Reading the Rhythm: How Cycle Length Variation Helps Diagnose Equine Arrhythmias

Send us Fan Mail One strange beat on a resting horse ECG can spark a surprisingly high-stakes question: are we seeing a normal sinus arrhythmia, or an atrial premature complex that changes how we think about risk and follow-up? We sit down with Drs. Kath Mitchell and Beth Williams to make that decision point less subjective, less stressful, and more repeatable for anyone interpreting equine ECGs. We dig into why the classic RR interval approach sometimes falls short, especially when subtle changes in AV node conduction and PR interval can blur the true prematurity of an atrial beat. Kath and Beth explain why PP intervals matter because they track atrial activity directly, and they share how focusing on timing can scale better than morphology when you move from a resting ECG to an exercise ECG. From coupling intervals to the “return cycle” and the post-ectopic pause, we unpack what to measure and what those measurements mean physiologically. We also talk about the study results, including how return cycle length variation cleanly separated rhythms in clear examples, why that produced near-perfect ROC performance, and what needs to happen next in the gray zone cases clinicians argue about every day. Along the way, we explore an unexpected finding: ventricular premature beats may influence several beats after the pause, raising new mechanistic questions about how the sinus node responds. If you want a practical, clinic-ready way to add confidence to your equine cardiology calls, this conversation delivers. Subscribe to Veterinary Vertex, share this with a colleague who reads horse ECGs, and leave a rating and review on Apple Podcasts. JAVMA article: https://doi.org/10.2460/javma.26.04.0253 INTERESTED IN SUBMITTING YOUR MANUSCRIPT TO JAVMA ®  OR AJVR ® ? JAVMA ® : https://avma.org/JAVMAAuthors AJVR ® : https://avma.org/AJVRAuthors FOLLOW US: JAVMA ® : Facebook: Journal of the American Veterinary Medical Association - JAVMA | Facebook Instagram: JAVMA (@avma_javma) • Instagram photos and videos Twitter: JAVMA (@AVMAJAVMA) / Twitter   AJVR ® :  Facebook: American Journal of Veterinary Research - AJVR | Facebook Instagram: AJVR (@ajvroa) • Instagram photos and videos Twitter: AJVR (@AJVROA) / Twitter JAVMA ®  and AJVR ®  LinkedIn: https://linkedin.com/company/avma-journals