Cardiovascular and respiratory responses to submaximal exercise training in the thoroughbred horse

Cardiovascular and respiratory responses to submaximal exercise training were investigated in 6 thoroughbred racehorses. Oxygen uptake, heart rate (HR) and arteriovenous oxygen content difference were measured during incremental treadmill exercise tests, before and after 7 weeks of treadmill training. Cardiac output during exercise was calculated by the direct Fick technique. Maximal oxygen uptake (VO2max) was increased by 23% after training, from 129.7 ml/kg/min to 160.0 ml/kg/min. The treadmill speed at which VO2max was attained increased by 19%. The increased aerobic power after training was associated with an increase in maximal cardiac output and stroke volume, a decrease in arteriovenous oxygen difference and no change in HR. There was no change in pulmonary ventilation during exercise at VO2max. Mean mixed venous oxygen content (CvO2) at VO2max before training was 2.8 +/- 1.0 ml/100 ml blood (mean +/- SE). After training the value was 8.6 +/- 1.4 ml/100 ml blood. It is concluded that the increase in VO2max after training in the horse is dependant on augmented blood flow, and is not dependant on either increased arterial oxygen content or arteriovenous oxygen content difference. Cardiac capacity to pump blood is therefore of primary importance as a determinant of increases in VO2max due to training in the horse.