Effect of ventilation equipment on imposed work of breathing.

OBJECTIVE To determine the imposed work of different ventilation systems at 3 levels of pressure support. DESIGN Laboratory study. SETTING Teaching hospital respiratory laboratory. PARTICIPANTS Healthy, human volunteers. INTERVENTIONS Measurement of imposed work of breathing (WOB(i)) in six ventilators with alteration of ventilatory settings, humidification device, and triggering mechanisms. RESULTS At 0 cmH2O CPAP and 0 cmH2O PSV, clinically significant (> 0.1 joules per litre or J/L) WOB(i )occurred in all systems. At 5 cmH2O pressure support ventilation (PSV) median WOB(i) ranged from 0.01 to 0.11 J/L. Removal of the pleated membrane heat and moisture exchanger (HME) significantly reduced WOB(i) (0.38 vs. 0.11 J/L, p < 0. 0001). Drawover humidification marginally increased WOB(i) (0.16 vs. 0.11 J/L, p = 0. 0001). Flow triggering reduced WOB(i) with the Servo (p < 0.0001) and Bennett ventilators (p = 0.001) but not with the Bear 1000 ventilator. CONCLUSIONS Up to 7 cmH2O of PSV may be required to reduce WOB(i) related to the ventilator, circuit and humdification devices. This pressure support does not address the additional resistive effect of the endotracheal or tracheotomy tube. Higher levels of PSV may therefore be required to offset WOB(i).