Topographic Distribution of Tidal Ventilation in Acute Respiratory Distress Syndrome: Effects of Positive End-Expiratory Pressure and Pressure Support

Objective: Acute respiratory distress syndrome is characterized by collapse of gravitationally dependent lung regions that usually diverts tidal ventilation toward nondependent regions. We hypothesized that higher positive end-expiratory pressure and enhanced spontaneous breathing may increase the proportion of tidal ventilation reaching dependent lung regions in patients with acute respiratory distress syndrome undergoing pressure support ventilation. Design: Prospective, randomized, cross-over study. Setting: General and neurosurgical ICUs of a single university-affiliated hospital. Patients: We enrolled ten intubated patients recovering from acute respiratory distress syndrome, after clinical switch from controlled ventilation to pressure support ventilation. Interventions: We compared, at the same pressure support ventilation level, a lower positive end-expiratory pressure (i.e., clinical positive end-expiratory pressure = 7 +/- 2 cm H2O) with a higher one, obtained by adding 5 cm H2O (12 +/- 2 cm H2O). Furthermore, a pressure support ventilation level associated with increased respiratory drive (3 +/- 2 cm H2O) was tested against resting pressure support ventilation (12 +/- 3 cm H2O), at clinical positive end-expiratory pressure. Measurements and Main Results: During all study phases, we measured, by electrical impedance tomography, the proportion of tidal ventilation reaching dependent and nondependent lung regions (Vt%(dep) and Vt%(nondep)), regional tidal volumes (Vt(dep) and Vt(nondep)), and antero-posterior ventilation homogeneity (Vt%(nondep)/Vt%(dep)). We also collected ventilation variables and arterial blood gases. Application of higher positive end-expiratory pressure levels increased Vt%(dep) and Vt(dep) values and decreased Vt%(nondep)/Vt%(dep) ratio, as compared with lower positive end-expiratory pressure (p < 0.01). Similarly, during lower pressure support ventilation, Vt%(dep) increased, Vt(nondep) decreased, and Vt(dep) did not change, likely indicating a higher efficiency of posterior diaphragm that led to decreased Vt%(nondep)/Vt%(dep) (p < 0.01). Finally, Pao(2)/Fio(2) ratios correlated with Vt%(dep) during all study phases (p < 0.05). Conclusions: In patients with acute respiratory distress syndrome undergoing pressure support ventilation, higher positive end-expiratory pressure and lower support levels increase the fraction of tidal ventilation reaching dependent lung regions, yielding more homogeneous ventilation and, possibly, better ventilation/perfusion coupling.