Critical Care: Original Research

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

Shailesh Bihari, Ubbo F. Wiersema

Chest 2024; 165: 583-589

https://doi.org/10.1016/j.chest.2023.09.032

Background

Studies investigating the effect of trunk inclination on respiratory mechanics in mechanically ventilated patients with ARDS 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 patients with obesity and ARDS is uncertain.

Research Question

Does the effect of change in posture on partition respiratory mechanics differ between patients with ARDS with and without obesity?

Study Design and Methods

In this single-center study, patients with ARDS with and without obesity 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 patients with obesity, 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 patients without obesity, only chest wall elastance was lower in supine vs semi-recumbent (P < .001).

Interpretation

In mechanically ventilated patients with ARDS and obesity, 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).