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

Atrial 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.

To quantify the effect of AF on maximum velocity (V_max ), maximum heart rate (HR_max ), heart rate recovery (T₁₀₀ ), hematologic parameters and development of abnormal QRS complexes.

Nine Standardbred trotters.

Two-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 HR_max were assessed using a linear mixed normal model. Abnormal QRS complexes were counted manually on surface ECGs.

Atrial fibrillation resulted in a 1.56 m/sec decrease in V_max (P < .0001). In the AF group, HR_max  ± SD increased from 226 ± 11 bpm at SET1 to 311 ± 27 bpm at SET 2. The AF group had higher HR_max 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.

Atrial 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.