Tidal volume reduction in patients with acute lung injury when plateau pressures are not high

Abstract

Use of a volume- and pressure-limited mechanical ventilation strategy improves clinical outcomes of patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS). However, the extent to which tidal volumes and inspiratory airway pressures should be reduced to optimize clinical outcomes is a controversial topic. This article addresses the question, "Is there a safe upper limit to inspiratory plateau pressure in patients with ALI/ARDS?" We reviewed data from animal models with and without preexisting lung injury, studies of normal human respiratory system mechanics, and the results of five clinical trials of lung-protective mechanical ventilation strategies. We also present an original analysis of data from the largest of the five clinical trials. The available data from each of these assessments do not support the commonly held view that inspiratory plateau pressures of 30 to 35 cm H2O are safe. We could not identify a safe upper limit for plateau pressures in patients with ALI/ARDS.

Figures

Figure 1.

Robust locally weighted regression and smoothing (Lowess) plot (bandwidth, 0.4) of mortality and Day 1 plateau pressure (Pplat; cm H2O) among patients enrolled in the ARDS Network study (n = 787) (4, 47). The Lowess method is a nonparametric smoother that uses overlapping neighborhoods of data to estimate a local effect. A bandwidth of 0.4 means 20% of the data on either side of a given Pplat will contribute to a local estimate of mortality at that Pplat. Data at the high and low ends of the curve therefore represent fewer observations. Data are smoothed using a tricubic weight function so that points furthest from the Pplat of interest are assigned the least weight (and approach zero).

Figure 2.

Mortality difference by quartile of Day 1 Pplat. The range of Pplat levels in cm H2O and the number of patients (n) is detailed in each bar of the graph. ARR = absolute risk reduction; CI = confidence interval.