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 H₂O at positive end-expiratory pressure (PEEP) of 0 and 5 cm H₂O. 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 H₂O: with ETTs of 5, 6, and 7 mm the PI values with ATC corresponded to PS of 0–4 cm H₂O; with the 8-mm ETT the PI values corresponded to PS of 0 cm H₂O; with the 9-mm ETT the PI values corresponded to PS of 0–2 cm H₂O. At PEEP of 5 cm H₂O, with all ETT sizes the PI values corresponded to PS of 0 cm H₂O. PTP_tot and PTP_supp of ATC corresponded to: PS of 2–4 cm H₂O with the 5-mm ETT; PS of 2 cm H₂O with the 6-mm ETT; PS of 0–2 cm H₂O with the 7-mm ETT; and PS of 0 cm H₂O with the 8- and 9-mm ETTs, at PEEP of 0 cm H₂O. PEEP of 5 cm H₂O 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 ≤ 4 cm H₂O, depending on ETT size, which was not enough to compensate for the work of breathing imposed by the ETT.