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Critical Care Medicine:
doi: 10.1097/CCM.0b013e318287f6e7
Clinical Investigations

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

Mauri, Tommaso MD1,2; Bellani, Giacomo MD, PhD1,2; Confalonieri, Andrea MD2; Tagliabue, Paola MD2; Turella, Marta MD1,2; Coppadoro, Andrea MD1; Citerio, Giuseppe MD2; Patroniti, Nicolo’ MD1,2; Pesenti, Antonio MD1,2

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Abstract

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 (Vtdep and Vtnondep), 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 Vtdep 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, Vtnondep decreased, and Vtdep did not change, likely indicating a higher efficiency of posterior diaphragm that led to decreased Vt%nondep/Vt%dep (p < 0.01). Finally, PaO2/FIO2 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.

© 2013 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins

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