Respiratory Mechanics, Lung Recruitability, and Gas Exchange in Pulmonary and Extrapulmonary Acute Respiratory Distress Syndrome

Abstract

Objectives: Acute respiratory distress syndrome is a clinical syndrome characterized by a refractory hypoxemia due to an inflammatory and high permeability pulmonary edema secondary to direct or indirect lung insult (pulmonary and extrapulmonary form). Aim of this study was to evaluate in a large database of acute respiratory distress syndrome patients, the pulmonary versus extrapulmonary form in terms of respiratory mechanics, lung recruitment, gas exchange, and positive end-expiratory pressure response.

Design: A secondary analysis of previously published data.

Patients: One-hundred eighty-one sedated and paralyzed acute respiratory distress syndrome patients (age 60 yr [46-72 yr], body mass index 25 kg/m [22-28 kg/m], and PaO2/FIO2 184 ± 66).

Interventions: Lung CT scan performed at 5 and 45 cm H2O. Two levels of positive end-expiratory pressure (5 and 15 cm H2O) were randomly applied.

Measurements and main results: Ninety-seven and 84 patients had a pulmonary and extrapulmonary acute respiratory distress syndrome. The median time from intensive care admission to the CT scan and respiratory mechanics analysis was 4 days (interquartile range, 2-6). At both positive end-expiratory pressure levels, pulmonary acute respiratory distress syndrome presented a significantly lower PaO2/FIO2 and higher physiologic dead space compared with extrapulmonary acute respiratory distress syndrome. The lung and chest wall elastance were similar between groups. The intra-abdominal pressure was significantly higher in extrapulmonary compared with pulmonary acute respiratory distress syndrome (10 mm Hg [7-12 mm Hg] vs 7 mm Hg [5-8 mm Hg]). The lung weight and lung recruitability were significantly higher in pulmonary acute respiratory distress syndrome (1,534 g [1,286-1,835 g] vs 1,342 g [1,090-1,507 g] and 16% [9-25%] vs 9% [5-14%]).

Conclusions: In the early stage, pulmonary acute respiratory distress syndrome is characterized by a greater impairment of gas exchange and higher lung recruitability. The recognition of the origin of acute respiratory distress syndrome is important for a more customized ventilatory management.