The effects of tidal volume demand on work of breathing during simulated lung-protective ventilation.

BACKGROUND: Lung-protective ventilation (LPV) can result in a ventilator tidal volume (V(T)) below patient V(T) demand, which may elevate work of breathing (WOB). Increasing the ventilator inspiratory flow may not sufficiently reduce WOB, because the patient's flow-time requirements may exceed the ventilator's flow-time delivery pattern. We investigated (1) the effects of V(T) demand on WOB during LPV and (2) which ventilator pattern best reduced WOB while achieving LPV goals.

METHODS: A standard WOB lung model simulated assisted breathing. Using 3 ventilators (Hamilton Veolar, Hamilton Galileo, and Dräger Evita 2 dura), we tested volume-control ventilation with a constant flow pattern (VCV-CF), volume-control ventilation with a decelerating flow (VCV-DF), and pressure-control ventilation (PCV). Simulated V(T) demand was increased from 50-125% of the ventilator-delivered V(T) (400 mL) as ventilator inspiratory time (T(I)) was decreased (0.95, 0.80, 0.65, and 0.45 s) relative to simulated T(I) (0.8 s). WOB was measured with a pulmonary mechanics monitor.

RESULTS: During VCV-CF and VCV-DF, a V(T) demand of > or = 100% drastically increased WOB, attributable to imposed WOB from the inspiratory valve. Increasing inspiratory flow by using the decelerating flow pattern and/or decreasing T(I) reduced WOB, but generally not to normal levels. "Double-triggered" breaths, with excessive V(T) delivery, often occurred when ventilator T(I) was well below simulated T(I). PCV was most effective in reducing WOB, but V(T) delivery exceeded the LPV target unless T(I) was reduced.

CONCLUSIONS: Given our dual goals of reducing both WOB and V(T) during LPV, VCV-DF with relatively brief T(I) appeared to be the best option, followed by PCV with a relatively brief T(I).