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Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 223

mal because it is not reported in healthy cats undergoing change in heart rate warrant immediate termination of
Holter monitoring (Hanås 2009; Ware 1999). Since AT the maneuver. Because cats very rarely suffer from
may occur in short bursts lasting 1–60 seconds or may carotid artery atherosclerosis, concern for thromboem-
be sustained (uninterrupted for >60 seconds), it may bolic consequences of carotid sinus massage as expressed
cause clinically relevant decreases in cardiac output and in human cardiology is unlikely to be relevant to small
even overt clinical signs. It is uncommon for AT to cause animal practice. The other form of vagal maneuver used
syncope in the cat or human (Delacrétaz 2006) but AT routinely in feline medicine is ocular pressure. It consists
has been diagnosed in a syncopal cat using an implant- of applying firm but controlled gentle digital pressure to
able cardiac event monitor (Ferasin 2009). Underdiagnosis both globes through closed eyelids. The distance of ret-
of AT in cats is likely, since the most common clinical ropulsion of the globes depends on the shape of the
signs in humans with AT are sensations without overt orbit, typically 5–15 mm in a cat (lower value for brachy-
manifestations: light-headedness, palpitations, and chest cephalic breeds with shallow orbits). Subjectively, the
pain (Delacrétaz 2006). amount of pressure applied is the best guide: it should
The diagnostic test of choice is ECG, which should be not exceed the digital pressure one would place on a ripe
a 10-lead baseline tracing initially since feline P waves grape without rupturing it. Likewise, this should be
and QRS complexes are often very small and difficult to acceptable to the patient, and signs that it is not warrant
interpret on single-lead tracings (see Figure 18.8). The immediate termination. Ocular pressure is contraindi-
sporadic occurrence of AT means that establishing the cated in patients with ocular problems. There appears to Arrhythmias
diagnosis may require extended ECG monitoring, such be interindividual variation in response to vagal maneu-
as in-hospital telemetry, or ambulatory ECG (Holter or vers, with some individuals demonstrating a stronger
event monitoring). The differential diagnosis for AT effect during carotid sinus massage and others during
includes sinus tachycardia (for which a distinct P wave ocular pressure. Overall, the effect of a vagal maneuver
of the same morphology as sinus P waves is present for should be manifested at some point during the applica-
every QRS complex at a fixed PR interval) and ventricu- tion of pressure (typically within 5–15 seconds of initia-
lar tachycardia (for which the QRS complexes should be tion of the maneuver). It must be remembered that vagal
different in morphology from sinus QRS complexes). maneuvers increase parasympathetic tone but are not a
Clinically, bradycardias may also cause syncope and are guarantee of parasympathetic override; that is, an
a differential diagnosis for AT in patients with undiag- anxious, distraught, or otherwise strongly sympatheti-
nosed syncope. Probably the most common ECG dif- cally stimulated cat may not respond to a vagal maneuver
ferential diagnosis for AT is motion artifact, notably simply because of the sympathetic stimulus, not because
purring (see Figure 18.10, later in this chapter) (Ware the arrhythmia is independent of the SA and AV nodes.
1992). Other forms of motion artifact (shivering/ Therefore, the value of vagal maneuvers lies mainly in
tremor) and electrical interference also may mimic AT the diagnostic help provided by slowing a very rapid
(see Figure 18.11, later in this chapter). heart rate to make the rhythm decipherable on ECG.
Confirmation of AT may be aided with a vagal maneu- Treatment of AT requires correction of the underlying
ver (Wright 2009). The purpose of a vagal maneuver is problem when possible (e.g., hyperthyroidism) and
to increase parasympathetic tone predominantly to the administration of medications that reduce the ventricu-
SA and AV nodes, since they receive a large proportion lar rate if it is excessive. Although formal guidelines do
of the autonomic inputs to the heart. Slowing of the not exist for the cat, ATs that produce a sustained heart
sinus rate, and/or transient physiologic AV block, are the (ventricular) rate of >260 beats/minute for several con-
most common and desired effects. Ventricular rhythms secutive minutes or longer, that are shown to be associ-
such as ventricular tachycardia are less easily influenced ated with overt signs of syncope or near-syncope via
by vagal maneuvers, if at all. ECG recordings during the clinical signs, or both, should
Vagal maneuvers can be performed safely and consis- be treated with antiarrhythmic medications. Beta-
tently. One form of vagal maneuver, carotid sinus blocking drugs (e.g., atenolol 1–1.5 mg/kg PO q 12h, or
massage, involves the simple application of gentle, sus- sotalol 2 mg/kg PO q 12h) are the first line of treatment
tained digital pressure by the clinician to the external except when AT is associated with hypovolemia/
region of the neck overlying one or both of a cat’s carotid dehydration, anemia, congestive heart failure, or other
sinuses, located just caudal to the dorsal aspect of the condition in which primary reduction of the heart rate
larynx (sometimes to the point of eliciting a gag reflex), might be detrimental. A cat that is unstable (unconscious
for 5 to 10 seconds, while the ECG tracing is being and/or recumbent with a weak or absent pulse) during
recorded. Such a maneuver should be tolerated by the AT should be approached as follows: 1) confirm AT, since
patient, and signs of discomfort, resentment, or a marked sinus tachycardia occurs much more commonly and