GAIT AND RESPIRATION IN STANDARD-BRED HORSES WHEN PACING AND GALLOPING

The relationship between gait and the respiratory response to exercise was examined in five standardbred racehorses which exercised on a tre admill at a pace and a gallop. After an initial warm-up, respiratory r ate and stride frequency were measured after one and two minutes of tr eadmill exercise at 80 per cent of maximal oxygen consumption VO2max), after one minute at 100 per cent VO2max and after two minutes at 100 per cent VO2max (galloping horses only). Exercise at 100 per cent VO2m ax continued until the horses showed signs of fatigue. Arterial blood was collected during exercise and when they were fatigued for the meas urement of oxygen and carbon dioxide tensions, haemoglobin saturation and pH. Venous blood temperature was also recorded. The mean (SE) time to signs of fatigue was significantly (P<0.05) less in the pacing hor ses (7.2 [0.4] minutes) than in the galloping horses (8.0 [0.4] minute s). The mean (SE) resting PCO2 was 47.7 (1.9) torr. During the pacing and galloping exercises at 80 per cent and 100 per cent VO2max the PCO 2 remained in the range of 41.1 to 66.8 torr, despite concurrent hyper thermia and acidosis. The PCO2 during exercise was not significantly d ependent on gait or exercise intensity. The PO2 was significantly high er in pacing horses during exercise at 80 per cent VO2max (111 [7] vs 96 [6] torr). The mean (SE) arterial blood pH decreased from 7.428 (0. 025) during pacing at 5 m sec(-1) to 7.250 (0.042) when the horses wer e fatigued. During the gallop test, the pH decreased from 7.451 (0.047 ) to 7.263 (0.029) after two minutes of exercise at 100 per cent VO2ma x. The horses' blood temperature was significantly lower when canterin g (39.3 +/- 0.3 degrees C) than when pacing (41.0 +/- 0.6 degrees C) a t 80 per cent VO2max (P<0.05), but there was no effect of gait on temp erature at 100 per cent VO2max. There was a close 1:1 ratio between st ride frequency and respiratory rate during the cantering and galloping exercises at 80 per cent VO2max and 100 per cent VO2max, whereas when the horses were pacing this ratio averaged 1.3 (range 1 to 1.5). The mean stride frequencies after one minute of exercise at 100 per cent V O2max while pacing and galloping were 130 (3.4) minute(-1) and 137 (4. 2) minute(-1), respectively, and the stride frequency was not dependen t on gait. Stride frequency and respiratory rate are not closely coupl ed during pacing. Eucapnea or hypercapnea occurs during strenuous exer cise in the racehorse exercising at the gallop or pace, and hypoxaemia does not occur in ail racehorses during intense exercise.