18.1 Electrocardiography

Though electrocardiography is routinely employed for monitoring cardiac rhythm, rate, conduction disturbances, and size of the chambers of the heart in man, dogs, and cats, its use in ruminants is restricted to monitoring of heart rate and rhythm only. Detection of cardiomyopathy from electrocardiogram is not feasible in ruminants (cows, buffaloes, sheep, goat) and other hoofed mammals (pigs and horses) due to deep penetration of Purkinje fibers from endocardium to epicardium and cross-fiber bridges and depolarization and repolarization occurring over multiple minor fronts at a time. Nevertheless, identification of arrhythmias and conduction disturbances is of immense clinical value in ruminants also. Early detection of arrhythmias of severe nature and electrolyte imbalance might enable large animal practitioners to take appropriate remedial measures and to assess an accurate prognosis of ailing animals.

The generation of electrocardiogram in ruminants is due to depolarization and repolarization of cardiac muscle fibers similar to canines and felines. The wave of depolarization begins in sinoatrial node (SA node) and passes through atrium producing “P” wave and reaches to AV node. The delay in conduction at AV node results into P-R segment. Then depolarization of ventricles occurs producing “QRS” complex followed by isoelectric period, i.e., S-T segment. This is followed by repolarization of the heart represented by “T” wave. The values of amplitude and duration of complexes and intervals in ruminants vary from that of dogs and cats.

Positioning of animals: The animals (cow, buffalo, goat, sheep, mithun) are kept standing on a rubber mat, and the sites for applying alligator clips are prepared by shaving and cleaning with alcohol. Gel is applied liberally on the areas of attachment of electrodes.

Placement of electrodes and recording of ECG: Traditional lead system in large animals was based on Einthoven’s triangle using standard bipolar limb leads and augmented unipolar limb leads as in case of humans, dogs, and cats. In this lead system, ECG wave forms are affected by the variations in limb position, and complexes are small. Other lead system, i.e., base-apex lead system, where such variations of limb positioning have less impact, is more appropriate in large animals. In base-apex lead system, the right arm electrode is attached to the neck in the left jugular furrow two third down in the neck, left arm electrode is placed over the apex of the heart just behind the left elbow, and earth electrode (right hind leg electrode) is attached at wither (Figs. 18.1, 18.2, 18.3, 18.4, and 18.5). Electrocardiogram is recorded in a calm and quite surroundings using electrocardiographic machine at a paper speed of 25 mm per second and amplitude of 1 mV per 10 mm. The recording is done in lead I. In hexaxial lead system, forearm electrodes are attached at anterior aspect of respective olecranon on medial side of the elbow joint, and hind limb electrodes are placed on medial aspect of respective stifle joint (Fig. 18.6) similar to canines.

18.1.1 Electrocardiogram in Buffalo and Cow Calves

Electrocardiogram of ruminants is having “P,” “QRS,” and “T” complexes, P-R interval, S-T interval, Q-T interval, and R-R intervals (Fig. 18.7) as also seen in other animals. But “QRS” complex in ruminants is predominantly negative in the form of “rS” (Figs. 18.8 and 18.13) or “QS” (Figs. 18.9 and 18.14), and the “T” wave is either positive (Fig. 18.9), negative (Fig. 18.10), or biphasic. “P” waves may be either round, peaked, or slightly isoelectric in configuration.

Complexes are significantly (P < 0.05) large in lead I of base-apex (Fig. 18.11) than those of lead II of hexaxial lead (Fig. 18.12) system.

18.2 Electrocardiographic Indices in Buffaloes

Values of electrocardiographic indices in healthy Surti buffalo calves are given in Table 18.1. Mean values of heart rate, “P” amplitude, “P” durations, “R” segment, “P-R” interval, “QRS” amplitude, “QRS” duration, “S-T segments” amplitude, “T” duration, “Q-T” interval, and “R-R” interval in buffaloes have been reported as 90.39 ± 3.44 bpm, 0.195 ± 0.010 mV, 0.0513 ± 0.0014 s, 0.11 ± 0.0558 s, 0.166 ± 0.056 s, 0.877 ± 0.062 mV, 0.051 ± 0.0017 s, 0.167 ± 0.034 s, 0.278 ± 0.043 mV, 0.065 ± 0.027 s, 0.278 ± 0.043 s, and 0.278 ± 0.043 s, respectively (Varshney et al. 2013).

18.3 Electrocardiographic Indices in Holstein Crossbreds

Values of electrocardiographic indices in healthy Holstein crossbred calves are given in Table 18.2. Mean values for heart rate, “P” amplitude, “P” durations, “R” segment, “P-R” interval, “QRS” amplitude, “QRS” duration, “S-T segments” amplitude, “T” duration, “Q-T” interval, and “R-R” interval have been reported as 85.2 ± 4.23 bpm, 0.234 ± 0.031 mV, 0.05 ± 0.0024 s, 0.125 ± 0.025 s, 0.884 ± 0.06 mV, 0.054 ± 0.0001 s, 0.182 ± 0.038 s, 0.318 ± 0.088 mV, 0.069 ± 0.0037 s, and 0.308 ± 0.0017 s, respectively (Varshney 2018c). Electrocardiographic complexes in buffaloes and cows are almost of similar pattern.

Table 18.2

Electrocardiographic indices in Holstein crossbred calves (Base-apex lead system)

ECG indices	Range	Mean ± S.E.	Median
Heart rate (BPM)	60.0–140.0	85.2 ± 4.23	80.0
P amplitude (mV)	0.10–0.30	0.234 ± 0.031	00.2
P duration (s)	0.04–0.08	0.05 ± 0.002	0.02
P-R interval (s)	0.08–0.18	0.125 ± 0.025	0.12
S amplitude (mV)	0.20–1.70	0.884 ± 0.06	0.90
QRS duration (s)	0.04–0.08	0.054 ± 0.0001	0.06
S-T segment (s)	0.12–0.28	0.182 ± 0.038	0.20
T amplitude (mV)	0.1–0.6	0.318 ± 0.088	0.30
T duration (s)	0.04–0.12	0.069 ± 0.0037	0.08
Q-T interval (s)	0.24–0.39	0.308 ± 0.0017	0.32

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