Spontaneously breathing lung model comparison of work of breathing between automatic tube compensation and pressure support.

INTRODUCTION Automatic tube compensation (ATC), a new ventilation mode that compensates for the work of breathing imposed by endotracheal tube resistance, recently became commercially available. METHODS We conducted a laboratory study with a lung model and a Nellcor Puritan Bennett 840 ventilator to compare ATC and pressure-support ventilation (PSV). A bellows-in-a-box lung model simulated spontaneous breathing with the following settings: respiratory rate 10 breaths/min, inspiratory time 1.0 s, peak inspiratory flow 60 L/min without connecting to the ventilator and endotracheal tube (ETT). At each ETT size (5, 6, 7, 8 and 9 mm inner diameter) 100% ATC was compared with pressure support (PS) of 0, 2, 4, 6, 8, and 10 cm H(2)O at positive end-expiratory pressure (PEEP) of 0 and 5 cm H(2)O. The negative deflection (PI) of the "alveolar" pressure (ie, pressure inside the bellows, P(alv)) and the delay time were measured. The PI and total pressure-time product (PTP(tot)) integrated from P(alv) were analyzed. PTP(tot) was subdivided into PTP(trig) (the PTP from the beginning of inspiration to the minimum P(alv)) and PTP(supp) (the PTP from the minimum P(alv) to the return to baseline P(alv)). RESULTS At PEEP of 0 cm H(2)O: with ETTs of 5, 6, and 7 mm the PI values with ATC corresponded to PS of 0-4 cm H(2)O; with the 8-mm ETT the PI values corresponded to PS of 0 cm H(2)O; with the 9-mm ETT the PI values corresponded to PS of 0-2 cm H(2)O. At PEEP of 5 cm H(2)O, with all ETT sizes the PI values corresponded to PS of 0 cm H(2)O. PTP(tot) and PTP(supp) of ATC corresponded to: PS of 2-4 cm H(2)O with the 5-mm ETT; PS of 2 cm H(2)O with the 6-mm ETT; PS of 0-2 cm H(2)O with the 7-mm ETT; and PS of 0 cm H(2)O with the 8- and 9-mm ETTs, at PEEP of 0 cm H(2)O. PEEP of 5 cm H(2)O was not tested for PTP. PTP(trig) with ATC showed comparable or greater values with each size of ETT. CONCLUSIONS ATC with a Nellcor Puritan Bennett 840 ventilator provided inspiratory ventilatory support corresponding to PS of <or= 4 cm H(2)O, depending on ETT size, which was not enough to compensate for the work of breathing imposed by the ETT.