Cardiac monitoring during exercise tests in the horse. 3. Changes in the electrocardiogram during and after exercise.

Changes that occur in the equine ECG during and after exercise have been described and compared with resting ECG's obtained from the same horses. When the speed of work equals or is greater than "three-quarter pace" (i.e. "even time" or 200 metres in 15 seconds) the high heart rates developed cause the loss of ECG waveforms that are readily discernible at rest. Although differences in the waveforms between horses with normal and abnormal resting ECG's have been observed, the more definitive useful information is the fact that abnormal horses showed significantly higher heart rates than normal horses at a particular speed of work. When this occurs in combination with a resting ECG classified as borderline or abnormal and the exercise ECG shows positive, low amplitude, broad rounded T waves or transient bursts of sine-waves suggestive of ventricular flutter, racing performance is likely to be impaired. Although more work needs to be done and improvements in technology are necessary, the exercise ECG has potential for improving understanding of physical fitness and the significance of resting cardiac abnormalities.