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.