Ventriculophasic response in a horse with atrioventricular block

Ventriculophasic response (VR) in atrioventricular nodal (AVN) block is characterized by a shortening of the sinoatrial cycle length when a QRS complex is interposed between two P waves. This has been well documented in humans and dogs with AVN blocks but not in horses. Herein, we describe a horse with second-degree AVN blocks and constant PR intervals that exhibited 5:3 conduction and suspected VR at rest but less consistently after submaximal exercise. Proposed VR mechanisms include (1) inhibition of the Bezold-Jarisch reflex from reduced end-systolic volume during ventricular contraction; (2) vagal inhibition from increased intra-atrial pressure (Bainbridge reflex); and (3) accelerated sinus node discharge from changes in sinus nodal blood supply during ventricular contraction. This case exhibited some electrocardiogram characteristics of the first two mechanisms to explain the PP interval alteration and suspected VR. Horses often exhibit physiologic AVN blocks, and clinicians should consider VR when interpreting equine electrocardiograms.