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

Abstract 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 compliance, 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. 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 in patients during passive ventilation reduces respiratory system, lung, and chest wall compliance. This effect was more pronounced in patients with obesity. Moreover, only this population exhibited an increase in PaCO₂. These findings highlight the importance of individualized respiratory management strategies, including optimizing bed inclination angles tailored to each patient's condition.