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

Overview

• This study investigated sudden, brief episodes of abnormal heart rhythm resembling paroxysmal atrial fibrillation (pAF) in racehorses during high-speed exercise.
• The researchers aimed to determine whether these pAF-like bursts impact the horses’ speed, stride, and overall racing performance.

Background and Purpose

• Paroxysmal atrial fibrillation (pAF) is a type of irregular heart rhythm that occurs sporadically and can negatively affect performance in horses.
• Previously, diagnosing pAF in racehorses relied mainly on electrocardiograms (ECGs) taken after exercise, potentially missing transient episodes during peak effort.
• This study sought to describe pAF-like bursts detected specifically during exercise and to assess their influence on racehorse speed and performance quality.

Study Design and Methods

• Retrospective case-control study reviewing records from a specialist equine ECG consultation practice focused on exercising horses.
• Cases were horses identified with pAF-like bursts on their ECGs recorded during exercise.
• Controls were horses from the same population with normal ECG recordings during exercise.
• Parameters analyzed included heart rate, occurrence and duration of pAF-like bursts, speed changes during and after these bursts, and heart rate variability measures such as:
• Root Mean Square of Successive Differences (RMSSD)
• Standard Deviation of Normal-to-Normal intervals (SDNN)
• Racing performance following the detected pAF-like episodes was also reviewed where available.

Results

• From 100 ECG consultations, 11 horses exhibited pAF-like bursts detected during peak exercise, while 10 controls showed normal ECGs.
• Key characteristics of pAF-like bursts:
• Occurred only during peak exercise speeds of 60-68 km/h, with elevated heart rates (206-217 bpm).
• Duration of bursts ranged from 10 to 90 seconds.
• ECGs returned to normal before the end of peak exercise, with normal rhythms during warm-up and recovery.
• Speed changes during pAF-like bursts varied:
• Five horses slowed down during the burst.
• Three horses accelerated during the burst.
• Three horses showed no change in speed.
• Immediately after the bursts:
• Five horses slowed down.
• Four maintained their speed.
• Two accelerated.
• Heart rate variability observations:
• RMSSD and SDNN values were elevated in all horses during peak exercise.
• During recovery, RMSSD normalized whereas SDNN remained high for both cases and controls.
• Racing performance post pAF-like bursts was poorer:
• Data from eight horses showed that seven did not race successfully after experiencing pAF-like bursts.

Conclusions and Implications

• pAF-like bursts in racehorses are transient arrhythmias that occur during high-intensity exercise and are not detectable on resting or post-exercise ECGs.
• These bursts potentially affect racing performance, as a majority of horses with such episodes did not race successfully afterward.
• The presence of these bursts during peak exercise highlights the importance of conducting ECG monitoring specifically during peak exertion rather than relying solely on post-exercise assessments.
• Monitoring for such arrhythmias could help trainers and veterinarians better manage racehorse health and performance, potentially leading to improved outcomes and early intervention strategies.