Page 177 - Small Animal Internal Medicine, 6th Edition

CHAPTER 7 Myocardial Diseases of the Dog 149

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FIG 7.4
Paroxysmal ventricular tachycardia at a rate of almost 300 beats/min in a Boxer with
arrhythmogenic right ventricular cardiomyopathy. Note the typical upright (left bundle
branch block–like) appearance of the ventricular ectopic complexes in the caudal leads.
Lead II, 25 mm/sec.

clear. However, Animal Registry of Certified Health guide- Treatment
lines (ARCH, http://www.archcertify.org/) suggest the fol- Antiarrhythmic treatment is indicated for Boxers with clini-
lowing classifications: fewer than 50 single monomorphic cal signs from tachyarrhythmias (i.e., syncope). Asymp-
VPCs/24-hour period is interpreted as normal; between 50 tomatic dogs with Holter findings of greater than 1000 single
and 300 single monomorphic VPCs/24-hour period is VPCs/24 hour, ventricular tachycardia, or close coupling of
equivocal (ARVC cannot be definitively diagnosed or VPCs to the preceding QRS on Holter monitoring are usually
excluded); and greater than 300 VPCs/24-hour period, or given antiarrhythmic therapy as well. However, the best
periods of couplets, triplets, or runs of ventricular tachycar- regimen(s) and when to institute therapy are still not clear.
dia (VT), is abnormal and consistent with a diagnosis of Sotalol and mexiletine have each shown efficacy in reducing
ARVC. Frequent VPCs or episodes of ventricular tachycar- VPC frequency and complexity. Either of these drugs is gen-
dia signal an increased risk for syncope and sudden death. erally considered acceptable first-line therapy for dogs with
The occurrence of ventricular arrhythmias appears to be ARVC, and combination therapy is common. Other drugs
widely distributed throughout the day, and there can be that can be used or added in refractory cases include amio-
enormous variability (up to 85%) in the number of VPCs darone, procainamide, or other β-blockers such as atenolol
between repeated Holter recordings in the same dog. Longi- (see Chapter 4). The addition of an omega-3 fatty acid sup-
tudinal studies of Boxer dogs show that the onset of ven- plement might also reduce VPC frequency. Magnesium
tricular arrhythmias in ARVC is relatively abrupt, with a supplementation (usually in the form of magnesium oxide)
sudden increase in VPC number from <50 VPCs/24 hour is used occasionally.
in the year preceding ARVC diagnosis to an average of The goals of antiarrhythmic drug therapy are to decrease
~1200 VPCs/24 hour at time of diagnosis. The biomarkers number and complexity of VPCs, reduce frequency of
cardiac troponin I and NT-proBNP do not reliably discrimi- syncope, and, ideally, reduce risk of sudden death. Holter
nate between normal and affected dogs, except possibly in monitoring is used to monitor efficacy of antiarrhythmic
dogs with DCM phenotype. therapy; given biologic variability in daily VPC number in
Annual Holter recordings are recommended beginning at dogs with ARVC, a reduction in VPCs of at least 85% gener-
age 2 to 3 years to screen for development and monitor pro- ally is needed to conclude adequate antiarrhythmic effect.
gression of ARVC in Boxer dogs. Once Holter criteria for Unfortunately, sudden death remains possible even with
ARVC are met, annual echocardiography is also recom- apparently good control of arrhythmias.
mended to screen for development of DCM phenotype. Therapy for dogs with DCM phenotype is similar to that
Genetic testing for the striatin mutation is highly recom- described for dogs with idiopathic DCM. Pimobendan and
mended for animals considered for breeding and may also ACEI are typically prescribed once LV systolic dysfunction
provide prognostic information about an individual dog is documented, although benefit of these drugs has not been
(particularly the likelihood of developing the DCM pheno- evaluated in this specific setting. Once CHF occurs, furose-
type). Homozygous-positive dogs should not be bred, and mide and spironolactone are added. Antiarrhythmics are
heterozygous-positive dogs should be bred only to dogs prescribed or continued as previously discussed; caution
negative for the striatin mutation. Dogs meeting Holter diag- should be used with sotalol in the setting of DCM phenotype
nostic criteria for ARVC (greater than 300 VPCs/24 hour or and acute CHF, because the mild β-blocking effects of this
dogs with complex ventricular ectopy) also should not be drug may exacerbate LV systolic dysfunction. Myocardial
bred, regardless of genotype. carnitine deficiency has been documented in some Boxers