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166 PART 3 CAT WITH SIGNS OF HEART DISEASE

RHYTHM STRIP: II
50 mm/sec; I cm/mV
APC

Figure 10.4. Atrial premature contraction (APC).

There is association between P waves and the QRS not likely to severely compromise the patient’s
complex. outcome.
Supraventricular premature beats cause re-setting of If the arrhythmia is frequent and is a result of severe
the sinoatrial node. left atrial enlargement, these beats may reflect atrial
electrical instability and precede the development of
Supraventricular tachycardia is always regular.
atrial fibrillation.
Differential diagnosis
VENTRICULAR PREMATURE BEATS
Marked respiratory sinus arrhythmia is rare in cats (VPBS)***
but may mimic the presence of premature beats. Sinus
arrhythmia is rare in cats and only occurs at slow
Classical signs
heart rates. Increasing the heart rate above 180 bpm
by excitement or atropine (0.04 mg/kg parenterally) ● Pulse deficits.
will abolish sinus arrhythmia. ● Interruptions of rhythm regularity.
Motion or electrical artifact may mimic premature
beats. Premature beats can be distinguished from motion Pathogenesis
artifact because they occur simultaneously in all leads.
Premature beats result from two mechanisms, alter-
Ventricular premature beats without a wide QRS ations in impulse formation and alterations in impulse
complex may be erroneously classified as supraventric- conduction.
ular premature beats. Supraventricular premature beats
Alterations of impulse formation depend on the
will usually reset the sinus rate.
intrinsic automaticity of diseased cardiac cells.
● Diseased myocardial cells outside the specialized
Treatment conduction system can acquire automaticity
because of changes in the resting membrane poten-
Isolated premature beats do not require therapy unless
tial.
they present with other more severe arrhythmias.
Alterations of impulse conduction result from re-
If there is suggestion of hemodynamic compromise
entry and by-pass tracts.
therapy may be considered:
● Re-entry, is a self-sustaining electrical circuit that
● Atenolol at 6.25–12.5 mg/cat PO every 12 hours.
repeatedly depolarizes surrounding tissue.
● Diltiazem at 7.5–15 mg/cat PO every 8 hours.
● By-pass tracts alter the normal pathway of con-
● Propranolol at 0.5–1 mg/kg PO every 8–12 hours.
duction by providing an alternative pathway
● Sotalol at 10–20 mg/cat PO every 12 hours.
around the AV node.
Prognosis
Clinical signs
The prognosis depends on the underlying cardiac
disease, but in general, this class of arrhythmias is Most patients show no evidence of clinical signs.