PULMONARY VASCULAR PRESSURES OF THOROUGHBREDS INCREASE RAPIDLY AND TO A HIGHER-LEVEL WITH RAPID ONSET OF HIGH-INTENSITY EXERCISE THAN SLOW ONSET

Previous studies of pulmonary vascular pressures have utilised gradual incremental step exercise protocols, but in competitive racing at the track, horses perform rapid acceleration high-intensity exercise. The rate of rise in pulmonary vascular pressures under conditions of quick onset high-intensity exercise is unknown. Catheter mounted manometers, whose in vivo signals were matched with pressure signals obtained via transducers connected to fluid-filled lumens from same cardiovascular sites, were used to compare right heart and pulmonary vascular pressures in 8 healthy Thoroughbreds performing 2 separate exercise protocols on a high speed treadmill (gradually incremental vs. rapid acceleration exercise protocol where the belt speed was raised from 8 m/s to 15 m/s in 8 s). Heart rate, right atrial and pulmonary vascular pressures at rest were similar for the 2 protocols. Rapid acceleration of horses from 8 to 15 m/s was attended by an equally rapid escalation in the right heart and pulmonary vascular pressures such that these pressures reached their zenith as belt speed approached 15 m/s. Although exercise at 15 m/s resulted in similar heart rate in the 2 protocols, the mean +/- s.e. values of mean right atrial pressure, mean pulmonary artery pressure, mean pulmonary artery wedge pressure and mean pulmonary capillary pressure (91.5 +/- 3.9 mmHg) in the rapid acceleration exercise were significantly (P<0.05) higher than respective values at 15 m/s in the gradual incremental step exercise protocol. It is concluded that pulmonary capillary blood pressure of Thoroughbreds rapidly accelerating to high-intensity exercise increases just as rapidly as their speed and that the values achieved are higher than in incremental step exercise protocol; thereby playing a potential role in initiating stress failure of pulmonary capillaries and exercise-induced pulmonary haemorrhage very early in a race.