Exercise ECG study links arrhythmias to sudden death in racehorses

A new Equine Veterinary Journal case series offers some of the clearest ECG evidence yet that exercise-associated sudden death in Thoroughbred racehorses may be preceded by identifiable rhythm abnormalities, particularly atrial fibrillation and malignant ventricular arrhythmias during exercise recovery. The study, led by Cristobal Navas de Solis, analyzed 24 readable exercise ECGs from 11 Thoroughbred racehorses that later experienced EASD, a fatal collapse during exercise or within about one hour afterward. (beva.onlinelibrary.wiley.com)

The question matters because EASD remains one of the most difficult fatal syndromes in equine sports medicine to explain with confidence. Prior work has shown that EASD accounts for roughly 20% to 25% of racing fatalities, and postmortem findings are often inconclusive when a fatal arrhythmia is suspected. Earlier international postmortem work in Thoroughbreds found that a definitive cause of sudden death could be identified in only about half of cases, while reviews and conference coverage over the past decade have repeatedly pointed to arrhythmia as a likely mechanism in many unexplained collapses. (beva.onlinelibrary.wiley.com)

In the new ECG study, researchers assembled cases through direct outreach to racing authorities and trainers in multiple jurisdictions, then had a single observer review available wearable-device ECGs. Four horses were wearing an ECG during the fatal event itself. In three of those horses, atrial fibrillation was present throughout the recording and then deteriorated into malignant arrhythmias and death. In the fourth, the ECG showed couplets and triplets, an R-on-T event, ventricular fibrillation, and death during late recovery. Of the seven horses with ECGs recorded 4 to 462 days before death, one had atrial fibrillation throughout the recording and died nine days later, while late-recovery arrhythmias were identified in five recordings from three additional horses. The authors noted important limitations, including the small sample size and reliance on single-lead ECGs, but still concluded that screening for atrial fibrillation before horses train or race has the potential to reduce EASD incidence. (beva.onlinelibrary.wiley.com)

The paper also fits with a broader shift in the field toward better characterization of which exercise arrhythmias are benign and which may signal risk. The authors note that arrhythmias are commonly observed in horses during high-intensity exercise, with prior studies reporting them in 20% to 84% of horses, which makes interpretation challenging in day-to-day practice. More recent work has focused on automated detection methods, recovery-phase ECG analysis, and race-day physiology to separate incidental ectopy from patterns that may deserve intervention or restriction from competition. (beva.onlinelibrary.wiley.com)

A second new paper, published online June 4, 2026, adds important pathology context. In that Journal of Veterinary Diagnostic Investigation study, investigators compared cardiac histopathology and toxicology in 29 horses with exercise-associated sudden cardiac death and 29 clinically healthy control horses euthanized for catastrophic musculoskeletal injury. They found acute cardiomyocyte injury was significantly more common and more severe in the left atrial appendage of sudden-death cases, but they did not find significant between-group differences in inflammation, fibrosis, or miscellaneous lesions. The authors said those latter findings may represent clinically meaningful lesions in some individuals, but can also be background changes that confuse interpretation. They also reported that elevated hepatic cobalt concentrations were overrepresented in sudden cardiac death horses, while toxicology for other performance-enhancing drugs or xenobiotics was negative or inconclusive. (journals.sagepub.com)

Industry and expert commentary over the past several years has emphasized the same practical problem: many horses that die suddenly do not have a clear structural lesion on necropsy, leaving clinicians and regulators to infer a fatal electrical event. AAEP necropsy guidance recommends detailed cardiac examination and toxicologic testing in sudden-death cases, underscoring how central standardized postmortem workups have become for welfare review and racetrack oversight. Educational commentary from equine cardiology experts has likewise stressed that ventricular arrhythmias in the immediate post-exercise period are of particular concern, even though not every exercise arrhythmia is clinically important. (aaep.org)

Why it matters: For veterinary professionals, this is less a story about a single diagnostic breakthrough than about triangulation. Exercise ECG data, pathology findings, and necropsy protocols are starting to point in the same direction: some EASD cases may be detectable as electrical instability before a fatal event, while structural pathology may be subtle, nonspecific, or absent. That has implications for pre-race and training evaluations, interpretation of poor performance, decisions about return to work after detected atrial fibrillation, and conversations with trainers, regulators, and pet parents following a collapse or sudden death. It also strengthens the case for capturing ECGs during training and recovery, not just at rest. (beva.onlinelibrary.wiley.com)

What to watch: The next phase will likely center on prospective screening studies, clearer risk stratification for late-recovery arrhythmias, and wider use of wearable ECG tools to identify which horses should be held from racing or investigated further with echocardiography, repeat exercise ECGs, or advanced rhythm analysis. (beva.onlinelibrary.wiley.com)