EFFECTS OF INSPIRATORY SUPPORT UPON BREATHING IN HUMANS DURING WAKEFULNESS AND SLEEP

We have compared the effects on breathing of inspiratory mechanical su pport during wakefulness and sleep in healthy subjects. Nine awake vol unteers breathed through a nose mask 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)) we re 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 re peated in 6 of the subjects during non-REM sleep. When awake, ''10 cmH 2O 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 slee p, ''10 cmH2O IPAP'' caused no significant change in VT (388 to 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 wi th diaphragmatic surface EMG recorded as an index of efferent inspirat ory activity. ''10 cmH2O IPAP'' had no consistent effect on EMG when a wake but caused a reduction in each subject during sleep. We conclude that during non-REM sleep, inspiratory mechanical support is associate d with a compensatory decrease in efferent inspiratory activity to ach ieve a similar tidal volume with maintenance of arterial P(CO2). When awake, a ''wakefulness drive to breathe'' may be associated with maint ained inspiratory activity such that mechanical support results in an increased tidal volume despite a fall in arterial P(CO2).