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Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 217

QRS

P T

Figure 18.2. Normal sinus rhythm in a 17-year-old male castrated domestic short-haired cat with peritoneopericardial diaphragmatic
hernia. Heart rate = 160 beats/minute. 25 mm/sec, 1 cm = 1 mV.

The result is an influx of calcium through the I Ca–T (tran- seen. Uncommonly, abnormal atrial repolarization may
sient) and I Ca–L (long-lasting) currents, causing depolar- produce a visible T a wave in the terminal part of the PR
ization. A spontaneous, inherent ability to depolarize is segment.
called automaticity, and this is a hallmark feature of the In the normal cat, this sequence of cardiac depolariza-
normal SA node. The end of depolarization is indicated tion and repolarization occurs consistently for each beat, Arrhythmias
by activation of the repolarizing outward rectifier and the interval from one beat to the next is constant,
current, I K, which evacuates potassium ions from the creating normal sinus rhythm (NSR). The ECG charac-
cell. teristics of NSR are monomorphic (same-shaped) P
The wave of electrical activity created in the SA node waves, monomorphic QRS complexes, the presence of a
spreads outward through the atria, creating the P wave P wave for every QRS complex and a QRS complex for
on the ECG. A portion of this electrical activity reaches every P wave with a consistent P-R interval, and a con-
the atrioventricular (AV) node in the floor of the right stant R-R interval (i.e., a regular rhythm) at a rate
atrium. The AV node is a functional filter that prevents between 110–180 beats/minute outside a stimulating or
excessive numbers of supraventricular impulses from stressful environment (Figure 18.2) (Côté and Harpster
reaching the ventricles and delays the conduction of 2009).
each normal impulse slightly in order to allow optimal
ventricular filling via the completion of atrial contrac- Sinus Tachycardia
tion. This AV node-associated delay is expressed as the Withdrawal of vagal tone, and an increase in sympa-
PR segment, the period of electrical quiescence between thetic tone, are the factors that underlie sinus tachycar-
the P wave and QRS complex. For purposes of nomen- dia in the cat (Diepstra et al. 1980; Tsuchimochi et al.
clature, note that the PR interval consists of the PR 2002). Sinus tachycardia is a rapid, regular rhythm in
segment plus the P wave proper. Passage through the AV which the ECG findings are identical to those of NSR
node triggers ventricular depolarization through the except for rate, which is defined as >180 bpm (Figure
His-Purkinje system’s arborization into the right and left 18.3). The upper limit of heart rate for sinus tachycardia
bundle branches, the latter of which may further divide in the cat is 260–280 bpm (Côté et al. 2004; Ware 1999),
into the left anterior and left posterior fascicles. This although anecdotal reports of sinus tachycardia at
division has been demonstrated in many species and 300 bpm exist in cats.
remains controversial in the cat, particularly as concerns Sinus tachycardia is essentially never the cause of
left anterior fascicular block. The ECG expression of clinical signs, nor is it frequently observed as an isolated
ventricular depolarization is the QRS complex. rhythm abnormality. Rather, sinus tachycardia is almost
Repolarization is necessary to prepare the myocardium always the consequence of a more generalized distur-
for a subsequent depolarization; in the ventricles, it bance, such as anxiety, pain, medication/toxicosis, hypo-
occurs transmurally (from the endocardial surface volemia, anemia, or hypoxemia. Therefore, the history
toward the epicardium, as well as from the epicardial of patients with sinus tachycardia is vital because it helps
surface to the endocardium, such that the components to pinpoint the underlying cause of the tachycardia.
of repolarization meet midmurally) and creates the T Sinus tachycardia itself is not expected to produce any
wave on ECG. An equivalent process occurs in the atria, owner-observed abnormalities.
producing the T a (“T sub-a”) wave, but the extremely The physical examination findings specific to sinus
small amplitude of this wave in the cat, and its frequent tachycardia consist essentially of an elevated heart rate.
overlaying by the QRS complex, mean that it is rarely The pulse is synchronous and may be of the same,