Post-hyperventilation hypopnea in humans during NREM sleep.

We wished to determine if mild hypocapnia above the "apneic threshold" would result in apnea or hypopnea during NREM sleep. Hypocapnia was induced by nasal mechanical hyperventilation for 1 min either under normoxia (51 trials, n = 7) or hyperoxia (43 trials, n = 5). Cessation of mechanical ventilation resulted in hypopnea due to reduced VT without a change in f. Central apnea occurred mostly under hyperoxic conditions (9/43 versus 2/51 trials under normoxic conditions), and only when complete inhibition of ventilatory motor output occurred during mechanical ventilation. Significant correlation between the magnitude of hypocapnia and nadir VE was noted under both normoxic and hyperoxic conditions. However, nadir VE was variable when hypocapnia was modest (-2 mmHg); further hypocapnia (-4 mmHg) was associated with consistent reduction in nadir VE below 30% of control under normoxic conditions, and central apnea under hyperoxic conditions. We conclude that: (1) Brief hyperventilation during NREM sleep is followed by hypocapnic hypopnea due to reduced VT and not breathing frequency; (2) Hypocapnia due to brief mild hyperventilation does not cause central apnea unless peripheral chemoreceptors are also inhibited; (3) Sustained hyperventilation or more severe hypocapnia may be required for the development of hypocapnic central apnea during NREM sleep.