The aim of the present study was to determine whether or not voluntary breath-holding time (BHT) changes with the time of the day. BHT with airways closed at end-expiration was measured in six male subjects in the sitting position during the morning (08.00–12.00 hours, on days 1, 6, 7 and 8) and evening (20.00–24.00 hours, on days 2 and 4). BHT increased with the number of days of testing and, at day 8, the morning values averaged 160% of those on day 1. Also, ΔPACO2 [the difference between end-tidal partial pressure of CO2 (PCO2) and alveolar PCO2 (PACO2) at the breaking point] increased in proportion to BHT. Hence the BHT/ΔPACO2 ratio remained nearly constant. Voluntary hyperventilation prolonged BHT and increased ΔPACO2. Conversely, in hypoxia (13% O2 for 1–2 h), BHT and ΔPACO2 were reduced proportionally. During the evening sessions, most of the BHT/ΔPACO2 ratios in normoxia, hypoxia or after hyperventilation were higher than the corresponding morning values, with the group difference reaching statistical significance for the measurements in normoxia and hypoxia. In conclusion, voluntary BHT varies in both duration and its relationship with ΔPACO2 between the morning and evening hours. The results should also imply that, with an interruption of breathing, changes in alveolar and arterial gases are not the same at different times of the day.

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