Lung Recruitment Assessed by Respiratory Mechanics and Computed Tomography in Patients with Acute Respiratory Distress Syndrome What Is the Relationship?

Rationale: The assessment of lung recruitability in patients with acute respiratory distress syndrome (ARDS) may be important for planning recruitment maneuvers and setting positive end-expiratory pressure (PEEP). Objectives: To determine whether lung recruitment measured by respiratory mechanics is comparable with lung recruitment measured by computed tomography (CT). Methods: In 22 patients with ARDS, lung recruitment was assessed at 5 and 15 cm H2O PEEP by using respiratory mechanics-based methods: (1) increase in gas volume between two pressure-volume curves (P-Vrs curve); (2) increase in gas volume measured and predicted on the basis of expected end-expiratory lung volume and static compliance of the respiratory system (EELV-Cst,rs); as well as by CT scan: (3) decrease in noninflated lung tissue (CT [not inflated]); and (4) decrease in noninflated and poorly inflated tissue (CT [not + poorly inflated]). Measurements and Main Results: The P-Vrs curve recruitment was significantly higher than EELV-Cst,rs recruitment (423 +/- 223 ml vs. 315 +/- 201 ml; P < 0.001), but these measures were significantly related to each other (R-2 = 0.93; P < 0.001). CT (not inflated) recruitment was 77 +/- 86 g and CT (not + poorly inflated) was 80 +/- 67 g (P = 0.856), and these measures were also significantly related to each other (R-2 = 0.20; P = 0.04). Recruitment measured by respiratory mechanics was 54 +/- 28% (P-Vrs curve) and 39 +/- 25% (EELV-Cst,rs) of the gas volume at 5 cm H2O PEEP. Recruitment measured by CT scan was 5 +/- 5% (CT [not inflated]) and 6 +/- 6% (CT [not + poorly inflated]) of lung tissue. Conclusions: Respiratory mechanics and CT measure under the same term, "recruitment"-two different entities. The respiratory mechanics-based methods include gas entering in already open pulmonary units that improve their mechanical properties at higher PEEP. Consequently, they can be used to assess the overall improvement of inflation. The CT scan measures the amount of collapsed tissue that regains inflation.