Long-term facilitation of breathing is absent after episodes of hypercapnic hypoxia in awake humans.

Despite the failure by many previous investigators to demonstrate a long-term facilitation of breathing following episodes of hypoxia in awake humans, we attempted to produce it using a pattern of hypercapnic hypoxic episodes similar to that experienced by obstructive sleep apnoea patients, reasoning that if long-term facilitation was relevant to these patients then it is appropriate to test the effectiveness of such episodes. Ten subjects drawn from the University student population were instrumented to measure ventilation, heart rate and end-tidal PCO2 and PO2 breath-by-breath while seated in a comfortable reclining chair. After an initial resting period breathing room air they experienced fifteen, 30-s episodes breathing 6% O2 and 5% CO2 separated by 90 s of breathing air. We examined the measured variables for an hour after the episodes but found no trends toward an increase in ventilation or decrease in end-tidal PCO2 that would indicate the presence of a long-term facilitation. We therefore concluded that long-term facilitation of ventilation was not demonstrated in awake humans using this pattern of stimuli.