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

Peak-effort ECGs detect pAF-like bursts missed by post-exercise ECGs; associated with poor racing performance.

This case-control study investigated paroxysmal atrial fibrillation-like (pAF-like) bursts in racehorses during exercise and their effects on performance. Among 100 ECG consultations, 11 cases with pAF-like bursts were identified and compared to 10 control horses with normal exercising ECGs. All pAF-like bursts occurred during peak exercise (60-68 km/h, heart rate 206-217 bpm), with normal rhythms during warm-up and recovery. Burst duration ranged from 10-90 seconds, self-terminating before peak exercise ended. Speed responses varied during bursts (5 decelerated, 3 accelerated, 3 unchanged) and immediately after (5 slowed, 4 unchanged, 2 accelerated). Heart rate variability parameters (RMSSD and SDNN) were elevated during peak exercise in all horses but normalized during recovery. Notably, 7 of 8 horses with available post-burst racing records did not race successfully. The findings highlight that pAF-like bursts are transient arrhythmias occurring specifically at high intensity, potentially missed on conventional post-exercise ECGs. The study emphasizes the clinical importance of peak-effort ECG monitoring for detecting these arrhythmias and their potential impact on equine athletic performance.

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