Correlation Between the %MinVol Setting and Work of Breathing During Adaptive Support Ventilation in Patients with Respiratory Failure

BACKGROUND: Adaptive support ventilation (ASV) is a new mode of mechanical ventilation that seeks an optimal breathing pattern based on the minimum work of breathing (WOB) principle. The operator's manual for the ventilators that provide ASV recommends that the %MinVol setting be started at 100% (the 100%MinVol setting), but it is unclear whether that setting reduces WOB in patients with respiratory failure.

METHODS: We studied 22 hemodynamically stable patients with respiratory failure who were on pressure-support ventilation. We switched the ventilation mode to ASV and started at the 100%MinVol setting. We then increased the %MinVol setting by 10% every 5 min until 1–3 mandatory breaths per min appeared, and called that setting the ASV target point. We then tested 2 additional %MinVol settings: 20% below the ASV target point (target-point–20%), and 20% above the ASV target point (target-point+20%). We tested each %MinVol setting for 10 min. At the end of each 10-min period we measured respiratory variables, pressure-time product (PTP), and airway occlusion pressure at 0.1 s after the onset of inspiratory flow (P_0.1).

RESULTS: In 18 patients (82%), at the 100%MinVol setting, the actual minute volume (V̇_E) was greater than the target V̇_E. At the ASV target point the mean ± SD %MinVol setting was 165 ± 54% and was associated with a 40% decrease in PTP and P_0.1, but V̇_E did not change. At target-point+20%, V̇_E increased slightly, primarily due to a small increase in tidal volume, and PTP and P_0.1 further decreased. At target-point–20%, PTP and P_0.1 were similar to those at the 100%MinVol setting. At the ASV target point the 6 patients with chronic obstructive pulmonary disease had a lower mean %MinVol setting (125 ± 23%) than the 16 patients who did not have chronic obstructive pulmonary disease (180 ± 55%).

CONCLUSIONS: The 100%MinVol setting was frequently not associated with lower WOB in patients with respiratory failure. The %MinVol setting that significantly reduced WOB could be detected by increasing the %MinVol setting until a few mandatory breaths began to appear, and was on average 165% of the MinVol setting.