Paroxysmal atrial fibrillation-like bursts in racehorses: do they affect speed and performance? A case control study

Background Paroxysmal atrial fibrillation (pAF) occurs sporadically in horses and has been implicated in poor performance. Diagnosis in large numbers of horses has previously relied on post-exercise ECGs. Hypothesis/Objectives Describe pAF-like bursts detected during exercise and determine their effect on speed, stride, and racing performance. Methods Retrospective case control study, evaluating records of exercising ECG consultations referred to a specialist practice to identify horses with pAF-like bursts. Controls were selected from the same population as racehorses with normal exercising ECGs. Results Eleven cases with pAF-like bursts and 10 controls with normal exercising ECGs were identified, among 100 ECG consultations. The pAF-like bursts occurred in all horses during peak exercise (60-68 km/h; heart rate, 206-217 beats per minute), with normal ECGs during warm-up and recovery. The duration of pAF-like bursts ranged from 10 to 90 s, returning to normal rhythm in all horses before the end of peak exercise. During the pAF-like bursts, 5 horses decelerated, 3 accelerated, and in 3 horses speed was unaffected. Immediately after the pAF-like bursts, 5 horses slowed down, 4 horses did not change their speed and 2 horses accelerated. Root mean square of successive differences (RMSSD) and standard deviation of normal-to-normal intervals (SDNN) during peak exercise were high in all horses. During recovery, RMSSD was normal and SDNN was high in both the pAF-like burst and control horses. Racing performance after the pAF-like burst was available for 8 horses and 7 did not race successfully. Conclusion and clinical importance Paroxysmal atrial fibrillation-like bursts occurred during high-intensity exercise in racehorses and may be missed on post-exercise ECGs. These findings emphasize the value of peak-effort ECGs for detecting transient arrhythmias.