EFFECT OF MECHANICAL LOADING ON EXPIRATORY AND INSPIRATORY MUSCLE-ACTIVITY DURING NREM SLEEP

We investigated the effect of acute and sustained inspiratory loading (IRL) on the activity of expiratory abdominal muscles (EMG(ab)) and the diaphragm (EMG(di)) and on ventilation during wakefulness and non-rapid-eye-movement (NREM) sleep in healthy subjects. EMG(di) and EMG(ab) were measured with esophageal and transcutaneous electrodes, respectively. During wakefulness, EMG(di) increased in response to acute loading (18 cmH2O·l-1·s) (+23%); this was accompanied by preservation of tidal volume (VT) and minute ventilation (V̇E). During NREM sleep, no augmentation was noted in EMG(di) or EMG(ab). Inspiratory time (TI) was prolonged (+5%), but this was not sufficient to prevent a decrease in both VT and V̇E (-21 and -20%, respectively). During sustained loading (12 cmH2O·l-1 s) in NREM sleep, control breaths (C) were compared with the steady-state loaded breaths (SS) defined by breaths 41-50. Steady-state IRL was associated with augmentation of EMG(di) (12%) and EMG(ab) (50%). VT returned to control levels, expiratory time shortened, and breathing frequency increased. The net result was the increase in V̇E above control levels (+5%, P < 0.01). No change was noted in end-tidal CO2 or O2. We concluded that 1) wakefulness is a prerequisite for immediate load compensation (in its absence, TI prolongation is the only compensatory response) and 2) during sustained IRL, the augmentation of EMG(di) and EMG(ab) can lead to complete ventilatory recovery without measurable changes in chemical stimuli.