The "mechanical paradox" unveiled: a physiological study

Abstract

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).

Keywords: Artificial [MeSH]; Electrical impedance tomography; Esophageal pressure; Respiration; Ventilator-induced lung injury [MeSH].

Fig. 1

Variation of Pplat_RS, Cstat_RS, and DP_RS with the different study phases and ΔPEEP. Each boxplot shows the median, 1st, and 3rd quartile of the variable value. Phase 1, phase 2, and phase 3 are depicted in red, blue, and black, respectively. Not all variables could be measured for each ΔPEEP level because of the fixed PEEP range explored in the decremental PEEP trial and the varying best PEEP value for each patient. The number of patients with available ΔPEEP values is displayed in the figure. Abbreviations: PEEP, positive end-expiratory pressure; Pplat, plateau pressure; RS, respiratory system; Cstat, static compliance; DP, driving pressure. The dotted vertical line identifies the ΔPEEP 0 cmH₂O value. *p < 0.05 of the pairwise comparison between phase 2 vs. phase 1 within each PEEP level

Fig. 2

Variation of Pplat_L, Cstat_L, and DP_L with the different study phases and ΔPEEP. Each boxplot shows the median, 1st, and 3rd quartile of the variable value. Phase 1, phase 2, and phase 3 are depicted in red, blue, and black, respectively. Not all variables could be measured for each ΔPEEP level because of the fixed PEEP range explored in the decremental PEEP trial and the varying best PEEP value for each patient. The number of patients with available ΔPEEP values is displayed in the figure. Abbreviations: PEEP, positive end-expiratory pressure; Pplat, plateau pressure; L, lung; Cstat, static compliance; DP, driving pressure. The dotted vertical line identifies the ΔPEEP 0 cmH₂O value. *p < 0.05 of the pairwise comparison between phase 2 vs. phase 1 within each PEEP level

Fig. 3

Variation of overdistention, collapse, and ΔEELI with the different study phases and ΔPEEP. Each boxplot shows the median, 1st, and 3rd quartile of the variable value. Phase 1, phase 2, and phase 3 are depicted in red, blue, and black, respectively. Not all variables could be measured for each ΔPEEP level because of the fixed PEEP range explored in the decremental PEEP trial and the varying best PEEP value for each patient. The number of patients with available ΔPEEP values is displayed in the figure. Abbreviations: PEEP, positive end-expiratory pressure; ΔEELI, difference of end-expiratory lung impedance compared to the value at 8 cmH₂O of PEEP during phase 2. The dotted vertical line identifies the ΔPEEP 0 cmH₂O value. *p < 0.05 of the pairwise comparison between phase 2 vs. phase 1 within each PEEP level