Cardiac arrhythmias during and after treadmill exercise in poorly performing Thoroughbred racehorses

Abstract

The prevalence and severity of cardiac arrhythmias during exercise in athletic horses presented for poor performance is not well described. To describe prevalence and severity of ventricular and supraventricular arrhythmias immediately before, during and immediately after standardised incremental treadmill exercise tests (IET) to fatigue in Thoroughbred horses during investigation of poor performance. The electrocardiograms (ECG) of 88 Thoroughbred racehorses, judged to be free of significant heart disease or arrhythmia at rest, were used. A modified base-apex ECG was recorded throughout an IET to fatigue. Recordings were analysed independently by 2 observers. Twenty-eight horses were diagnosed with dorsal displacement of the soft palate, 25 had varying degrees of soft palate instability and aryepiglottic fold collapse, 8 had other respiratory problems and, in 27 cases, no definitive diagnosis was reached. Fifty-five horses had at least one ventricular (VPD) or supraventricular (SVPD) depolarisation, 23 had only VPDs, 17 had only SVPDs and 15 had both in at least one exercise period. Premature depolarisations were seen predominantly during the first min of recovery from IET. The range of premature beats after exercise was 1-30 VPDs, and 1-9 SVPDs. No significant associations were observed between age, sex, race type, diagnosis, peak heart rate or run time to fatigue during IET and occurrence of either > or =1 premature beat or of more severe arrhythmias (multiple singles [>5] or pairs or paroxysms of premature depolarisations during peak exercise or immediately after exercise). However, a larger sample size would be required to have greater confidence in these associations. Isolated VPDs and SVPDs are frequently detected in poor performing racehorses during IET but their clinical relevance remains to be determined. The guidelines for interpretation and clinical relevance of premature depolarisations observed during and immediately after treadmill exercise tests in poor performing Thoroughbred racehorses deserves further evaluation.