Page 227 - Feline Cardiology

232 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

T
P

QRS

Figure 18.16. Sinus tachycardia mimicking VT in a cat. A quick glance of this monomorphic rhythm with QRS complexes of predomi-
nantly negative morphology due to marked axis deviation, might fail to reveal that each QRS complex has a P wave preceding it at a
regular PR interval. This is normal sinus rhythm, not VT.

Identifying features that distinguish these impostors electrolytes is the quickest method for ruling out this
Arrhythmias A ventricular escape rhythm always occurs at a slower- Finally, macroreentrant tachycardia caused by an acces-
from PVCs are
possibility.
sory or bypass tract (see the section “Ventricular
than-normal rate for that individual (by definition),
uncommonly in the cat; when it exists, it may be con-
such as during third-degree AV block; the QRS com- Preexcitation and Macroreentry ,” below) occurs
plexes are wide and bizarre in shape and occur typi- verted to normal sinus rhythm with a vagal maneuver,
cally at a rate of 60–120 beats/minute in the cat. and during sinus rhythm a consistently short PR inter-
Right bundle branch block and right axis deviation due to val (or absent PR segment altogether) is possible.
hypertrophy or displacement (Figure 18.16) represent
a deviation in the orientation of intraventricular con- Treatment of PVCs with antiarrhythmic drugs is gener-
duction, with no effect on the rhythm (sequence) of ally not undertaken unless the PVCs occur consecutively
the heartbeats; therefore, consistent R-R and P-R and for a prolonged duration, meeting the criteria for
intervals are observed for all beats and the P waves ventricular tachycardia (VT; see below). Rather, a cat
and QRS complexes are of an unchanging morphol- with PVCs should have a complete diagnostic evaluation
ogy (exception: QRS complexes may be of 2 mor- (see the section “Evaluating the Arrhythmic Feline
phologies if bundle branch block is intermittent, but Patient,” above) to identify and address the underlying
the PR interval remains constant). cause. For example, since PVCs in cats frequently are
Accelerated idioventricular rhythm (AIVR) is the term associated with underlying cardiomyopathy, then identi-
given to ventricular tachycardia that fails to meet the fying the type of cardiomyopathy and degree of struc-
criterion for rate (i.e., 4 or more PVCs but occurring tural change may lead to treatment for the cardiomyopathy
at a rate >100 but <240 beats/minute). The recogni- that is itself also antiarrhythmic. A very common sce-
tion of AIVR as such, and not VT, is only a matter of nario is the cat with PVCs and asymptomatic hypertro-
correctly identifying the rate, but the clinical signifi- phic cardiomyopathy causing PVCs. The presence of
cance is essential: the rate of AIVR is sufficiently PVCs in such a patient adds support for treating with a
similar to sinus rates that diastolic filling is not altered beta blocker like atenolol, though never at the expense of
from beat to beat, and therefore antiarrhythmic drugs the patient’s tolerance and tractability: the proven
are not warranted to treat AIVR. arrhythmogenicity of endogenous catecholamines like
With motion artifact, the pseudo-PVCs often exceed epinephrine in the cat (Hikasa et al. 1996) signifies that in
the range of expected tracing with PVCs, and evi- difficult-to-treat cats, asymptomatic PVCs may best be left
dence of normal QRS complexes is superimposed on untreated beyond identification and control of concurrent
the pseudo-PVCs; nevertheless, some instances of inciting causes such as hypokalemia or hyperthyroidism.
motion artifact may be extremely deceptive, and The prognosis for PVCs in cats is not known to be
careful analysis, ideally of several simultaneously different from the prognosis of the concurrent or under-
recorded ECG leads, becomes essential for an accurate lying disorder. Syncope or sudden cardiac death, not
diagnosis. gradual deterioration, are the hallmarks of acute arrhyth-
Severe hyperkalemia may produce consistently wide mic decompensation. Sudden cardiac death is a well-
QRS complexes (see below), and analysis of serum recognized occurrence that occurs in a small but