Obesity Modifies the Effect of Peep on Mechanics of Breathing in Acute Hypoxemic Respiratory Failure

Abstract Rationale: Obesity has detrimental effects on the respiratory system—including decreased tidal volume and lung compliance, and increased atelectasis and work of breathing. These effects are especially pronounced in patients who are sedated and mechanically ventilated. The effect of obesity on spontaneous breathing effort and lung-distending pressure in acute hypoxemic respiratory failure (AHRF) is unknown. This study aims to establish the effects of obesity on respiratory mechanics during spontaneous breathing and the success of a lung- and diaphragm-protective ventilation strategy in patients with AHRF. Methods: This is a secondary analysis of a randomized cross-over trial that tested various interventions on lung-distending pressure and respiratory effort in an attempt to achieve lung- and diaphragm-protective (LDP) targets to treat invasively mechanically ventilated patients with AHRF. Ventilation and sedation were titrated to achieve moderate respiratory effort and safe lung-distending pressures at lower and higher PEEP levels (applied in random order). High PEEP was defined as end-expiratory transpulmonary pressure of 0-2 cm H2O. Low PEEP was defined as the lowest tolerable level (FiO2 ≤0.9). Bayesian statistical methods were used to assess the influence of BMI on the selected PEEP, the proportion of success in achieving LDP targets, esophageal pressure swings, and dynamic transpulmonary pressure. Results: BMI modified the effect of increasing PEEP on the probability of achieving LDP targets (posterior probability of interaction: 99%; see figure). In patients with a BMI≥30, the probability of achieving LDP targets was greater when applying higher PEEP compared to lower PEEP (median posterior probability 100% vs 21%). In patients with a BMI<30, the probability of achieving LDP targets was lower at higher PEEP compared to lower PEEP (median posterior probability 40% vs. 87%). In patients with BMI ≥30, esophageal pressure swing was lower under high PEEP conditions (median -9 cm H2O, IQR -10 − -7) compared with low PEEP conditions (median -10 cm H2O, IQR -14 − -7); transpulmonary pressure swing was also lower under high PEEP conditions (median 15 cm H2O, IQR 14−17) compared with low PEEP conditions (median 19 cm H2O, IQR 13−21). Patients with BMI≥30 required more PEEP to achieve the high PEEP target: in patients with BMI≥30, PEEP was a median of 16 cm H2O (IQR 14−17); in patients with BMI<30, PEEP was a median of 14 cm H2O (IQR 12−14). Conclusion: Patients with higher body mass index may require high PEEP to achieve lung- and diaphragm-protective targets for spontaneous breathing in AHRF.

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