Abnormal Peripheral Pulse
Palpation of the arterial pulse is an important aspect of the examination of the patient with cardiovascular disease. Arterial pulse strength and contour (how fast pressure rises and falls) are the objectives of the examination and are determined by the cardiac output, heart rate, and vascular impedance.
The arterial pressure pulse begins with the opening of the aortic valve and ventricular ejection and rises rapidly in early systole. The pulse pressure reaches a peak and then declines as ventricular ejection slows. During isovolumic relaxation (before AV valve opening), there is a transient reversal of flow in the arterial system, and an incisura or dicrotic notch (Fig. 6.6) is inscribed on the descending limb of the pressure curve. Following the incisura, there is a small positive wave that is attributed to elastic recoil of the aorta and the aortic valve and the summation of reflected waves from more distal arteries.19 After the positive wave, the pulse pressure declines because there is peripheral runoff of blood in diastole. The incisura and secondary positive wave are not usually palpable. Palpation of peripheral arteries (facial, transverse facial, and digital arteries in the horse and median and coccygeal arteries in ruminants) normally reveals a smooth, rapid upstroke, a dome-shaped summit, and a downstroke that is slightly more prolonged than the upstroke.Pressure values and pulse wave configurations are altered as the pressure waves are transmitted through the peripheral arterial tree. With increasing distance from the heart, the dicrotic notch and second positive wave disappear, the systolic pressure gets higher (loss of distensibility in the distal arteries and summation of reflected pulse waves from the distal vascular bed), and the diastolic pressure gets lower. The difference between the systolic and diastolic pressure determines pulse pressure and can be evaluated by an impression of pulse strength.
■ BOX 6.18
■ BOX 6.19
Causes of Abnormal Peripheral Pulse in Horses
Common Causes
Dehydration
Shock
Toxemia
Congestive heart failure Electrolyte imbalances
Acid-base disorders
Hypertension
Hypotension Exercise
Fever
Laminitis
Aortic regurgitation
Cardiac arrhythmias
Uncommon Causes
Aortic cardiac fistula
Aortopulmonary fistula
Peripheral arteriovenous shunt Patent ductus arteriosus
Pulse pressure increases as one moves to more peripheral arterial sites.
The mean arterial pressure changes little but decreases slightly as one moves downstream in the arterial system from the pressure source. Systolic blood pressure, as measured indirectly at the tail or on a limb, is higher than that measured in the ascending aorta. In smaller arterial beds (e.g., arteries of the ear), the pulse wave is gradually dampened and pulsatile characteristics are lost on the capillaries and small veins.Mechanisms of Abnormal Peripheral Pulse
Hyperkinetic arterial pulses occur in patients with increased cardiac output (e.g., fever, exercise, excitement), increased stroke volume, or bradycardia (Boxes 6.18 and 6.19). They may also occur when there is rapid runoff of blood in the arterial system, as occurs with aortic valve regurgitation, patent ductus arteriosus, aortic cardiac fistulas, or aortopulmonary fistulas. In aortic valve regurgitation the rapidly rising, hyperdynamic pulse is caused by increased stroke volume (regurgitated blood in the left ventricle), followed by a rapid runoff of pressure later in systole as a result of regurgitation (see Fig. 6.6).
Hypokinetic pulses are present in patients with diminished stroke volume caused by hypovolemia, left ventricular failure, or, rarely in large animals, mitral or aortic valve stenosis.
Abnormal peripheral pulses are detected in patients with cardiac arrhythmias. With premature ventricular complexes (PVCs), the compensatory pause that occurs after the PVC allows a longer period of time for ventricular filling, which results in a greater end-diastolic volume, increased contractile force, and a stronger pulse in the beat that follows the PVC. The strength of the peripheral pulse is variable in arrhythmias such as atrial fibrillation because the irregular rhythm is associated with variable time for ventricular filling. Certain arrhythmias, particularly tachyarrhythmias, allow inadequate ventricular filling to generate a peripheral arterial pulse, and a pulse deficit is palpated.
This is particularly likely with ventricular tachycardia at high heart rates.Approach to Diagnosis of Abnormal Peripheral Pulse
1. Take history. Note changes in appetite, attitude, milk production, or ability to exercise; determine whether there have been signs of previous illness and duration and progression of the problem.
Causes of Abnormal Peripheral Pulse in Ruminants
Common Causes
Dehydration
Shock
Toxemia
Congestive heart failure
Electrolyte imbalances
Acid-base disorders
Fever
Cardiac arrhythmias
Uncommon Causes
Patent ductus arteriosus
Aortic regurgitation
Peripheral arteriovenous shunt
2. Perform a physical examination. Determine vital signs. Note whether there is a cardiac arrhythmia, murmur, or other evidence of heart disease (e.g., jugular venous distention or pulsation, edema). Palpate the pulse in multiple sites and bilaterally to rule out occlusive arterial disease; check patient's hydration. Evaluate for potential causes of hypovolemia (e.g., diarrhea, blood loss) and sepsis/toxemia.
3. Obtain blood for the following:
a. CBC; look for evidence of toxemia or anemia
b. Blood gases to determine acid-base balance
c. Electrolyte concentration, especially Ca and K
4. Record an ECG to characterize any arrhythmia.
5. Determine blood pressure. The site for indirect blood pressure measurement in the standing animal is the tail over the coccygeal artery; a limb can be used in a recumbent animal. Take the mean of several (minimum of three) readings in which the blood pressure cuff is gradually inflated and deflated at approximately 2 to 4 mm Hg/sec; values obtained should be corrected for the difference in height between the site measured and the heart, which is considered to be at the level of the shoulder; the difference in height (centimeters) between the heart and the site of pressure measurement is multiplied by 0.77 (constant used to convert centimeters of blood to millimeters of mercury).
In standing horses, approximately 27 mm Hg is added to the indirectly measured coccygeal artery pressure to give the corrected value22; no correction factor is necessary if the measurement is made in a recumbent animal. An appropriately sized blood pressure cuff is considered to be one fourth of the tail circumference23; in horses a 4.5- to 5.6-cm cuff has been recommended for electronic oscillometric devices, and a 10.6-cm cuff is used for ultrasonic flowmeters.22 Values for normal horses by blood flow detection methods are 79/49 to 145/106 mm Hg (uncorrected for the height of the tail).21 Mean arterial blood pressure (MAP) obtained noninvasively (NIBP) correlates well with invasive blood pressure (IBP) measurements in normotensive horses but not in hypotensive or hypertensive horses.24,256. Blood pressure can be evaluated by Doppler scan, or measurements can be recorded by direct cardiac catheterization (see Chapter 30).
7. Perform an echocardiogram to evaluate size of chambers, myocardial function, presence of valvular or pericardial disease, or congenital defects.