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 eupnoeic P_CO2 and the P_CO2 required to produce hypopnoea or apnoea (hypopnoea/apnoeic threshold) in sleeping humans.
• 2
  The effect of hypoxia on eupnoeic end-tidal partial pressure of CO₂ (P_ET,CO2) and hypopnoea/apnoeic threshold P_ET,CO2 was examined in seven healthy, sleeping human subjects. A bilevel pressure support ventilator in a spontaneous mode was used to reduce P_ET,CO2 in small decrements by increasing the inspiratory pressure level by 2 cmH₂O every 2 min until hypopnoea (failure to trigger the ventilator) or apnoea (no breathing effort) occurred. Multiple trials were performed during both normoxia and hypoxia (arterial O₂ saturation, S_a,O2= 80 %) in a random order. The hypopnoea/apnoeic threshold was determined by averaging P_ET,CO2 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,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,CO2 (42.1 ± 1.4 vs. 43.0 ± 1.2 mmHg, P > 0.05) or the apnoeic threshold P_ET,CO2 (41.3 ± 1.2 vs. 41.6 ± 1.0 mmHg, P > 0.05). Thus, the difference in P_ET,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,CO2 and the hypopnoea/apnoeic threshold P_ET,CO2, which could increase the likelihood of ventilatory instability.