Post-hyperventilation apnoea in conscious humans.

1. In nine normal subjects, analysis was performed of the number, length and location of apnoeic pauses during 20 min of recovery following voluntary overbreathing (VHV). Four different rates of recovery of end-tidal PCO2 (PET,CO2), studied in randomized order, were induced by overbreathing to 15 or 25 mmHg, each for 3 or 6 min. Subjects breathed mildly hyperoxic gas mixtures (inspired PO2 approximately 250 mmHg) to and fro into an open circuit via a mouthpiece and pneumotachograph. 2. Apnoeic pauses rarely occurred immediately after the end of VHV but gradually increased in number and length. When averaged across all subjects and protocols, the largest pauses occurred 2.0 +/- 0.3 min (S.D.; range 1.6-2.4 min) after the end of VHV. Based on a definition of apnoea as expiratory time greater than 6 s, apnoeas occurred between mean times of 0.8 and 5.6 min after the end of VHV, the end of this period being associated with a mean PET,CO2 value of 36.4 mmHg, which was below the initial mean resting value of 39.8 mmHg. 3. Within this apnoeic period, 80% of experiments produced apnoeas of less than 10 s duration, 61% of between 10 and 20 s duration and 42% of between 20 and 30 s duration. Only one out of nine subjects consistently failed to show apnoeas. 4. The range of lengths of individual apnoeas and the number per minute were independent of the length and level of VHV and were not significantly different between the four protocols. 5. The number and length of apnoeas did not change in repeated runs in each subject. We were not able to confirm previous reports that apnoeas occurred more frequently in subjects familiar with the experiment. 6. These results reconciled previous studies showing either apnoea or hyperpnoea following voluntary overbreathing in conscious humans. They showed an initial period of heightened breathing lasting about a minute with few apnoeas, consistent with 'after-discharge'. Beyond that, apnoeas occurred as an 'all-or-nothing' phenomenon as long as PET,CO2 was on average less than 3.4 mmHg below resting PET,CO2. The occurrence and length of apnoeas was consistent in individual subjects with no evidence of a learning effect.