THE NATURE OF BREATHING DURING HYPOCAPNIA IN AWAKE MAN

We have studied post-hyperventilation breathing pattern in eight, awak e, healthy, naive volunteers after 5 min voluntary or mechanical hyper ventilation during normocapnia (PET(CO2) = 38mmHg) 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 b reathing following hypocapnic voluntary hyperventilation with that fol lowing hypocapnic mechanical hyperventilation indicated that ventilati on was elevated following voluntary hyperventilation; this would sugge st that 'after-discharge' exists in man following active hyperventilat ion, even during hypocapnia. In the absence of 'after-discharge' (i.e, following mechanical hyperventilation), hypocapnia was clearly associ ated 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 PET(CO2) Most notable, was the marked increase in breath-by-breath variability of TI, TE and VT during hypo capnia. The increased variability of breathing during hypocapnia may r eflect fluctuations in behavioural drives associated with wakefulness.