Page 235 - Feline Cardiology

240 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

information (Fox and Harpster 1999; Côté 2010; Kellum maker implantation in cats where the arrhythmia is
and Stepien 2006; Côté and Harpster 2009; Harvey et al. responsible for overt clinical signs, but this intuitive
2005; Ferasin et al. 2002; Fox et al. 1991; Kaneshige et al. impression remains unproven in the cat. Interestingly,
2006; Penning et al. 2009; Côté et al. 1999; Harpster sudden death is not documented in cats with third-
1987, 1992). In summary, cats with third-degree AV degree AV block, perhaps because cats commonly express
block show no clear gender tendency (18 male, 12 a more rapid ventricular escape rate (e.g., 100–140 beats/
female), a varied breed distribution (20 mixed breed, 4 minute) compared to other species, because overt clini-
Siamese, 3 Burmese, 1 each Himalayan, Russian Blue, cal signs become sufficiently dramatic to elicit veterinary
Persian), and an age range of 4–19 years. Chief com- attention while a ventricular escape mechanism remains
plaints are stated for 34 cats; 7 (21%) had chief com- functional, and because retrospective reports often do
plaints suggestive of cardiac arrhythmia (e.g., syncope, not provide follow-up information to the time of death.
collapse), 19 (56%) were presented because of nonspe- The causes of AV block are diverse, but in cats an
cific signs (e.g., lethargy, inappetence, dyspnea), and in association with cardiomyopathy is common (Kaneshige
8 (23%), there was no complaint and third-degree AV et al. 2006; Liu et al. 1975). First-degree AV and Mobitz
block was an incidental finding. Heart rate on physical type I second-degree AV blocks often are functional
exam ranged from 70–160 beats/minute, and ECG iden- (high vagal tone in healthy individuals, negative dromo-
Arrhythmias rates between 55 and 160 beats/minute. There was no 2-stimulating anesthetics) and thus are normal physio-
tropic effects of digitalis, antiarrhythmics, or alpha
tified third-degree AV block with ventricular escape
logic variants or resolve with drug discontinuation. Less
appreciable association between ventricular rate and
presenting complaint; third-degree AV block was an
nodal lesions may be present as a cause of first-degree
incidental finding in cats with ventricular escape rates commonly, cardiac disease with atrial dilation and AV
ranging from 80–140 beats/minute (Côté and Harpster or Mobitz type I second-degree AV block. Mobitz type
2009; Harpster 1992), for example, and cats with conges- II second-degree AV block and third-degree AV block
tive heart failure did not have a significantly different are sometimes functional (hyperkalemia, digitalis toxic-
heart rate than cats without congestive heart failure ity, alpha 2-stimulating anesthetics) but are more com-
(Kellum and Stepien 2006). The QRS complexes of the monly associated with a structural lesion: inflammatory
ventricular escape rhythm were wide in 5 cats (19%) and (endocarditis, Lyme myocarditis, traumatic myocardi-
narrow in 21 (81%). When echocardiographic or post- tis) or degenerative (physical disruption of the AV node
mortem results were reported (n = 25), many cats had arising from cardiomyopathy with or without degenera-
underlying structural heart disease (HCM/DCM/RCM/ tion and fibrosis, as indicated above). Treatment is there-
UCM [n = 7; 28%], mitral valve disease [n = 4; 16%], fore aimed at the underlying cause when possible. In
excessive LV moderator bands/false tendons [n = 2; 8%], clinically overt, advanced, Mobitz type II second-degree
ARVC [n = 2; 8%]). The association between cardiomy- AV blocks or third-degree AV blocks, response to para-
opathy and AV block is consistent with the finding that sympatholytic or sympathomimetic drugs tends to be
cats with third-degree AV block and HCM commonly fairly disappointing and potentially dangerous.
have fibrosis of the branching portion of the AV bundle Pacemaker implantation is a better choice.
and the upper portion of the left bundle branch, exten- Although spontaneous conversion from third-degree
sive fibrosis of the central fibrous body, and endocardial AV block to normal sinus rhythm has been documented
and myocardial fibrosis in the upper border of the ven- in the cat (Kellum and Stepien 2006), in most cases the
tricular septum (Kaneshige et al. 2006). However, in 10 block is permanent, and treatment is necessary if clinical
cats (40%), echocardiographic findings were described signs attributable to the bradycardia are present. Medical
as normal or consistent with mild chamber dilation therapy is usually unrewarding for third-degree AV
expected due to bradycardia-induced, neurohormonally block but may be attempted in stable patients (see dis-
mediated sodium and water retention. cussion of second-degree AV block, above). Pacemaker
Pacemaker implantation was performed infrequently implantation is the treatment of choice for third-degree
(11/36 cases; 31%). Cats with untreated third-degree AV AV block (Figure 18.21). It is performed less frequently
block had a wide range of survival times (median: 386 than in dogs, and with some important differences. At
days in 1 study; range: 1 day–5.7 years postdiagnosis), least 12 cases of therapeutic feline pacemaker implanta-
and third-degree AV block does not automatically confer tion have been described (Kellum and Stepien 2006;
a poor prognosis in the cat. Congestive heart failure or Ferasin et al. 2002; Fox et al. 1991; Kaneshige et al. 2006;
indeed structural heart disease did not confer a worse Penning et al. 2009; Stamoulis et al. 1992). For cats, a
prognosis in the largest of these case series (Kellum and human pediatric pacemaker generator (the “can”) is the
Stepien 2006). Survival is likely increased with pace- implant of choice. It is generally placed in the abdomen