The Effects of Tidal Volume Demand on Work of Breathing during Simulated Lung-Protective Ventilation

Abstract

BACKGROUND: Lung-protective ventilation (LPV) can result in a ventilator tidal volume (VT) below patient Vr 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 VT 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 ventila-tion with a constant flow pattern (VCV-CF), volume-control ventilation with a decelerating flow (VCV-DF), and pressure-control ventilation (PCV). Simulated V₁ demand was increased from 50-125% of the ventilator-delivered VT (400 mL) as ventilator inspiratory time (T₁) was decreased (0.95, 0.80, 0.65, and 0.45 s) relative to simulated T₁ (0.8 s). WOB was measured with a pulmonary mechanics monitor. RESULTS: During VCV-CF and VCV-DF, a V₁ demand of ≥ 100% drastically increased WOB, attributable to imposed WOB from the inspiratory valve. Increasing inspiratory flow by using the decelerating flow pattern and/or decreasing T₁ reduced WOB, but generally not to normal levels. "Double-triggered" breaths, with excessive Vr delivery, often occurred when ventilator T₁ was well below simulated T₁. PCV was most effective in reducing WOB, but Vr delivery exceeded the LPV target unless T₁ was reduced. CONCLUSIONS: Given our dual goals of reducing both WOB and V₁ during LPV, VCV-DF with relatively brief T₁ appeared to be the best option, followed by PCV with a relatively brief T₁.