How do I distinguish arrhythmogenic right ventricular cardiomyopathy in Boxers from other cardiac arrhythmias?

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Boxer ARVC is distinguished from other ventricular arrhythmias primarily by the combination of left bundle branch block (LBBB)-morphology ventricular premature complexes (VPCs) on ECG, a VPC count ≥300/24 hours on Holter monitoring, absence of other identifiable cardiac disease, and breed predisposition — with genetic testing for the striatin mutation providing additional confirmation. The hallmark ECG finding is VPCs with LBBB morphology — a positive net deflection in leads II, III, and aVF — consistent with right ventricular origin.18 Multiple VPC morphologies (both LBBB and right bundle branch block patterns) have been reported, and polymorphic ventricular tachycardia can occur.1 The key differential to exclude is idiopathic outflow tract ventricular tachycardia (OTVT), which also produces LBBB-pattern VAs but is nonfamilial and lacks histopathological abnormalities; OTVT is adrenergically mediated via triggered activity (delayed afterdepolarizations), whereas ARVC arises from fibrofatty replacement providing a reentrant substrate.8 Holter monitoring is the cornerstone of antemortem diagnosis. ARVC is defined as ≥300 VPCs/24 hours in the absence of other disease; dogs with ≤20 VPCs/24 hours of right ventricular origin are classified as unaffected.914 VPC frequency is relatively constant throughout the day, though slightly higher during 0800–1200 and 1600–2000 hours, and lowest between midnight and 0400 hours — a pattern that supports full 24-hour recording rather than spot ECG.13 Because day-to-day arrhythmia variability is recognized, a single Holter result may not capture the full arrhythmia burden.3 Exercise testing improves diagnostic classification. Dogs with >100 VPCs/24 hours show greater increases in both VPC count and high-sensitivity cardiac troponin-I (hs-cTnI) after brief moderate-intensity exercise compared with dogs with ≤100 VPCs/24 hours.4 Dogs carrying the striatin mutation show greater post-exercise increases in both VPC count and hs-cTnI than n