Background: Cardiac arrhythmias are common in horses, but their clinical relevance remains controversial. Objectives: To describe prevalence and identify risk factors for arrhythmias that may warrant additional screening in a sedentary mixed‐breed population of horses. Study Design: Prospective cross‐sectional. Methods: Ninety‐four clinically healthy, university‐owned, sedentary horses underwent 24‐h ambulatory electrocardiograms and echocardiograms. Potential risk factors were recorded for all horses. Affected individuals were defined as those with >1 supraventricular premature complex/h or with any ventricular premature complexes for risk factor analysis. Forward stepwise logistic regression was used to identify factors associated with the presence of arrhythmia. The p value <0.15 was considered significant in univariable screening and p ≤ 0.05 was considered significant overall. Results: During recording, 92.6% (87/94) of horses experienced an arrhythmia. Supraventricular premature complexes were present in 86.2% (81/94) of the horses, and ventricular complexes were present in 24.5% (23/94) of the horses. Of the sample, 38.3% (36/94) were considered affected by arrhythmias. Increased heart girth score was associated with detection of arrhythmias (OR 1.06, 95% CI 1–1.12, p = 0.05) and age was retained in the final model as a confounder. Main Limitations: The study sample included a limited range of body condition scores and number of stallions for risk factor analysis. Conclusion: Arrhythmias occurred with high frequency in this group of horses despite no known history or clinical signs of cardiovascular disease. Increased heart girth is a potential risk factor for arrhythmia in the horse.

Mules and donkeys are the 5th largest equine population in the U.S., yet basic information such as heart rate and arrhythmia prevalence in mules has not been described. Analysis of heart rate variability (HRV) provides valuable insight into physiologic and autonomic differences between mules, horses, and donkeys. We aimed to pilot the evaluation of heart rate and variability in a healthy population of mules using a 24 h electrocardiogram. Twenty-four-hour ambulatory electrocardiogram (aECG) recordings were collected from an athletic population of seven privately owned healthy mules in Northern California. Vision 5 Holter Analysis Software was utilized for ECG review and analysis. An average heart rate of 32–42 bpm, with a mean of 36 ± 3 bpm for 23.8 ± 0.2 h, was recorded. Minimum heart rate was 16–24 bpm, and maximum was 70–156 bpm. Individual mules had supraventricular complexes, sinoatrial heart block, and second-degree atrioventricular block during the observation period. Electrocardiographic evaluation of mules over 24 h found heart rate (HR) to be similar to horses and lower than in donkeys. Arrhythmias were consistent with the range of cardiac rhythms observed in horses and more frequent than reported in donkeys. Further investigation with a larger sample size of mules is warranted to determine if reference values for horses should continue to be used to assess mules and to investigate the prevalence of arrhythmias and their clinical relevance in the mule.