Discussion

Atrial fibrillation (AF) is the most important dysrhythmia affecting performance in horses. Aims: To investigate ventricular response in AF horses during exercise. Methods: A modified base-apex ECG (Televet100®) was recorded from 47 horses diagnosed with AF. All horses performed a standardised lungeing exercise test consisting of 5 min walk, 10 min trot, 4 min canter and 1 min gallop. ECGs were digitally stored and analysed by an experienced observer (T.V.). Results: Individual average heart rate during walk ranged from 42 to 175 beats/min, during trot from 89 to 207 beats/min, during canter from 141 to 269 beats/min, and during gallop from 181 to 311 beats/min.

Individual beat-to-beat maximal heart rate ranged from 224 to 492 beats/min. QRS broadening was present in 87% of the horses: at rest (34%), during exercise (88%), and during cool down (5%). In 34% of the horses broad QRS complexes with Ron-T morphology were present. Conclusions and practical significance: In exercising AF horses, heart rate can raise high above the normal maximal heart rate and QRS broadening is often found. QRS broadening may originate from ventricular ectopy or from aberrant intra-ventricular conduction, for example due to bundle branch block. In 34% of the AF horses QRS broadening with R-on-T morphology was found, which is a well-known trigger for development of ventricular tachycardia and ventricular fibrillation. QRS broadening and especially R-on-T are considered risk factors that might explain signs of weakness, collapse or sudden death that have been reported in AF horses.