Inspiratory work imposed by continuous positive airway pressure (CPAP) machines: the effect of CPAP level and endotracheal tube size.

Inspiratory work imposed by Continuous Positive Airway Pressure (CPAP) machines has been a matter of concern. The imposed inspiratory work of CPAP machine circuits (Wcir) and the effect of the total breathing apparatus with endotracheal tube (ETT) and connector included in the circuit (Wapp), were measured in three continuous flow (CF) and various configurations of three demand flow (DF) CPAP machines. The performance was assessed at 0, 5, 10 and 15 cmH2O CPAP using a Michigan Instruments Test Lung Model 1600, internal compliance set at 50 ml/cmH2O, driven at square wave inspiratory flows (VI) of 20, 40 and 60 l/min at a tidal volume of 500 ml. Work, expressed in mJ/l, was calculated from the area of pressure-volume loops. Inspiratory work, Wcir and Wapp, was dependent upon the particular CPAP machine, VI and ETT size, but not upon CPAP level, being maximum at VI 601/min and with ETT 7.0 mm i.d. Work values (Wcir) varied from 50 to 325 mJ/l with both CF and DF machines and up to 1100 mJ/l with ETT and connector (Wapp). No consistent advantage of CF over DF machines was demonstrated. There was little advantage of high gas flows (greater than 5 l/min) in various DF circuits. Within an individual machine maximum negative pressures generated during inspiration correlated with both Wcir and Wapp.