Effect of hypoxia on the hypopnoeic and apnoeic threshold for CO2 in sleeping humans

1. Rhythmic breathing during sleep requires that P-CO2 be maintained above a sensitive hypocapnic apnoeic threshold. Hypoxia causes periodic breathing during sleep that can be prevented or eliminated with supplemental CO2. The purpose of this study was to determine the effect of hypoxia in changing the difference between the eupnocic P-CO2 and the P-CO2 required to produce hypopnoea or apnoea (hywpopnoea/apnoeic threshold) in sleeping humans. 2. The effect of hypoxia on eupnoeic end-tidal partial pressure of CO2 (P-ET,P-CO2) and hypopnoea/apnoeic threshold P-ET,P-CO2 was examined in seven healthy, sleeping human subjects. A bilevel pressure support ventilator in a spontaneous mode was used to reduce P-ET,P-CO2 in small decrements by increasing the inspiratory pressure level by 2 cmH(2)O every 2 min until hypopnoea (failure to trigger the ventilator) or apnoea (no breathing effort) occurred. Multiple trials were performed (luring both normoxia and hypoxia (arterial O-2 saturation, S-a,S-O2 = 80%) in a random order, The hypopuoea/apnoeic threshold was determined by averaging P-ET,P-CO2('01-1 Of the last three breaths prior to each hypopnoea or apnoea. 3. Hypopnoeas and apnoeas were induced in all subjects during both normoxia and hypoxia. Hypoxia reduced the eupnoeic P-ET,P-CO2 compared to normoxia (42.4 +/- 1.3 vs, 45.0 +/- 1.1 mmHg, P < 0.001). However, no change was observed in either the hypopnoeic threshold P-ET,P-CO2 (42.1 +/- 1.4 vs. 43.0 +/- 1.2 mmHg, P > 0.05) or the apnoeic threshold P-ET,P-CO2 (41.3 +/- 1.2 vs. 41.6 +/- 1.0 mmHg, P > 0.05). Thus, the difference in P-ET,P-CO2 between the eupnoeic and threshold levels was much smaller during hypoxia than during, normoxia (-0.2 +/- 0.2 vs. -2.0 +/- 0.3 mmHg, P < 0.01 for the hypopnoea threshold and -1.1 +/- 0.2 vs. -3.4 +/- 0.3 mmHg, P < 0.01 for the apnoeic threshold). We concluded that hypoxia causes a narrowing of the difference between the baseline P-ET,P-CO2 and the hypopnoea/apnoeic threshold P-ET,P-CO2, which could increase the likelihood of ventilatory instability.