Intermittent ventricular pre-excitation in a dog with orthodromic reciprocating atrioventricular tachycardia.

Ambulatory ECG is essential to detect intermittent delta waves confirming accessory pathways in dogs with OAVRT.

This case report describes a four-year-old female neutered Labrador Retriever presented for paroxysmal tachycardia, ultimately diagnosed with orthodromic atrioventricular reciprocating tachycardia (OAVRT) with intermittent ventricular pre-excitation. Physical examination revealed a heart rate of 220 beats/min without murmurs, and cardiac troponin I was significantly elevated, suggesting myocardial stress secondary to the tachyarrhythmia. Six-lead electrocardiography demonstrated alternating phases of narrow QRS complex tachycardia with regular RR intervals and a QP'/P'Q ratio of 0.80, consistent with OAVRT, and normal sinus rhythm. Ambulatory (Holter) electrocardiography further characterized the arrhythmia, showing sinus beats intermittently interrupted by wide QRS complexes with shortened PR intervals and delta waves—hallmark features confirming an accessory atrioventricular pathway. The intermittent nature of ventricular pre-excitation is clinically significant, as delta waves may not be present on all recordings, potentially complicating diagnosis. Laddergrams were utilized to illustrate the underlying conduction patterns, serving as a valuable educational tool. OAVRT occurs when an accessory pathway allows reentrant conduction, with antegrade conduction through the AV node and retrograde conduction via the accessory pathway. This case highlights the importance of ambulatory ECG monitoring in detecting intermittent arrhythmias and accessory pathway-mediated tachycardias in dogs, and underscores the diagnostic value of combining standard and ambulatory electrocardiography for accurate characterization of complex arrhythmias.

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