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Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 233

devastating proportion of feline cardiomyopathy ing cause eliminated, or monthly, or even more often if
patients, either spontaneously or in association with a overt clinical signs such as syncope are present). The
stressful episode such as general anesthesia. Stratification purpose of follow-up is to update the history (has the
of risk of life-threatening episodes such as these is poorly client observed a change in clinical signs?) and physical
understood in cats at the present time. The presence of examination; perform an ECG to identify new arrhyth-
clinical signs (dyspnea, lethargy, hiding, or other signs mias if present; and perform diagnostic tests (blood or
of congestive heart failure; syncope; pulselessness or radiographic examinations) as indicated by history of
other signs of aortic thromboembolism) and the degree clinical signs, current medications, etc. Subsequent
of ventricular distortion (hypertrophy or other change) rechecks similarly evaluate these parameters at a
probably are negative general prognostic indicators with minimum and include follow-up telemetry or Holter/
PVCs, markers that may occur more commonly in event monitoring if done previously.
arrhythmic cats that have a somewhat worse long-term
prognosis. Long-term clinical studies are needed to Accelerated Idioventricular Rhythm
confirm these suspicions, and currently a markedly A rhythm that consists of 4 or more PVCs occurring
worse prognosis (and ensuing decision not to pursue consecutively but where the rate is less than the defined
treatment or to euthanize) for PVCs cannot be sup- rate for ventricular tachycardia (VT; 240 beats/minute)
ported by degree of concurrent echocardiographic is called accelerated idioventricular rhythm (AIVR)
change, for example: there are likely too many other (see Figure 18.15). Initially termed “slow ventricular Arrhythmias
variables to pin a prognosis on these parameters tachycardia”—a contradiction in terms—AIVR occurs
alone and a great deal of overlap exists between cats for the same reasons as PVCs and VT. The major differ-
that do well despite these parameters and cats that do ence is that the slower rate of AIVR provides more time
poorly. for diastolic ventricular filling, and therefore is a more
A likely prelude to syncope or sudden cardiac death hemodynamically effective rhythm than VT. AIVR and
is PVCs, and one of the greatest misconceptions in car- VT likely represent different parts of one continuum, and
diology is that proactive control or elimination of ven- the lower end of this continuum is approximately 120
tricular arrhythmias with antiarrhythmic medications beats/minute in the cat; ventricular rhythms lower than
could reduce the risk of sudden death. Indeed, as in this rate are in fact a ventricular escape rhythm, beneficial
human cardiology, “rendering treatment to suppress a expressions of spontaneous ventricular activity that save
risk factor [asymptomatic PVCs] because of the tempta- the heart from catastrophic bradycardia or asystole (see
tion to equate causality with association between the the discussion of third-degree AV block, below). AIVR is
risk factor and a putative clinical outcome has never generally well-tolerated, and treatment is therefore
been more misguided than with nonsustained ventricu- directed at the underlying cause; antiarrhythmic medica-
lar tachycardia (Marinchak et al. 1997).” Thus, in feline tions are not considered unless management of the
heart disease, the prognosis associated with PVCs is not underlying cause is ineffective at abolishing the arrhyth-
conclusively worse than that of normal sinus rhythm mia and the rate increases to the point that the criteria for
given that both can occur with cardiomyopathy, and no VT are met.
study has evaluated the benefit or harm associated with
treating ventricular arrhythmias with antiarrhythmic Ventricular Tachycardia
drugs in asymptomatic cats. Certainly, treating inciden- As mentioned earlier, ventricular tachycardia (VT) is the
tally discovered PVCs in asymptomatic humans has occurrence of 4 or more PVCs consecutively at a rate of
been shown conclusively to be harmful with several anti- 240 beats/minute or greater (Figure 18.17). Accordingly,
arrhythmic drugs (causing worsening arrhythmias or a major difference between PVCs and VT is that PVCs,
“proarrhythmia”) (Echt et al. 1991; Cardiac Arrythmia since they occur as only 1 or a few at a time, have little
Suppression Investigators 1992)). In cats, the prognosis or no negative hemodynamic impact. Organ perfusion
of the concurrent cardiomyopathy is likely the dominant and oxygen delivery are not affected to a clinically mean-
factor rather than the prognosis of the arrhythmia. Cats ingful extent by individual PVCs. Conversely, VT may
with PVCs but no associated signs of syncope or collapse occur for any length of time and at any rate, including
should be treated with medications that reverse or rapid VT (>280 beats/minute) wherein diastolic filling
correct triggers that exist (e.g., concurrent hyperthy- of the ventricles is compromised and a reduced, possibly
roidism, hypokalemia, or other), and followed up rou- inadequate, cardiac output occurs.
tinely: typically within 1–2 weeks of diagnosis at first, The causes, mechanisms, associated conditions, sig-
and then as dictated by clinical signs (e.g., every 3–6 nalment, differential diagnoses, and diagnostic evalua-
months if the arrhythmia is asymptomatic and underly- tion of patients with VT are the same as for PVCs (above)