Prevalence of cardiac arrhythmias and R-R interval variation in healthy Thoroughbred horses during official Chuckwagon races and recovery.

The prevalence and severity of cardiac arrhythmias in healthy racehorses undergoing competition is not well defined. The aim was to characterize arrhythmias in Thoroughbreds participating in official Chuckwagon races and to determine normal beat-to-beat (R-R) variability during supramaximal exercise. Electrocardiograph (ECG) recordings were obtained during pre-race, race, and active-recovery from 82 clinically healthy Thoroughbreds. ECG recordings were analyzed for arrhythmias and mean percent R-R deviation. Plasma lactate and high-sensitivity troponin (hs-cTnT) were also measured. Fifty-two ECGs were included in the analysis. Arrhythmias were seen in 48/52 horses (92%) and were predominantly isolated events. No complex rhythms were observed. During the race, 92% of horses had arrhythmias (81% supraventricular premature complex [SVPC]; 33% ventricular premature complex [VPC]). Eleven percent of racing arrhythmias were VPCs (all singlets except for two couplets). During active-recovery, 58% of horses had arrhythmias (56% SVPC; 15% VPCs): Three horses had VPC couplets and one horse had a VPC triplet. All plasma hs-cTnT were within normal limits. The measured lactate was 28.5 ± 4.5 mmol/L, confirming supramaximal exercise. R-R variation ranged between -9.5 to +18.8% during pre-race (mean heart rate [HR], 155 ± 22 beats per min [bpm]), -27.8 to +45.3% during racing (mean HR, 200 ± 9 bpm) and -16.4 to +40.1% during active-recovery (mean HR, 165 ± 14 bpm). Maximal and 1st percentile R-R shortening and lengthening were significantly greater at race than pre-race and active-recovery (P < 0.0001). Racing and active-recovery maximal R-R lengthening were significantly greater than pre-race (P = 0.0003). Supraventricular premature complexes and VPCs are prevalent in healthy horses undergoing Chuckwagon racing. R-R variation is greater during racing than has previously been described.