APNEA AFTER NORMOCAPNIC MECHANICAL VENTILATION DURING NREM SLEEP

We determined whether normocapnic mechanical ventilation at high tidal volume (VT) and breathing frequency (f) during non-rapid-eye-movement (NREM) sleep would cause apnea. Seven normal sleeping subjects were p laced on assist-control mechanical ventilation (i.e., subject initiate s inspiration) and VT was gradually increased to 2.1 times eupneic VT (1.17 +/- 0.04 liters). This high VT was maintained for 5 min, the ven tilator mode was switched to controlled mechanical ventilation, and f was increased gradually from 9.5 +/- 1.0 (during assist-control mechan ical ventilation) to 14.0 +/- 0.7 breaths/min. Normocapnia (end-tidal PCO2 = 44 +/- 1.2 Torr) was maintained throughout the trials. Inspirat ory effort was completely inhibited during the period of sustained hig h VT and f, and apnea occurred immediately after cessation of the pass ive mechanical ventilation. The duration of the apnea preceding the fi rst inspiratory effort was 20.3 +/- 2.3 s or 7.1 times the eupneic exp iratory duration and 5 times the expiratory duration chosen by the sub ject during assist-control mechanical ventilation. We conclude that in hibition of inspiratory motor output occurs during and after normocapn ic mechanical ventilation at high VT and f during NREM sleep. These ne uromechanical inhibitory effects may serve to initiate and prolong apn ea.