For which of the following patients would capillary refill be a reliable sign of circulatory status?

Capillary refill time

Evaluation of capillary refill time (CRT) provides further information on peripheral perfusion. Used in conjunction with pulse quality, respiratory effort, heart rate, and mucous membrane color, the CRT can help assess a patient's blood volume and peripheral perfusion and provide information on shock etiology. Normal CRT is 1 to 2 seconds. This is consistent with a normal blood volume and perfusion. A CRT longer than 2 seconds suggests poor perfusion due to peripheral vasoconstriction. Peripheral vasoconstriction is an appropriate response to low circulating blood volume and reduced oxygen delivery to vital tissues. Patients with hypovolemic and cardiogenic shock should be expected to have peripheral vasoconstriction. Peripheral vasoconstriction is also commonly associated with cool extremities, assessed by palpation of the distal limbs. Significant hypothermia will also cause vasoconstriction. A CRT of less than 1 second is suggestive of a hyperdynamic state and vasodilation. Hyperdynamic states can be associated with systemic inflammation, distributive shock, and heat stroke or hyperthermia.

Cardiac Output

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Capillary refill time, body temperature, and mentation are the physical findings that best reflect cardiac output. However, these are not always accurate.

Urine output is a good indicator of cardiac output. When cardiac output is reduced, sympathetic nervous system activity may maintain blood pressure within normal limits but may decrease renal blood flow. Consequently, urine output will be decreased (<1 ml/kg/hr). However, other causes of reduced urine formation must be considered.

Cardiac rhythm influences cardiac output and can be monitored by ECG.

2 Capillary Refill Time

Normal values of capillary refill time, based on observation of thousands of persons, average approximately 2 seconds.17,18 Women have slightly longer times compared with men, and capillary refill times normally increase in elderly patients and in cooler ambient temperatures.

In the studies of capillary refill of patients with suspected peripheral vascular disease, investigators applied firm pressure for 5 seconds to the plantar skin of the great toe and then timed how long it took for normal skin color to return after releasing the pressure. Measurements of 5 seconds or more were regarded abnormal.16 In studies of capillary refill of critically ill patients, investigators tested the patient’s finger (usually index finger) by applying firm pressure for 15 seconds and regarded times of 5 seconds or more as abnormal.19-21

Capillary Refill Time

Evaluation of capillary refill time (CRT) is also subject to interpretation. You may even notice a normal CRT in a recently deceased patient. However, used in conjunction with pulse quality, respiratory effort, heart rate, and gum color, the CRT can help assess a patient's blood volume and peripheral perfusion, and give an insight into causes of a patient's shock. Normal CRT is 1 to 2 seconds. This is consistent with a normal blood volume and perfusion. A CRT longer than 2 seconds is a subjective sign of poor perfusion or peripheral vasoconstriction. Peripheral vasoconstriction is an appropriate response to low circulating blood volume and reduced oxygen delivery to vital tissues. Patients with hypovolemic and cardiogenic shock should be expected to have peripheral vasoconstriction. Peripheral vasoconstriction is also commonly associated with cool extremities, assessed by feeling the distal limbs. A CRT less than 1 second is indicative of a hyperdynamic state and vasodilation (as are bright red mucous membranes). Hyperdynamic states can be associated with systemic inflammation, heat stroke, distributive shock, and hyperthermia.

Venous distention can be a sign of volume overload or right-sided congestive heart failure. Palpation of the jugular vein may demonstrate distention, although it may be easier to appreciate by clipping a small patch of hair over the lateral saphenous vein. With the patient in lateral recumbency, if the lateral saphenous vein in the upper limb appears distended (as if the vessel is being held off), slowly raise the rear leg above the level of the heart. If the vein remains distended, the patient likely has an elevated central venous pressure, and volume overload or diseases causing right-sided congestive heart failure (dilated cardiomyopathy, tricuspid insufficiency, pericardial effusion) should be considered. A patient with pale mucous membranes from vasoconstriction in response to hypovolemia would not be expected to have venous distention. In comparison, cardiogenic shock is more likely to cause pale mucous membranes and increased venous distention.

Mucous membrane colour and capillary refill time

The membrane colour and capillary refill time (CRT) reflect the circulatory status of the animal. The normal membrane appearance is moist and pink. Dry, congested membranes suggest dehydration and circulatory disturbance. The CRT, observed by blanching out the gum adjacent to an incisor tooth and judging the time to colour restoration, indicates whether perfusion, hydration and vascular tone are impaired. In health, the normal CRT occupies less than 2 seconds. Increasing refill times indicate progressively inadequate perfusion and are usually accompanied by dryness and discoloration of the membranes.

SKIN ASSESSMENT

In addition to acrocyanosis and capillary refill delay discussed above, other findings can be seen in the skin. Fine downy hair, called lanugo, can develop on the trunk, arms, and face in undernourished patients as a method to maintain body heat. It will resolve as nutrition improves. Scalp hair can fall out from malnutrition and as it starts to reverse (the new, healthy hair pushes out the old). Cuts, abrasions, and calluses from using the hands to stimulate vomiting occur on the dorsum of the hand, called “Russell’s sign,” in honor of the first author on BN. Skin can also have pigment changes from excess intake of certain foods. A high proportion of carrots and pumpkin can cause an orange tint, and squash can cause a yellow discoloration. Conjunctival hemorrhages and swelling of the face and eyes can occur in those who vomit forcefully. Fortunately, all of these skin changes will resolve in time.

Mucous Membranes

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Normal membranes are pink with a capillary refill time of <2 seconds. Membrane pallor suggests poor perfusion or anemia (evaluate for intestinal parasitism, especially hookworm infection). Pallor and prolonged refill time suggest heart failure or reduced blood pressure with reactive vasoconstriction.

Cyanosis (blue-colored mucous membranes) develops from a low arterial oxygen tension with >5 g/dl of desaturated hemoglobin. Pulmonary dysfunction due to left-sided CHF or concurrent bronchopneumonia is the most common cause of cyanosis in CHD. Lesions that allow right-to-left shunting, such as tetralogy of Fallot, can lead to persistent or exercise-induced cyanosis in the absence of pulmonary dysfunction.

Right-to-left shunting requires a source of high right-sided resistance, and a communication or shunt proximal to the obstruction. With this combination, desaturated right-sided blood may enter the left side of the circulation.

Reasons for high resistance include PH from high vascular resistance, PS, mid-RV obstruction, and tricuspid valve disease (either stenosis or severe regurgitation that raises right atrial pressure).

The lesion allowing shunting can be a patent foramen ovale, ASD, VSD, or PDA. Additionally, in certain complex defects, there may be only a single great vessel exiting the heart; one ventricle that serves each great vessel; or transposition of the great vessels. Each of these situations allows mixing of pulmonary venous and systemic venous blood and may lead to cyanosis.

The term “differential cyanosis” generally refers to the condition of pink oral membranes and cyanotic caudal membranes (best seen in the vulva or prepuce). This is most typical of reversed PDA caused by a large ductus and severe PH.

Perfusion

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Heart rate

Mucous membrane color, CRT, palpable pulse pressures

Urine production: should be normal or increased after administration of intravenous fluids. Urine specific gravity can be used to help determine appropriate administration volume.

Cardiac contractility: M mode may be used to estimate this value. Contractility should be 35% to 50%, and chamber size should appear normal in addition to clinical evidence of euvolemia and/or normal CVP. In some hospitals, cardiac output can be measured by lithium dilution or sonographic method.

Arterial pressure: tail cuff or subjective digital pulse pressure. An arterial line can be established for recumbent foals (mean arterial pressure should be >65 mm Hg, ideally 120 to 130 mm Hg systolic pressure). The accuracy of the indirect monitoring of blood pressure using oscillometric measurements can vary depending on the following:

The ratio of bladder cuff width to tail circumference. No ideal ratio is known; however, a bladder width of 20% to 25% and length of 50% to 80% of the circumference of the tail is recommended. For foals, a 5.2-cm bladder width is recommended. The cuff can alternatively be placed over the metatarsus (great metatarsal artery) or the forearm (median artery) in foals.

The positional location of the cuff in relation to the level of the base of the heart. Practice Tip: This affects blood pressure measurements, as does the standing patient's head position; if possible keep the head in the same neutral position each time blood pressure measurements are performed.

At best, the indirect measurement gives an acceptable mean pressure and an indication of trends when performed intermittently in the identical manner and on the same patient. An accurate heart rate on the monitor should be displayed when blood pressure measurements are computed.

Practice Tips:

Mean arterial pressure <60 mm Hg without urine production is an indication for enhanced treatment and further monitoring.

Fluid therapy is the number 1 way of improving cardiac output and perfusion.

CVP should be 5 to 15 cm H2O for adults and 2 to 12 cm H2O for foals. Lower values are an indication for increased fluid rate, whereas high values are often, but not always, an indication for decreased fluid rate, pump therapy, and/or the possibility of renal failure. See Chapter 10, p. 35, for measurement of CVP.

Administer plasma protein, to a goal of ≥4.2 g/dL, to maintain oncotic pressure and prevent edema formation.

Practice Tip: Oncotic (osmotic) pressure should remain greater than 18 mm Hg in adults and 15 mm Hg in foals in order for crystalloid therapy to be most effective and to prevent edema formation.

Packed cell volume should be 30% to 45%.

Evaluation of the Cardiovascular System.

A full evaluation of the mucous membranes, capillary refill time, pulse rate, pulse character and rhythm, heart rate and rhythm, and auscultatory findings is an important part of the evaluation of the performance horse. Failure to maintain cardiac output because of an inability to regulate either heart rate or stroke volume is the mechanism through which cardiac diseases induce exercise intolerance. Many cardiac dysrhythmias and valvular dysfunctions are apparent on auscultation of the resting horse. However, the contribution of mild abnormalities to exercise intolerance may require exercise testing. In addition, some arrhythmias and valvular or myocardial dysfunctions can be detected only during or shortly after exercise. Thus resting and exercising electrocardiography and echocardiography before and immediately after exercise are indicated.99,122 The heart rate can be monitored during exercise using either telemetric electrocardiography or commercial heart rate monitors. Electrocardiograms (ECGs) provide the additional benefit of evaluating both heart rate and rhythm. Supraventricular tachyarrhythmias, the most important of which is atrial fibrillation (AF) in horses, can lead to heart rates exceeding 240 beats/min at submaximal exercise.126 Under these circumstances, cardiac output may be limited by the decreased time available for diastolic perfusion of the myocardium; the absence of atrial contraction; and the reduced time for passive ventricular filling, leading to reduced stroke volume. Many horses with AF maintain efficient circulation at rest and during light exercise but are intolerant to strenuous exercise because they are unable to increase cardiac output sufficiently at rapid heart rates.127 Some horses show transient paroxysmal AF during exercise but not at rest.127 These are often easiest to observe in ECG tracings obtained within 60 seconds after an exercise test. Conversion to sinus rhythm, either spontaneously with quinidine sulfate or with electroconversion,128 usually leads to a return to normal performance in horses with AF.129,130 The ability to maintain cardiac output can be compromised by other cardiac arrhythmias such as ventricular tachycardia and ventricular premature depolarization.99,131 The frequency of premature depolarization can increase with exercise, and the timing of resultant abnormal extra systoles can reduce cardiac output even at submaximal heart rates.131 Intraatrial block, second-degree atrioventricular (AV) block, and intraventricular block have also been documented in exercise-intolerant, poorly performing horses.132,133 Horses with cardiac arrhythmias may show abnormal elevations in lactate concentration in response to exercise, indicating a lowered anaerobic threshold, which contributes to exercise intolerance.111

Electrocardiography is believed by some veterinarians to be of value in identifying myocarditis. T wave abnormalities (positive and peaked T waves, in contrast to the normal diphasic T waves) in multiple leads on resting ECG traces have been found in a high percentage of horses with a history of fading during the final portion of a race.132 T waves are highly labile, affected by training status, and the mechanism by which abnormal T waves are generated is uncertain. However, T wave changes have been identified in some horses with myocarditis confirmed at necropsy.119,133,134 Myocarditis may be more definitively documented by evaluating cardiac troponin I.135 A positive correlation has been demonstrated between heart size and racing performance.133 Electrocardiography has been used to assess performance potential by heart score measurement. The heart score represents the mean QRS duration in leads 1, 2, and 3 expressed in milliseconds and has been strongly correlated with heart weight and prize money won by racehorses.119,133 The physiologic basis for heart score remains in dispute, and expertise is required to standardize leads and measure QRS complexes.

An echocardiogram provides essential information in many cases in which clinical evidence of valvular or myocardial dysfunction is present. The pericardium, the size of the heart chambers, the presence of congenital defects, the function of valvular leaflets, and myocardial contractility all can be assessed (see Chapter 30). Decreased myocardial contractility, regurgitant leaks caused by valvular incompetence, left-to-right shunts, and increases in afterload such as occur in aortic stenosis result in systolic dysfunction and a drop in cardiac output.136 A pulsed, continuous, or color flow Doppler technique may be necessary to determine the size and significance of any disturbances to flow. Cardiac conditions such as effusive pericarditis and myocardial fibrosis and peripheral vascular conditions that inhibit venous return may interfere with ventricular filling during diastole, resulting in decreased end-diastolic volume, stroke volume, and cardiac output.136 Measurement of fractional shortening before and immediately after treadmill exercise, when pulse rates are above 100 beats/min, permits documentation of resting and exercise-induced myocardial dysfunction.99 Contrast angiographic studies may be indicated if congenital or acquired cardiac outflow problems are suspected. Nuclear angiocardiography is also useful for evaluating myocardial contractility, cardiac chamber enlargement, outflow problems, and other abnormalities. Hemodynamic studies, which measure pulmonary capillary wedge pressure, pulmonary driving pressure, and pressure in the right side of the heart and pulmonary artery, have proven useful in detecting early cardiac and pulmonary failure in poorly performing trotting horses. These studies involve the introduction of flow directional balloon-tipped catheters through the jugular vein into awake horses.137

7 Blood Pressure

Crude indicators of blood pressure are pulse strength and capillary refill time. Blood pressure is best monitored via direct or indirect methods. Direct measurement of central venous pressure (CVP) requires the placement of a central catheter into the cranial vena cava, with the tip lying near the base of the heart. CVP reflects intravascular volume when right heart function and intrathoracic pressure are normal. This type of invasive monitoring is not practical for most nondomestic ruminant patients but may be indicated in rare cases. Indirect (noninvasive) blood pressure measurements are less accurate but more easily obtained. Blood pressure cuffs in ruminants may be placed around the distal portion of an unflexed limb (usually forelimb) if the animal is recumbent or around the tail over the coccygeal artery. The optimal width of the cuff bladder is 40% to 60% of the circumference of the extremity to which it is applied.7

Extrapolating from domestic animals, mean arterial blood pressure should be maintained higher than 60 mm Hg and systolic pressure higher than 90 mm Hg. Cardiac failure, hypovolemic shock, and drugs such as sedatives, opioids, and certain anesthetics decrease blood pressure. Conditions that increase cardiac output, such as fever, exertion, and septic shock, also increase blood pressure. Blood pressure should be evaluated together with the animal's perfusion parameters, urinary output, and disease state. As with any other monitoring parameter, repeated measurements are needed to detect a trend. Hypotension may be treated by hemostasis, oxygen supplementation, crystalloid or colloid infusion, and potentially positive inotropes or vasopressors. Hypertension may be treated by pain relief, diuretics, or possibly vasodilators.4,7,9