The nature of breathing during hypocapnia in awake man

We have studied post-hyperventilation breathing pattern in eight, awake, healthy, naive volunteers after 5 min voluntary or mechanical hyperventilation during normocapnia (PETCO2 = 38 mmHg) and 'hypocapnia (24 mmHg). Breathing was monitored for 10 min post-hyperventilation, 'non-invasively', using calibrated respiratory inductance plethysmography; wakefulness was confirmed with electroencephalography. Comparison of breathing following hypocapnic voluntary hyperventilation with that following hypocapnic mechanical hyperventilation indicated that ventilation was elevated following voluntary hyperventilation; this would suggest that 'after-discharge' exists in man following active hyperventilation, even during hypocapnia. In the absence of 'after-discharge' (i.e. following mechanical hyperventilation), hypocapnia was clearly associated with hypoventilation. Apnoeas (increased TE) were present during hypocapnia; but neither the duration nor the occurrence of apnoea was related to the absolute level of PETCO2. Most notable, was the marked increase in breath-by-breath variability of TI, TE and VT during hypocapnia. The increased variability of breathing during hypocapnia may reflect fluctuations in behavioural drives associated with wakefulness.