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 n ormoxia (51 trials, n = 7) or hyperoxia (43 trials, n = 5). Cessation of mechanical ventilation resulted in hypopnea due to reduced VT witho ut a change in f. Central apnea occurred mostly under hyperoxic condit ions (9/43 versus 2/51 trials under normoxic conditions), and only whe n complete inhibition of ventilatory motor output occurred during mech anical ventilation. Significant correlation between the magnitude of h ypocapnia and nadir Vover dotE was noted under both normoxic and hyper oxic conditions. However, nadir Vover dotE was variable when hypocapni a was modest (-2 mmHg); further hypocapnia (-4 mmHg) was associated wi th consistent reduction in nadir Vover dotE below 30% of control under normoxic conditions, and central apnea under hyperoxic conditions. We conclude that: (1) Brief hyperventilation during NREM sleep is follow ed by hypocapnic hypopnea due to reduced VT and not breathing frequenc y; (2) Hypocapnia due to brief mild hyperventilation does not cause ce ntral apnea unless peripheral chemoreceptors are also inhibited; (3) S ustained hyperventilation or more severe hypocapnia may be required fo r the development of hypocapnic central apnea during NREM sleep.