Effect of induced chronic atrial fibrillation on exercise performance in Standardbred trotters

BackgroundAtrial fibrillation (AF) is the most common arrhythmia affecting performance in horses. However, no previous studies have quantified the performance reduction in horses suffering from AF. ObjectivesTo quantify the effect of AF on maximum velocity (V-max), maximum heart rate (HRmax), heart rate recovery (T-100), hematologic parameters and development of abnormal QRS complexes. AnimalsNine Standardbred trotters. MethodsTwo-arm controlled trial. Six horses had AF induced by means of a pacemaker and 3 served as sham-operated controls. All horses were subjected to an exercise test to fatigue before (SET1) and after (SET2) 2 months of AF or sham. The V-max and HRmax were assessed using a linear mixed normal model. Abnormal QRS complexes were counted manually on surface ECGs. ResultsAtrial fibrillation resulted in a 1.56 m/sec decrease in V-max (P<.0001). In the AF group, HRmaxSD increased from 226 +/- 11 bpm at SET1 to 311 +/- 27 bpm at SET 2. The AF group had higher HRmax at SET2 compared with controls (P<.0001), whereas no difference between the control and AF groups was observed at SET1 (P=.96). Several episodes of wide complex tachycardia were observed during exercise in 3 of the AF horses during SET2. Conclusions and Clinical ImportanceAtrial fibrillation resulted in a significant reduction in performance, an increase in HR and development of abnormal QRS complexes during exercise, which may be a risk factor for collapse or sudden cardiac death.