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Effect of hypoxia on the hypopnoeic and apnoeic threshold for CO 2 in sleeping humans
Author(s) -
Xie Ailiang,
Skatrud James B.,
Dempsey Jerome A.
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.00269.x
Subject(s) - hypoxia (environmental) , apnea , anesthesia , cardiology , medicine , blood pressure , oxygen , chemistry , organic chemistry
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 CO 2 . 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 CO 2 ( 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 cmH 2 O 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 O 2 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.