Computed Tomography Assessment of Positive End-expiratory Pressure-induced Alveolar Recruitment in Patients with Acute Respiratory Distress Syndrome

Abstract

Computed tomography (CT) assessment of positive end-expiratory pressure (PEEP)-induced alveolar recruitment is classically achieved by quantifying the decrease in nonaerated lung parenchyma on a single juxtadiaphragmatic section (Gattinoni's method). This ap- proach ignores the alveolar recruitment occurring in poorly aer- ated lung areas and may not reflect the alveolar recruitment of the entire lung. This study describes a new CT method in which PEEP- induced alveolar recruitment is computed as the volume of gas pen- etrating in poorly and nonaerated lung regions following PEEP. In 16 patients with acute respiratory distress syndrome a thoracic spi- ral CT scan was performed in ZEEP and PEEP 15 cm H 2 O. Accord- ing to the new method, PEEP induced a 119% increase in func- tional residual capacity (FRC). PEEP-induced alveolar recruitment was 499 6 279 ml whereas distension and overdistension of previ- ously aerated lung areas were 395 6 382 ml and 28 6 6 ml, respec- tively. The alveolar recruitment according to Gattinoni's method was 26 6 24 g and no correlation was found between both meth- ods. A significant correlation was found between PEEP-induced al- veolar recruitment and increase in Pa O2 only when recruitment was assessed by the new method (Rho 5 0.76, p 5 0.003), suggesting that it may be more accurate than Gattinoni's method. Thoracic computed tomography (CT) enables an accurate evaluation of the volume of gas and tissue present in the lungs of patients with acute respiratory distress syndrome (ARDS) (1). Using CT, it is possible to assess the pulmonary distribu- tion of the increase in gas volume resulting from tidal ventila- tion and positive end-expiratory pressure (PEEP) (2) and to separate PEEP-induced lung overdistension from alveolar re- cruitment (3, 4). In the mid-1990s, Gattinoni and coworkers measured PEEP-induced alveolar recruitment on a single jux- tadiaphragmatic CT section by quantifying the decrease in nonaerated lung parenchyma characterized by CT attenua- tions ranging between 2 100 and 1 100 Hounsfield units (HU) (2, 5, 6). This approach ignores the alveolar recruitment occur- ring in poorly aerated lung areas characterized by CT attenua- tions ranging between 2 100 HU and 2 500 HU and, therefore, tends to underestimate PEEP-induced alveolar recruitment. In addition, by considering a single CT section, it may under- estimate or overestimate alveolar recruitment occurring in the entire lung. In fact, PEEP-induced alveolar recruitment may transform nonaerated lung areas into poorly aerated lung ar- eas and poorly aerated lung areas into normally aerated lung areas. Because of these complex gas transfers between the dif- ferent lung compartments, it appears quite difficult to quantify alveolar recruitment using a global approach. Clinically, PEEP- induced alveolar recruitment can be defined as the volume of gas penetrating in poorly and nonaerated alveolar structures fol- lowing the administration of PEEP, whereas PEEP-induced alveolar distension can be defined as the volume of gas pene- trating in previously normally aerated alveolar structures. The aim of the present study was to describe a new CT method for assessing PEEP-induced alveolar recruitment based on this sim- ple definition. METHODS