Afferent and cardiodynamic drives in the early phase of exercise hyperpnea in humans

The ventilatory and cardiac responses to voluntary and passive exercise were studied in 20 healthy subjects. These responses to passive leg exercise were also studied in 23 patients with spinal cord transection at the level of T5-T12. In the normal subjects, minute ventilation (VE) increased abruptly from the first breath after the onset of the two types of exercise. In contrast, cardiac output (Q) increased gradually in voluntary exercise, exhibiting significant augmentation from the fifth breath. Q changed insignificantly in passive exercise. In the patients with spinal cord transection, neither VE nor Q changed with passive exercise. These results suggest that ventilatory responses at the onset of mild exercise are related to drives from the moving limbs. We could not detect any evidence to support cardiodynamic hyperpnea at the onset of exercise.