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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

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

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.

Author Information

1Department of Health Sciences, University of Milan-Bicocca, Monza, Italy.

2Department of Perioperative Medicine and Intensive Care, San Gerardo Hospital, Monza, Italy.

*See also p. 1811.

This study has been performed in the General and Neurosurgical Intensive Care Units of the university-affiliated San Gerardo Hospital, University of Milan-Bicocca, Monza, Italy.

Supported, in part, by Institutional and FSE Regione Lombardia, Milan, Italy. Dräger Medical GmbH, Lübeck, Germany, provided free of charge electrical impedance tomography monitoring device and healthy controls data.

Dr. Coppadoro received grant support from Draeger. The remaining authors have disclosed that they do not have any potential conflicts of interest.

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