VENTILATORY CONTROL DURING EXERCISE IN CALVES WITH ARTIFICIAL HEARTS

To determine the role of cardiac reflexes in mediating exercise hyperpnea, we investigated ventilatory responses to treadmill in seven calves with artificial hearts and seven controls. In both groups, the ventilatory responses were adequate for the metabolic demands of the exercise; this resulted in regulation of arterial PCO2 and pH despite the absence of cardiac output increase in the implanted group. In this group, there was a small but significant reduction of arterial PO2 by 4 ± 3 Torr and a rise of blood lactate by 1.1 ± 1 mmol/L. When cardiac output was experimentally increased in the implanted calves to a level commensurate with that spontaneously occurring in the control calves, ventilation was not affected. However, experimental reductions of cardiac output led to an immediate augmentation of exercise hyperpnea by 4.56 ± 4.3 l/min and a further significant lactate increase of 1.2 ± 1.22 mmol/l that was associated with a significant decrease in the exercise O2 consumption (0.32 ± 0.13 l/min). These observations indicate that neither cardiac nor hemodynamic effects of increased cardiac output constitute an obligatory cause of exercise hyperpnea in the calf.