Adaptive support ventilation may deliver unwanted respiratory rate-tidal volume combinations in patients with acute lung injury ventilated according to an open lung concept
Background: With adaptive support ventilation, respiratory rate and tidal volume (VT) are a function of the Otis least work of breathing formula. We hypothesized that adaptive support ventilation in an open lung ventilator strategy would deliver higher VTs to patients with acute lung injury. Methods: Patients with acute lung injury were ventilated according to a local guideline advising the use of lower VT (6-8 ml/kg predicted body weight), high concentrations of positive end-expiratory pressure, and recruitment maneuvers. Ventilation parameters were recorded when the ventilator was switched to adaptive support ventilation, and after recruitment maneuvers. If VT increased more than 8 ml/kg predicted body weight, airway pressure was limited to correct for the rise of VT. Results: Ten patients with a mean (±SD) Pao2/Fio2 of 171 ± 86 mmHg were included. After a switch from pressure-controlled ventilation to adaptive support ventilation, respiratory rate declined (from 31 ± 5 to 21 ± 6 breaths/min; difference = 10 breaths/min, 95% CI 3-17 breaths/min, P = 0.008) and VT increased (from 6.5 ± 0.8 to 9.0 ± 1.6 ml/kg predicted body weight; difference = 2.5 ml, 95% CI 0.4-4.6 ml/kg predicted body weight, P = 0.02). Pressure limitation corrected for the rise of VT, but minute ventilation declined, forcing the user to switch back to pressure-controlled ventilation. Conclusions: Adaptive support ventilation, compared with pressure-controlled ventilation in an open lung strategy setting, delivers a lower respiratory rate-higher VT combination. Pressure limitation does correct for the rise of VT, but leads to a decline in minute ventilation. Copyright © 2011, the American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins. Anesthesiology.