Density-dependent airflow and ventilatory control during exercise

Abstract

The influence of respired gas density on ventilatory control during cycle-ergometer exercise was investigated in six healthy subjects. They underwent constant-load exercise for 10 min both at 50% and 90% of the anaerobic threshold, inhaling air for the first 5 min followed abruptly by 80% helium-20% oxygen (HeO 2) for the remaining 5 min (and vice versa). The HeO 2 breathing elicited no discernible effect on ventilation (V̇ i) or mean alveolar P CO 2 (P a CO 2 ) at rest or at the lower work rate. However, at the higher work rate, HeO 2 breathing resulted in a clear and sustained hyperventilation in all subjects. A compensatory response to the hypocapnia, consequent to the helium-induced hyperventilation, was not evident even though all subjects demonstrated a normal ventilatory responsiveness to inhaled CO 2 while in this condition. These observations suggest that turbulent airflow normally imposes a constraint on the magnitude of the hyperpnea of high-intensity exercise.