO2 = 100 PO2 = 100 ++ 20 20
Patients > 60 y. o.Patients > 60 y. o.
PO2 = 80 – (# years > 60)PO2 = 80 – (# years > 60)
Steps for Analyzing Steps for Analyzing Oxygenation StatusOxygenation Status
1. Is the patient hypoxemic or normoxemic?1. Is the patient hypoxemic or normoxemic?
Indices of Oxygenation:Indices of Oxygenation:a. AaDOa. AaDO22 = PAO = PAO22 – PaO – PaO22
PAOPAO22 = FiO = FiO22 (713) – (713) – PaCOPaCO22
0.80.8PaOPaO22 = obtained from blood gas determination = obtained from blood gas determination
b. aAOb. aAO22 = = PaOPaO22
PAOPAO22
c. P/F ratio = c. P/F ratio = POPO22
FiOFiO22
Normal Value:Normal Value: patients patients << 60 y. o. > 400 60 y. o. > 400patients > 60 y. o. expected P/F = 400 patients > 60 y. o. expected P/F = 400 – – [(age in years – 60) x 5][(age in years – 60) x 5]
Actual P/F Ratio < expected =Actual P/F Ratio < expected = hypoxemic hypoxemicActual P/F Ratio Actual P/F Ratio >> expected = expected = normoxemicnormoxemic
2. If hypoxemic, is it uncorrected, 2. If hypoxemic, is it uncorrected, corrected, or overcorrected? corrected, or overcorrected?
With OWith O2 2 supplementationsupplementationPaOPaO2 2 (mmHg)(mmHg)
Uncorrected hypoxemiaUncorrected hypoxemia < 80< 80Corrected hypoxemiaCorrected hypoxemia 80 – 12080 – 120OvercorrectedOvercorrected > 120> 120
FiOFiO22 to PaO to PaO22 Relationship in Normal Lungs Relationship in Normal LungsFiOFiO22 PaOPaO22 (mmHg) (mmHg)0.300.30 > 150> 1500.400.40 > 200> 2000.500.50 > 250> 2500.800.80 > 400> 4001.001.00 > 500> 500
Room Air (patient Room Air (patient << 60 y. o.) 60 y. o.)
PaOPaO22 (mmHg) (mmHg)
Mild hypoxemiaMild hypoxemia 60 to < 8060 to < 80
Moderate hypoxemiaModerate hypoxemia 40 to < 6040 to < 60
Severe hypoxemiaSevere hypoxemia < 40< 40
For each year > 60 subtract 1 mmHg for limits of For each year > 60 subtract 1 mmHg for limits of mild & mild &
moderate hypoxemia.moderate hypoxemia.
At any age, PaOAt any age, PaO2 2 < 40 mmHg indicates severe < 40 mmHg indicates severe hypoxemia.hypoxemia.
3. If normoxemic, is oxygenation3. If normoxemic, is oxygenation adequate or more than adequate or more than
adequate? adequate?
Thank you !Thank you !