Background Information of Clinical Importance

Anatomy and Physiology

Structure of the Heart and Vessels

The heart of the goat extends from the third to the sixth rib and may contact the diaphragm at its caudal edge. The position and orientation within the thorax are similar to those of other ruminants.

Goat Medicine, Third Edition. Mary C. Smith and David M. Sherman. © 2023 John Wiley & Sons, Inc. Published 2023 by John Wiley & Sons, Inc.

Anomalies of the heart reported in goats include ven­tricular septal defects (VSD) in the lower, middle, and upper ventricular septum (Parry et al. 1982) and ectopia cordis, with the heart exposed to the outside through a fis­sured sternum (Narasimha Rao et al. 1980). Kids with ecto­pia cordis may be born alive and survive for hours or days (Upadhye and Dhoot 2001), or they may be born dead with structural abnormalities of the externalized heart such as a single ventricle (Dadich 2000).

There are no major differences in the structure and dis­tribution of the great vessels of goats compared with other ruminants. Anomalies of the great vessels are uncommon, but persistence of the left cranial vena cava (Waibl 1973), dextroposition of the aorta (Parry et al. 1982), and aortic stenosis (Scarratt et al.

Parameters of Blood Flow

Information has been published on cardiac output, stroke volume, systolic and diastolic arterial pressures, pulmo­nary arterial pressure, pulmonary arterial flow rates, and central venous pressure, but the numbers of animals involved in the various studies have been limited (Jha et al. 1961; Hoversland et al. 1965; Foex and Prys- Roberts 1972; Ivankovich et al. 1974; Vesal and Karimi 2006).

Mean cardiac outputs in goats of various breeds, age, and sex have been reported in the range of 2.8 ± 0.7 to 4.8 ± 1.4 L/ min (Hoversland et al. 1965; Foex and Prys-Roberts 1972; Ivankovich et al. 1974; Olsson et al. 2001). Significant dif­ferences in cardiac output have been confirmed in dairy goats during pregnancy (6.73 ± 0.72) compared to lactation (6.12 ± 0.52) and the dry period (4.39 ± 0.27) in the same individuals (Olsson et al. 2001).

Mean stroke volumes derived from a measurement of cardiac output and heart rate or measured directly by dye dilution technique have been reported in the range of 20.3 ± 3.1 to 46.9 ± 23 mL (Foex and Prys-Roberts 1972; Ivankovich et al. 1974). Mean systolic arterial pressure recorded from goats of varying breeds, age, and sex ranged from 122 to 124.9 mmHg, while mean diastolic pressures in the same goats ranged from 85 to 97.8 mmHg (Jha et al. 1961; Hoversland et al. 1965). Mean central venous pressure in goats of both sexes was reported as 1.25 ± 0.14 cm H₂O, but there was a statistically significant difference between males (0.80 ± 0.11) and females (1.9 ± 0.26). There was no difference noted, however, between standing goats and those in lateral recumbency (Vesal and Karimi 2006).

Normal Heart Rate and Rhythm

The heart is heard most audibly over the left thoracic wall at the fourth intercostal space. Heart rate varies considerably in goats with age and level of activity. Mean heart rates for small numbers of goats at different ages have been reported from India (Upadhyay and Sud 1977).

Using telemetric recording of heart rate, investigators demonstrated a significant increase in heart rate of normal young goats during physical examination (116 ± 11.7 bpm) compared with before (92.2 ± 6.3 bpm), and a significantly higher rate when eating (114.1 ± 2.1 bpm) compared to standing (84.2 ± 0.9 bpm) or lying down (76.5 ± 1.1 bpm) (Vesal et al. 2000). Other investigators have reported sig­nificant differences in heart rates in goats of different breeds kept under similar conditions (Medeiros et al. 2001), and elevated heart rates in pregnant does compared to the same does when lactating and when non-pregnant and non-lactating (Olsson et al. 2001).

Two heart sounds are normally heard in the goat, S₁, and S₂. A normal respiratory sinus arrhythmia is common, with acceleration occurring in late inspiration. It is more pro­nounced in younger goats. Second degree A-V block, though uncommon, has been recorded in a normal goat (Szabuniewicz and Clark 1967).

Diagnostic Methods

Electrocardiography

Parameters for the electrocardiogram (ECG) in normal goats have been established using standard and augmented limb leads (Szabuniewicz and Clark 1967; Upadhyay and Sud 1977).

Interval durations, in seconds, have been reported for the P-wave, P-R interval, QRS complex, Q-T interval, and

Table 8.1 Electrocardiographic parameters for lead II reported in normal goats.

T-wave (Jha et al. 1961; Szabuniewicz and Clark 1967; Schultz and Pretorius 1972; Upadhyay and Sud 1977; Ogburn et al. 1977). Minimal differences occur between various leads (Szabuniewicz and Clark 1967). The ampli­tudes of ECG deflections are small in the goat. Mean ampli­tudes for various leads as measured in 100 goats have been reported (Szabuniewicz and Clark 1967).

The P-wave is most often peaked in standard and aug­mented leads, but is occasionally flat or rounded. Biphasic P-waves are rarely seen. The P-wave is usually positive in the standard leads and the augmented leads, except for aVR. Changes in the shape of P-waves may be observed sometimes in normal goats, representing wandering of the pacemaker in the sinoatrial node.

The shape of the QRS complex is quite variable within and between the different leads. It is usually mono- or diphasic and infrequently triphasic. In lead I, a QS pattern dominates. In lead II, QS or Qr patterns are most often recorded. In lead III, no single pattern is predominant, and the Qr, qR, R, RS, and Rs patterns commonly occur. Q waves are rarely observed in the aVR lead, and the R and Rs patterns predominate. In the aVL lead, the QS pattern is most common, followed in frequency by the rS pattern. In lead aVF, either the Qr or the QS pattern may predominate, with the R, RS, Rs, and rS patterns also common. Because of the variability of the QRS complex in particular, it is gen­erally considered that there is no typical morphologic pat­tern that is characteristic for the ECG test results of the goat using the common lead systems.

The T-wave occurs most commonly in the peaked form. Flat, round, or diphasic T-waves are uncommon. In most cases, the T-wave is of opposite polarity or deflection to the accompanying QRS deflection, although concurrent posi­tive deflections occur in lead III and concurrent negative deflections in lead aVL.

Vectorcardiography is of limited clinical value in goats. As is the case with other ruminants, Purkinje fibers fully penetrate the ventricular wall with extensive ramification. This permits rapid excitation of both ventricles, resulting in near cancellation of potentials within the wall. As a result, subtle changes in vector orientation or QRS wave structure resulting from cardiac derangements are difficult to distinguish.

Radiography

The radiographic anatomy of the caprine thorax has been described (Singh et al. 1983). In dorsoventral radiographs, the normal heart lies between the second or third intercos­tal space and the sixth or seventh space, with the base cen­tered over the midline and the apex shifted to the left of midline.

While not universally accepted, measurement of contact area of the heart with the sternum has been used to evalu­ate cardiac enlargement secondary to VSD in the goat. Normal hearts had a mean contact area of 3.3 sternebrae; abnormal hearts had a mean contact area of 6 (Parry et al. 1982).

Echocardiography

While M-mode, two-dimensional (B-mode), and Doppler echocardiography are being applied more frequently to the diagnosis of suspected heart disease in goats, published information on normal echocardiographic parameters in the goat remains limited. In one study, mitral, aortic, and tricuspid valves were identified at the right third and fourth intercostal spaces slightly dorsal to the olecranon. The pul­monic valve was identified at the same location on the left (Yamaga and Too 1984). In a later study, technical difficul­ties were noted with regard to echocardiographic examina­tion of the goat. The cranial location of the heart, partly covered by the olecranon and the caudal brachial muscles,

Table 8.2 Echocardiographic and Doppler measurements and calculated stroke volume and cardiac output during pregnancy, lactation, and dry period in the same eight goats (Capra hircus).

AO, aortic root; FS, fractional shortening of the left ventricle; HR, heart rate; LA, left atrium; LVEDD and LVESD, left ventricular end-diastolic and end-systolic diameter, respectively; LVWd and LVWs, left ventricular wall thickness in diastole and systole, respectively; V_max, maximal aortic flow; VTI, velocity trace integral. Values are means ± standard error of the mean.

P < 0.001 vs. dry period

^t P < 0.05 vs. lactation

* P < 0.01

Source: Olsson, K., Hansson, A., Hydbring, E. et al. (2001). A serial study of heart function during pregnancy, lactation and the dry period in dairy goats using echocardiography. Experim. Physiol. 86(1):93-99. Reproduced with permission of Wiley-Blackwell.

along with the narrow intercostal spaces made it difficult to position the ultrasound transducer and limited the acoustic window (Olsson et al. 2001). Nevertheless, echo­cardiographic and Doppler measurements were obtained in eight normal Swedish domestic goats during pregnancy, lactation, and the dry period. These measurements are pre­sented in Table 8.2. A separate study of eight normal goats in the Philippines also reports normal caprine cardiac parameters measured by B-mode and M-mode ultrasonog­raphy (Acorda et al. 2005).

A more recent study established reference values in 12 normal adult Saanen goats using two-dimensional and M-mode echocardiography, while also demonstrating good interday repeatability of measurements (Leroux et al. 2012). Application of pulse wave Doppler echocardiography in 20 healthy Markhoz goats in Iran demonstrated poor interday repeatability and moderate variability in measurements (Sadi and Alizadeh 2018).

There is one published case report on the use of echocar­diography for the diagnosis of heart disease in a goat (Gardner et al. 1992). In that report, a 3-year-old Pygmy buck was evaluated for a systolic murmur. E Ichocardiography revealed an enlarged right atrium and ventricle, an atrial sep­tal defect, and a dysplastic tricuspid valve, while color-flow Doppler echocardiography revealed severe tricuspid regurgi­tation and a right-to-left shunt through the atrial septal defect. A diagnosis of Ebstein’s anomaly of the tricu sppd valve was made, the first such report in goats. Ullrason.ography has ako been used to identify cardiac and pulmonic abnormalities associated with enzootic calcinosis in goats (Gufler et ah 1999).

A relatively new technique, two-dimensional speckle­tracking echocardiography, has been applied in the goat to assess systolic left ventricular function in relation to general anesthesia (Berli et al. 2015), and to measure cardiac response to exercise and to experimental myocardial infarc­tion (Leroux et al. 2020). The technique is considered to be an improvement over tissue Doppler imaging and is particu­larly useful for measuring myocardial velocities and defor­mation parameters such as strain and strain rate, providing important information on systolic and diastolic function, ischemia, myocardial mechanics, and other pathophysio­logic processes of the heart (Blessberger and Binder 2010).

Pericardiocentesis

Indications for pericardiocentesis in the goat are limited. Traumatic reticuloperitonitis, which is a well-known con­dition in cattle, is rarely reported in goats in the veterinary literature. Pericardial effusions may occur in caprine heart­water (cowdriosis), but the fluid is not useful for diagnostic purposes. Pericarditis also occurs in conjunction with sys­temic mycoplasmosis, but the organism is more easily iso­lated from other sites. If attempted, pericardiocentesis is best performed at the right fourth intercostal space low on the chest wall to avoid lung and coronary arteries. The ani­mal can be standing or in lateral recumbency. Chemical restraint such as diazepam is advisable. Surgical access to the pericardium can be accomplished by resection of the left fourth rib.

Computed Tomography

Computed tomography of the caprine thorax has been described (Smallwood and Healey 1982). The heart is visi­ble in cross-section beginning at the caudal aspect of tho­racic vertebra T1 and ending at the middle of T5. The right side of the heart is more visible in the anterior cross­sections and the left side in the posterior sections. At the level of the caudal aspect of T2, the heart silhouette is in contact with both the right and left thoracic walls.

Angiography

Left ventricular angiocardiograms in goats have been reported for the diagnosis of VSD. Passage of a catheter was performed via the left and right carotid arteries to acquire angiographic and hemodynamic data (Parry et al. 1982; Scarratt et al. 1984). Arteriography and ultrasonography also have been applied to studies of the carotid artery diam­eter and blood flow velocity in goats (Lee et al. 1990).

Clinical Chemistry

The reported use of clinical chemistry in the diagnosis of caprine heart disease is limited. Myocardial damage or necrosis can lead to release of various enzymes into the cir­culation, including aspartate aminotransferase (AST), lac­tate dehydrogenase (LDH), and creatine kinase (CK). However, skeletal muscle damage can also result in meas­urable increases in serum concentrations of these enzymes. In the case of LDH and CK, measurement of cardiac­specific isoenzymes might confirm the involvement of heart muscle, but the cardiac specificity and reference val­ues for caprine isoenzymes of LDH (LDH₁) and CK (CK MB) are not well documented.

In recent years, measurement of the myofibrillar contrac­tile protein troponin in plasma has become the accepted standard biomarker for acute myocardial infarction in human patients, and troponin is also increased in patients with decreased left ventricular function without acute myo­cardial infarction (Ammann et al. 2003). Reference values for measurement of cardiac troponin (cTN T and cTN I) in healthy cattle, sheep, and goats have been reported. No cTN T was measurable in the serum of any of the species. In cat­tle, cTN I levels were 0-0.23 ng/mL, (mean 0.18 ng/mL), in sheep 0-0.21 ng/mL (mean 0.15 ng/mL), and in goats 0-0.24 ng/mL (mean 0.20 ng/mL) (Basbugan et al. 2010).