Effect of He-O2 breathing on blood gases and ventilation during exercise in normal man.

Temporal changes in ventilation (VI) and arterial blood gases after substitution of helium (He) for nitrogen were studied in normal man during constant load exercises of 14 min duration (30 and 90 W). An abrupt switch of helium for air breathing (protocol 1; 5 subjects), or vice-versa (protocol 2; 4 subjects), was made at the 7th min. Whatever the work loads, the effect of He appeared rapidly: higher values of VI (protocol 1) were observed throughout the 7 min period of He-O2 breathing, but were only significant (p less than or equal to 0.05) during the first minute after substitution at 90 W. Reverse pattern was observed in protocol 2. Helium induced alveolar hyperventilation: sustained and significant hypocapnia (p less than or equal to 0.05) was observed during helium breathing. This effect does not seem to be a consequence of pulmonary gas exchange disturbance, in that concomitant Po2 was normal. It is suggested that He could have evoked a reflex which overrode humoral regulation. Significant increase in ventilatory CO2 responses at rest during He-O2 compared to air breathing in seven subjects (p less than or equal to 0.01) seems to confirm this hypothesis.