Effects of inspiratory support upon breathing in humans during wakefulness and sleep

We have compared the effects on breathing of inspiratory mechanical support during wakefulness and sleep in healthy subjects. Nine awake volunteers breathed through a nose mark connected to a machine supplying variable levels of inspiratory positive airway pressure (IPAP). Tidal volume (Vt), breath duration (Ttot) and end-tidal P_CO2 (Pet_CO2) were measured over 1 min steady state periods with IPAP set at a minimal level (approx. 2 cmH₂O) and at approx. 10 cmH₂O. This protocol was repeated in 6 of the subjects during non-REM sleep. When awake, "10 cmH₂O IPAP" caused a significant increase in mean Vt from 513 to 842 ml and a significant fall in Pet_CO2 from 39.7 to 32.7 mmHg. During sleep, "10 cmH₂O IPAP" caused no significant change in Vt (388 tp 390 ml) or Pet_CO2 (41.8 to 39.8 mmHg). In each state, "10 cmH₂O IPAP" had no significant effect on Ttot. Three subjects repeated the protocol with diaphragmatic surface EMG recorded as an index of efferent inspiratory activity. "10 cmH₂O IPAP" had no consistent effect on EMG when awake but caused a reduction in each subject during sleep. We conclude that during non-REM sleep, inspiratory mechanical support is associated with a compensatory decrease in efferent inspiratory activity to achieve a similar tidal volume with maintenance of arterial P_CO2. When awake, a "wakefulness drive to breathe" may be associated with maintained inspiratory activity such that mechanical support results in an increased tidal volume despite a fall arterial P_CO2.