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Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 243

finding, especially if the diagnosis was reached partly or atrial myopathy and hyperkalemia using the ECG alone
entirely in error due to incorrect limb position during is difficult. If P waves are not evident, immediate mea-
restraint for the ECG (see Chapter 9). Clinical manifes- surement of serum electrolytes is warranted.
tations of BBBs alone generally do not occur. These dis-
turbances therefore do not warrant specific treatment Sinus Node Dysfunction (Sick Sinus Syndrome,
beyond that of their underlying cause if one exists. The Bradycardia-Tachycardia Syndrome)
importance of recognizing BBB lies in the fact that they This polyarrhythmic disorder, which is well-recognized
could be the first indicators of underlying cardiac in humans and dogs, has never been reported as such in
disease, which itself warrants further diagnosis and the cat. An autoimmune disturbance directed at the
treatment, and that they can—and should not—be mis- heart’s nodal tissues, or at autonomic receptors, has been
interpreted as ventricular arrhythmias. shown in humans with this disorder, but such mecha-
nisms have not been evaluated in veterinary medicine
Atrial Standstill (Silent Atrium) (Côté 2010). The hallmark ECG findings are a mixture
This rhythm disturbance is characterized by the total of sinus pause/sinus arrest with a failure of escape
absence of atrial depolarization. The three differential rhythm to emerge (causing asystole of up to several
diagnoses for atrial standstill are 1) moderate to marked seconds’ duration) and AV block, as bradycardias; PVCs
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hyperkalemia (K >7.5 mEq/l; see below), 2) atrial and/or PACs as tachycardias; and generally a variety of
myopathy (Gavaghan et al. 1999; Richig and Tilley these arrhythmias occurring unpredictably and some- Arrhythmias
1984), and 3) ECG artifact (P waves too small, or iso- times simultaneously on the same ECG strip. Based on
electric, preventing them from being seen properly on a cardiac event monitoring in dogs, it is bradycardia that
chosen ECG lead). Given the very small size of normal is associated with the clinical signs, and therefore optimal
feline P waves, a 10-lead ECG (with reduced or no noise treatment includes pacemaker implantation. The prog-
filtration and an increased gain of 20 mm/mv [Schrope nosis in cats is unknown, as it has not been reported in
et al. 1995]) is always warranted when atrial standstill is this species. Interrestingly, cats with third-degree AV
suspected (Figure 18.23); the presence of P waves on block causing syncope often have a failure of escape
even 1 lead rules out atrial standstill. While hyperkale- mechanism and prolonged periods of ventricular
mia is the most common cause of atrial standstill (and asystole corresponding to the syncopal episodes (see
the only reversible one; see below), atrial standstill may Fig 18.20). This failure of ventricular (or junctional)
occur due to marked atrial stretch, as occurs in severe escape mechanism emergence, together with AV block,
feline cardiomyopathies (Gavaghan et al. 1999; Richig could be consistent with the criteria for sinus node
and Tilley 1984). Atrial myopathy is not common, and dysfunction. However, the underlying cardiomyopathy
it carries a grave prognosis; severe right-sided CHF is that often exists in these cats (see discussion of AV
usually responsible for the major clinical signs, and it is blocks, above), and histologic demonstration of seg-
worsened by the bradycardia. Regardless of cause, the mental degeneration and fibrosis of the intraventricular
ECG appearance is of a regular rhythm, usually with conduction system with no histologic lesions in the SA
QRS complexes that are of a supraventricular appear- or AV nodes (Kaneshige et al. 2006), suggest that a
ance, and with a normal rate but without detectable P mechanism associated with cardiomyopathy is the cause,
waves in any lead on the ECG. Differentiating between rather than multifocal or diffuse lesions affecting the

I II III aVR aVL aVF rV2 V2 V4 V10

Figure 18.23. Atrial standstill in a young cat with extreme left atrial enlargement (LA : Ao >3 : 1) associated with hypertrophic cardio-
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myopathy. Two heartbeats are shown for each lead, and no P waves are seen. The serum K level was 4 mEq/l (within normal limits).
Because feline P waves are often very small, all 10 ECG leads are needed to identify atrial standstill. T waves (positive in III, aVF, rV2;
negative in I, aVL, V4, V10) should not be mistaken for P waves. 25 mm/sec, 1 cm = 1 mV.