Cardiac Catheterization in Large Animals

Cardiac and great vessel catheterization can be performed in standing unsedated large animals to determine the following data:

• Pressure and waveforms (shape of the pressure curve)

• Oxygen tension, oxygen saturation, oxygen content

• Cardiac output and other indicators of ventricular size and function

Cardiac catheterization is also used for special diagnostic studies such as angiocardiography, nuclear angiocardiography, and indicator dilution studies.

Cardiac catheterization is usually reserved for specialty practices and referral institutions, owing to the equipment needed and skills required for acquiring and interpreting accurate data. Results are not always specific, but catheterization can add quantitative measurements that increase diagnostic and prognostic accuracy for certain cardiac conditions and may provide a direction for therapy.

Blood Pressure Measurements

Normal blood pressures for cattle and horses are listed in Table 30.2.^33-36 The values for horses represent a summary of data from numerous authors as cited in the references given. Accuracy of pressure recordings is greatly influenced by the

FIG. 30.4 Schematic representation of ventricular pressure curves recorded from a normal horse and a horse with pericarditis, showing the relationship between pressure changes and the electrocardiogram. With pericarditis, the ventricular end-diastolic and systolic pressures are elevated and pressure declines sharply in early diastole.

■ TABLE 30.2

Cardiac Pressure Measurements in Normal Horses and Cattle^a

^aPressure ranges are reported as systolic/diastolic (mean) in mm Hg unless otherwise designated.

AO, Aorta; CA, carotid artery; LV, left ventricle; PA, pulmonary artery; PAW, pulmonary arterial wedge; RA, right atrium; RV, right ventricle.

choice of catheter and recording equipment used. While an oscillometric blood pressure device placed over the coccygeal artery corrected to the height of the heart is good for estimating blood pressure noninvasively and assessing marked trends, its accuracy and precision is low when compared with invasive blood pressure monitoring in standing horses.³⁷ Pulmonary artery wedge pressure (PAWP) is an indicator of left atrial mean pressure so long as balloon inflation of the catheter occludes flow in the segment of the pulmonary artery that is catheterized. PAWP is superior to central venous pressure (CVP) as a monitor of left ventricular function and fluid therapy. However, CVP is a good indication of intravascular volume in horses with dehydration.³⁸

The shape of the pressure curve in the ventricles, aorta, or pulmonary artery may have diagnostic significance in conditions like constrictive pericardial disease (Fig. 30.4); pulmonic stenosis (giant A wave), rare as an isolated defect in large animals; or tricuspid or mitral valve regurgitation (large V wave). These conditions are more commonly assessed by echocardiography. An abnormal rise in pressure going from one chamber to the next indicates a stenotic lesion (uncommon in large animals) at the level of the pressure gradient. The size of the pressure gradient can be used to determine the severity of the lesion.

Blood Oxygen Measurements

Blood oxygen measurements are taken from the chambers on the right side of the heart to detect abnormal elevations indica­tive of a left-to-right shunt (ASD, VSD, patent ductus arteriosus [PDA]).

• ≥1.9 volume percent from the superior vena cava to the right atrium

• ≥0.9 volume percent from the right atrium to the right ventricle

• ≥0.5 volume percent from the right ventricle to the pulmonary artery

Because oxygen content depends on hemoglobin concentra­tion, oxygen saturation, which is independent of hemoglobin concentration, may be a more accurate indicator of shunts in anemic or polycythemic patients. Changes in the animal's physiologic status during sampling (cardiac output, ventilation, oxygen consumption); incomplete mixing of shunted blood; and variable time intervals between sampling can be potential sources of error. Several samples within a single chamber improve reliability of results. Small shunts and shunts in animals with low systemic arterial oxygen tension may not be detected by this technique.

Shunt calculations can be made once oxygen saturation or content has been measured in each of the right heart chambers, the pulmonary artery, and a systemic artery. For a left-to-right shunt, the pulmonary flow/systemic flow ratio (QP/QS) is determined as follows:

in which SAO₂ is the arterial blood oxygen content, MVO₂ is the mixed venous blood oxygen content, and PAO₂ is the pulmonary artery oxygen content. A 2:1 QP/QS represents a 50% left-to-right shunt, indicating that 50% of the pulmonary flow is from the left side of the heart.

For a right-to-left shunt, the QP/QS is determined as follows:

in which PVO₂ is the pulmonary venous oxygen content (assumed to be 98% of oxygen capacity plus 0.3 mL of dissolved oxygen).⁴²

Cardiac Output and Ventricular Function Assessment

Cardiac output is determined by indicator dilution methods (usually by dye dilution or thermodilution or, more recently, by lithium dilution), by the Fick method, or by using 2D or Doppler echocardiography.

Cardiac output values in the resting horse range from 32 to 40 L/min.⁴⁷ Cardiac index is the cardiac output divided by a measure of body size (body weight in kilograms) and is expressed in mL/kg/min.⁴⁷ The normal cardiac index for the adult horse ranges from 72 to 88 mL/kg/min.^47,48 Cardiac output values of 20.9 to 23.6 L/min have been reported in normal anesthetized horses.⁴³ In cattle a cardiac index of approximately 110 mL/kg/min has been reported.³⁶ Cardiac output measure­ments in clinical patients vary with heart rate, excitement, hydration, and many other factors and are best determined in the pulmonary artery. Electronic integration and computation of area under the curve by means of battery-powered units that can display results instantly provide the most reliable results.⁴⁹

Cardiac output results or indicator dilution curves can provide quantitative and qualitative assessment of cardiac shunts. Characteristic changes in the temperature-time curve (ther­modilution methods) or dye concentration-time curve indicate the presence of a left-to-right, right-to-left, or bidirectional shunt. Calculation of the cardiac output in the chamber just proximal to the shunt and distal to it can give a quantitative estimate of the size of the shunt.⁴⁰

Angiocardiography

Angiocardiography is used in neonates or animals small enough to have the entire cardiac silhouette visualized on a single radiograph cassette.

Nuclear Angiocardiography

In nuclear angiocardiography, specialized equipment captures sequential digitized images of the right side of the heart, lung, and left side of the heart after rapid injection of radiographic tracer into peripheral circulation. A more extensive review of this subject has been published.⁵⁰ Nuclear angiocardiography can be used to confirm valvular dysfunction, which is manifested by chamber enlargement or prolonged washout of affected vessels or cardiac chambers and is quantitated by the regurgitant fraction. It also can reveal enlargement of chambers and prolonged washout resulting from cardiac failure. The presence of intracardiac or extracardiac shunts is demonstrated by the simultaneous visualization of left- and right-side cardiac chambers or slow washout downstream of the shunt. In addition, nuclear angiocardiography is used to calculate cardiac output and ejection fraction (EF) and other indices of cardiac function.