Haemodynamic and metabolic responses of laboratory rabbits to near-maximal treadmill exercise

1. The haemodynamic and metabolic responses of four laboratory rabbits to horizontal treadmill exercise at 8, 12, 16 and 20 m/min were studied. The primary haemodynamic variables measured were heart rate, cardiac output and arterial pressure. The metabolic variables measured were arteriovenous oxygen difference, whole-body oxygen consumption and central venous plasma lactate concentration. 2. Only at 8 m/min did the haemodynamic and metabolic variables reach an approximate steady state. This was the greatest speed at which aerobic exercise could be sustained for 4 min in all rabbits. 3. As the speed of the treadmill was increased the rabbits showed signs of exhaustion after progressively shorter periods of exercise. Exhaustion was associated with steep rises in central venous plasma lactate concentration. Exhaustion was also usually associated with secondary rises in systemic vascular resistance and arterial pressure, especially at 20 m/min. 4. The asymptotic (maximal) values for heart rate, cardiac index and O2 consumption were 386 beats/min, 263 ml/kg per min and 21.7 ml/kg per min, respectively. heart do not permit it to run continuously at other than a slow rate. Exhaustion is associated with increasing oxygen debt and, especially at near-maximal rates of exercise, with intense vasoconstriction in as yet unidentified vascular beds.