The "mechanical paradox" unveiled: a physiological study

Background: Recent studies report that chest wall loading may reduce airway pressures and increase respiratory system compliance, contrary to the anticipated effect of this maneuver ("mechanical paradox"). Aim of this physiological study is to clarify the mechanism underlying this phenomenon. Methods: Twenty patients receiving invasive mechanical ventilation for acute hypoxemic respiratory failure were studied during a decremental PEEP trial. Variable weights were placed on the patients' abdomen to achieve a 5-mmHg increase in intra-abdominal pressure. Three consecutive phases for each PEEP level were performed: weight-off, weight-on, and weight-off. Esophageal pressure measurement and electrical impedance tomography (EIT) were used. Results: The abdominal weight decreased end-expiratory lung impedance (EELI) and overdistention and increased collapse for all PEEP values (all p-values < 0.001). For PEEP values higher than the EIT-based optimal PEEP, the abdominal weight reduced respiratory system and lung plateau pressures (coefficient [standard error] - 1.26 [0.21] and - 5.51 [0.28], respectively, both p-values < 0.001) and driving pressures (- 1.47 [0.22] and - 1.62 [0.22], respectively, both p-values < 0.001). For PEEP values lower than the optimal, the effect of the application of the abdominal weight was the opposite (all p-values < 0.001). Conclusions: The improvement in respiratory system and lung mechanics following abdominal loading is consequent to the reduction of end-expiratory lung volume. This effect, however, only occurs at PEEP levels associated with prevalent overdistention. This simple and safe maneuver could be applied at the bedside to identify lung overdistension and titrate PEEP. Trial registration: ClinicalTrials.gov (NCT06174636, July 9th 2023).