Respiratory effects of trunk inclination in obese and non-obese patients mechanically ventilated for ARDS.

BACKGROUND: Adjusting trunk inclination in patients with acute respiratory distress syndrome directly affects physiological variables such as respiratory mechanics and PaCO2 levels. These effects may vary according to the body mass index (BMI) due to differences in lung and chest wall mechanics, highlighting the need for further investigation to clarify the clinical relevance of body position across patient subgroups. METHODS: A secondary analysis compared the physiological effects of increasing trunk inclination angles between mechanically ventilated patients with obesity (BMI ≥ 30 kg/m2) and those without obesity (BMI < 30 kg/m2). RESULTS: Data from 159 patients collected across seven individual studies were analyzed. The following physiological changes were observed in response to increased trunk inclination: Sixty-five patients with obesity presented a greater decrease in respiratory system compliance (-7.5 [-10; -5] mL/cmH2O; p < 0.001) compared to ninety-four patients without obesity (-3.5 [-7; -0.08] mL/cmH2O; p = 0.045). Lung compliance decreased in obese patients (-7.8 [-12.4; -3.3] mL/cmH2O; p < 0.001), whereas no significant changes were observed in patients without obesity (-5.9 [-14.2; 2.3] mL/cmH2O; p = 0.160). Chest wall compliance decreased by -42.9 [-63.2; -22.6] mL/cmH2O (p < 0.001) in obese patients and by -47.7 [-95.3; -0.15] mL/cmH2O in non-obese patients (p = 0.049). PaCO2 increased in obese patients by 4.6 [1.4; 7.8] mmHg (p = 0.004) but not in patients without obesity (2.5 [-0.6; 5.6] (p = 0.113). No significant differences were observed in PaO2/FIO2 between phases. CONCLUSIONS: Increasing the trunk inclination angle during passive ventilation reduces respiratory system, lung, and chest wall compliance. This effect was more pronounced in obese patients. Moreover, only this population exhibited an increase in PaCO2. We acknowledge the methodological heterogeneity across the included studies, which may have influenced the results. Overall, our results highlight the importance of considering BMI as a significant variable that influences individual physiological responses to changes in bed inclination.