Seminars in Aeromedical Transport

Hemodynamic Assessment in Aeromedical Evacuation

by Frank Thomas, MD and Terry P. Clemmer, MD Proper hemodynamic assessment is

essential during the transport of the critically-ill. However, space constraints and high background noise during aeromedical transport limits one's ability to use the physical examination to monitor the patient. Hemodynamic evaluation during transport is largely dependent upon palpation and visual clues. In this seminar we will provide simple, rapid methods for clinically assessing the hemodynamics of a patient during aeromedical evacuation.

Preload, contractility, and afterload are three factors which influence the stroke volume. Stroke volume and heart rate determine the cardiac output (Figure 1). The adequacy of cardiac output should be interpreted in terms of these parameters. An accurate quantification of these parameters frequently necessitates pulmonary arterial catheter pressure and cardiac output determinations. However, the utilization of these catheters is not without difficulty. The placement and data interpretation of pulmonary arterial catheters requires specialized training and a significant amount of time. Even in the intensive care setting, data generated from these devices must be closely scrutinized if data collection errors are to be avoided. Aircraft vibrations frequently make pulmonary artery waveform interpretation difficult

during the transport. Thus hemodynamic measurements by pulmonary arterial catheters are seldom available, interpretable, or useful during aeromedical transport.

Aeromedical evaluation of the patient's hemodynamic status is largely restricted to the physical examination of the patient's afterload (capillary refill, quality of the radial pulse, and blood pressure), heart rate (ECG or pulse rate), stroke volume (radial or carotid pulse pressure), and preload (venous filling pressure and the size of the veins).

Capillary Refill The body prioritizes tissues and

makes every effort to preserve those cells necessary for the continued maintenance of body functions. The organs most important in the preservation of life are located within the central axis. Here we find the brain, heart, bowels, kidneys, as well as other organs necessary in maintaining body functions. Destruction of any of these organs will lead to increased morbidity or death. Teleologically, the body behaves in a manner to preserve this central axis at the expense of peripheral areas such as the arms and legs. A reduction in cardiac output curtails the body's ability to perfuse all organs adequately. Therefore, blood is

Figure 1 CARDIAC OUTPUT

PRELOAD

" ' ' ~ V O L U M E / ,# (Quant i t y of

AFTERLOADJ Pulse) (Capillary Refill)

(BLood Pressure) (Quality of Radial Pulse)

X HEART RATE

(Radial Pulse Rate)

= CARDIAC OUTPUT

shunted away from the peripheral areas by increasing peripheral resistance toward these vital organs. The fingers and toes, which are located furthest from the central axis, are first to see a decrease in blood flow. Examining the capillary refill provides early evidence that vasoconstriction is occurring in the peripheral sites.

The technique for assessing capillary refill is simply accomplished by pressing on a digital nailbed until it blanches white, then releasing the pressure on the nailbed and saying "capillary refill." By the time one completes saying "capillary refill" the nailbed should return from the blanched white background seen with the applied pressure, to its usual pink color. If not, the capillary refill test is said to be positive. If it requires more than 2 seconds to return to its pink color it is classified as delayed. If the color does not return it is absent.

False positive capillary refill tests, suggesting generalized vasoconstriction, can occur when blood flow to that arm is restricted (i.e., disruption or spasm of the brachial artery), during cold weather evaluations, or if the patient is hypothermic. The impediment of unilateral arterial flow is easily ruled out by merely checking the other hand, since it is unlikely that bilateral arterial occlusion or impingement has occurred.

Occasionally when assessing for a capillary refill test one finds that the nailbed cannot be observed because of colored fingernail polish. The remedy for this problem is easily solved by rotating the finger and performing a capillary refill test on the finger pad. A normal capillary refill test indicates that adequate peripheral perfusion and adequate cardiac output exist. A delayed or absent capillary refill test suggests early vasoconstriction is present and that additional hemodynamic evaluation is necessary.

18 HOSPITAL AVIATION, NOVEMBER 1984

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Seminars...

Radial Pulse

Attention is next directed to the radial pulse. Heart rate and pulse contour are the two parameters which are assessed in evaluating the radial pulse.

Heart Rate

Heart rate in conjunction with stroke volume determines the cardiac output . An increase in the cardiac output requires an increase in the stroke volume, heart rate, or both. When stroke volume begins to decline because of decreased preload filling, decreased contractility, or an increase in afterload, an increase in the heart rate usually occurs in order to maintain adequate cardiac output . Indeed, tachycardia is one of the earliest signs of shock.

In severe respiratory or circulatory failure, acidosis and p ro found tissue hypoxia may depress myocardial pacemaker activity resulting in a slowing of the heart rate. Severe hypotens ion (less than 40 m m H g systolic) and/or severe hypoxia is

u sua l l y present in this setting. However , vasoconstrictive agents may mask this associated hypotension. A decline in heart rate to less than 60 beats per minute in the face of p ro found shock is an ominous sign. On the other hand, a decline in the heart rate from the tachycardic range is consistent with the reversal of the shock state and can be used to monitor the efficacy of therapy. Thus a high, normal and low heart rate can all be consistent with an improvemen t or decompensat ion of the patient 's hemodynamic status at various stages. Heart rate is best in terpre ted as to its relationship to stroke volume, venous pressure and per ipheral resistance.

Stroke Volume

The quality of the contour of the radial pulse is one of the most important aspects to measure w h e n evaluating a patient 's hemodynamics . Estimating a normal contour requires practice. Palpatation of one's own radial pulse will assist in distinguishing a normal or abnormal pulse contour. Learning how to estimate the quality of the radial pulse allows one to estimate the stroke volume and amount of blood flow

Seminars in Aeromedical Transport,..

being del ivered th rough the radial artery to per iphera l tissues. Stroke vo lume is a m e a s u r e m e n t of the hear t ' s capacity to p u m p blood. A declining stroke v o l u m e usually indicates decreased cardiac contractility, r educed diastolic filling, or increased afterload. A poor radial pulse contour fo rewarns that per iphera l constrict ion is progressing. In mos t forms of h e m o d y n a m i c compromise a decrease in stroke vo lume is associated with an increase in pulse rate. Howeve r , during heart block or p ro found shock, the heart rate m a y also decrease. A slow pulse rate in the face of a poor radial pulse contour is a fo reboding sign since it suggests that normal compensa to ry mechan i sms are failing. A good strong radial pulse and good capillary refill in association with a low heart rate does not have the poor prognos is of bradycardia with signs of inadequate per iphera l perfusion.

Poor radial pulse contours can also be seen in pat ients who are ulnar artery dominant . A good capillary refill in the face of a poor ly palpable radial pulse should suggest ulnar arterial dominance. In such cases the other radial ar tery or the ulnar pulse contour should be examined.

Table I gives the order in which the body pulses are lost to palpation. The progress ive loss of these pulses toward the central axis is indicative of increasing circulatory failure.

Blood Pressure

With a delayed capillary refill and a d iminished radial pulse contour, one is obligated to examine the blood pressure . Blood pressure m e a s u r e m e n t s dur ing aeromedical t ranspor t f requently require the use of mechanical dopple r augmenta t ion of sound, b lood pressure display modules , or indwel l ing arterial catheter monitor ing. The impor tance of examining the b lood pressure is to de termine the extent of hypotens ion. Hypo tens ion does not need to be present to see a decreased capillary refill and poor radial pulse contour. However , a recorded low blood pressure in the face of a normal capillary refill and good radial pulse contour should make one suspect the accuracy of the m e a s u r e d blood pressure , or suggest an etiology of a vasodialated hypo tens ive state such as neurogenic hypo tens ion or sepsis.

H y p o t e n s i o n should be regarded as a late and threa tening sign of i m p e n d i n g circulatory collapse w h e n it is associated with per iphera l vasoconstr ict ion. When hypo tens ion occurs, aggress ive therapeut ic m a n e u v e r s mus t be initiated immediate ly .

V e i n s

Lastly, the presence or absence of veins is clinically impor tan t w h e n one is a t t empt ing to assess the pa t ien t ' s int ravascular vo lume. Since 75 % of our blood vo lume is located in our venous sys tem, w h e n hypovo lemia occurs the venous sys tem begins to constrict. To proper ly evaluate the venous sys tem, veins that are level wi th or be low the heart should be observed. Sites f requent ly used to examine the v e n o u s filling pressure are the h a n d and neck veins. Veins will be col lapsed if they are evalua ted above the level of the hear t giving a false impres s ion of venous constriction. This f requent ly occurs w h e n hands are folded across the chest on pat ients secured supine to the stretcher. In this case, examina t ion of the presence of neck veins will p rov ide a bet ter a s se s smen t of intravascular status. Ano the r false posit ive in which veins m a y be absent is w h e n pat ients are exposed to the vasoconstr ic t ive effects of a cold env i ronment . Apply ing a venous tourn ique t on the forearm of normal volemic individuals" will usual ly dis tend their veins whereas individuals in p ro found hypovo lemia will mainta in a venous constr ic ted state.

An absence of neck and h a n d veins should sugges t that a hypovo lemic state exists even though other etiologies such as h y p o t h e r m i a and cold env i ronmen ta l condit ions are also possible causes. Inadequa te venous filling pressures lead to a decline in stroke vo lume. However , the presence of veins does not assure that adequate filling pressures exist in

Table 1

DESCENDING ORDER OF LOST PULSES

Radial Brachial Femoral Carotid

the left ventricle. In the critically ill a poor correlation be tw een the central venous pressure and p u l m o n a r y arterial w e d g e is not u n c o m m o n . The presence or absence of veins should be cons ide red indicators of the vo lume state on the right side of the pa t ien t ' s hear t and not necessari ly a reflection of end diastolic vo lume in the left ventricle.

S u m m a r y

Delayed capillary refill, poor radial pulse contours , and hypo tens ion are signs which indicate progress ive h e m o d y n a m i c failure. Their presence denotes that forward flow from the heart to the per iphera l area t issues is deteriorat ing. Examinat ion of the hand and neck veins is useful in assess ing if an adequate intravascular vo lume to the right heart is present . Absence of veins should sugges t h y p o v o l e m i a and that fluid resusci ta t ion is indicated.

When faced with a poor capillary refill, poor radial pulse with or wi thout a d rop in b lood pressure , plus large neck or hand veins, a t tent ion should be localized to the thoracic p u m p . Disorders which would cause i m p e d a n c e of blood flow th rough the thoracic p u m p include tension p n e u m o t h o r a x , cardiac t a m p o n a d e and cardiac failure. Chest wall a s symet ry , a unilateral decrease in brea th sounds , and assymetr ic hype r - r e sonance suggest a possible tension pneumotho rax . Decreased heart tones, d i s tended neck veins, evidence of anterior chest wall injury, and decreased EKG voltage sugges t the possibil i ty of pericardial t a m p o n a d e . Bilateral rales and chest pain wou ld hint of a recent lef t-sided myocard ia l infarction with left and right s ided heart failure. This can usual ly be conf i rmed by a 12 lead ECG. A recent history of chest pa in and clear lungs might suggest a right ventr icular infarction or p u l m o n a r y embolus .

If p roper ly per formed, a comple te a s ses smen t of the hear t rate, capillary refill, radial pulse contour, arterial b lood pressure and the venous vo lume and filling pressure can be accompl i shed in one to two minutes . This m e t h o d thereby offers a simple, rapid and relatively helpful means of assess ing a pa t ien t ' s h e m o d y n a m i c status in the field and dur ing aeromedica l t ransports .

HOSPITAL AVIATION, NOVEMBER 1984 21