Influence of respiratory mechanics, lung recruitability and hemodynamics on pulmonary and extrapulmonary effects of PEEP in ARDS patients: a prospective observational study.

BACKGROUND: The pulmonary and extrapulmonary effects of positive end-expiratory pressure (PEEP) in patients with acute respiratory distress syndrome (ARDS) according to radiological features, respiratory mechanics, or hemodynamics are not fully elucidated. We evaluated the effects of low and high PEEP on pulmonary, haemodynamic, and renal function according to lung recruitability, lung-chest wall transmission ratio, and cardiac output. METHODS: Thirty-eight deeply sedated, paralyzed, and mechanically ventilated ARDS patients underwent quantitative lung CT and measurements of partitioned respiratory mechanics, gas exchange, hemodynamics, urinary function, and electrolytes at 5 and 15 cmH2O of PEEP within the first 48 hours from ICU admission. RESULTS: Increasing PEEP from 5 to 15 cmH2O significantly increased PaO2/FiO2 (123 [88-153] vs 189 [117-219], p <0.001) while venous admixture (43 ± 14 vs. 33 ± 13%, p <0.001) and cardiac output decreased (6.6 ± 19 vs. 5.9 ± 1.4 L/min, p <0.001). Urine output (40 [30-60] vs. 30 [22-49] mL, p =  0.011) decreased at high PEEP levels. Patients stratified by lung-chest wall transmission ratio at 5 cmH2O of PEEP or by potential for lung recruitment showed similar changes in respiratory mechanics, gas exchange, hemodynamics, and urine variables. Patients with higher cardiac output at 5 cmH2O of PEEP had lower oxygenation and a greater reduction in cardiac output with high PEEP. Urinary output changes were similar, regardless of baseline cardiac output. CONCLUSIONS: Although increasing PEEP from 5 to 15 cmH2O improved oxygenation, it reduced cardiac output and diuresis independently of elastance transmission ratio and lung recruitability.