Changes in Respiratory Mechanics With Trunk Inclination Differs Between Patients With ARDS With and Without Obesity

Background

Studies investigating the effect of trunk inclination on respiratory mechanics in mechanically ventilated ARDS patients have reported postural differences in partition respiratory mechanics. Compared with more upright positions, the supine-flat position provided lower lung and chest wall elastance, allowing reduced driving pressures and end-inspiratory transpulmonary pressure. However, the effect of trunk inclination on respiratory mechanics in obese patients with ARDS is uncertain.

Research Question

Does the effect of change in posture on partition respiratory mechanics differ between obese and non-obese patients with ARDS?

Study Design and Methods

In this single-center study, obese and non-obese patients with ARDS were randomized into two 15-minute steps in which trunk inclination was changed from semi-recumbent (40° head up) to supine-flat (0°), or vice versa. At the end of each step partition respiratory mechanics, airway opening pressure and arterial blood gases were measured. Paired t test was used to examine respiratory mechanics and blood gas variables in each group.

Results

Forty consecutive patients were enrolled. Twenty were obese (BMI, 38.4 [34.5-42.3]), and 20 were non-obese (BMI, 26.6 [25.2-28.5]). In the obese patients, lung and chest wall elastance, driving pressure, inspiratory transpulmonary pressure, Paco₂, and ventilatory ratio were lower supine than semi-recumbent (P < .001). Airways resistance was greater supine (P = .006). In the non-obese patients, only chest wall elastance was lower in supine vs semi-recumbent (P < .001).

Interpretation

In mechanically ventilated obese patients with ARDS, supine posture provided lower lung and chest wall elastance, and better CO₂ clearance, than the semi-recumbent posture.

Clinical Trial Registration

This study was registered with Australian New Zealand Clinical Trials Registry (ACTRN12623000794606).