How to recruit the lungs?

Recruitment may be defined as the inflation of previously collapsed pulmonary units applying a sufficient pressure during the inspiration. The distribution of opening pressures throughout the lung parenchyma is Gaussian or bimodal and may vary from 4–5 up to 30–40cmH2O or higher. The application of recruitment maneuver may have a clinical impact if a part of the lung was collapsed before the application of airway pressure. In general, the main reasons for lung collapse are gas re-absorption, surfactant deficit, and/or compressive forces, including gravity. It follows that the greater is the lung edema is associated to greater weight and regional collapse. Consequently, higher lung edema is associated to greater lung recruitability. Indeed, the gravitational mechanism is likely the primary reason of lung collapse in human ARDS, whereas surfactant deficit and airway occlusion play a minimal role. Therefore, we may expect that recruitment maneuver is very effective when lung recruitability is high and less effective, or even nil and dangerous, when lung recruitability is low. We found that lung recruitability in unselected ARDS patients widely varies from 0 to nearly 70%, with a median value of 9%. Not surprisingly, in a clinical scenario, in a high percentage of patients, recruitment maneuvers may have deleterious effects. Lung recruitment may be quantified as a gain of aeration of previously non aerated lung tissue. CT scan quantitative analysis may detect and measure lung recruitability by acquiring the images of the lung at two different levels of airway pressure. However, in daily practice, this technique is rarely employed for clinical. Two approaches are commonly used to quantify lung recruitment at bedside. The first one relies on pulmonary mechanics using the pressure–volume curve However this model is too simplistic, as it has been repeatedly shown by CT scan that lung recruitment occurs along the entire pressure–volume curve. A second tool largely employed to investigate the effectiveness of lung recruitment is by the detection of gas exchange variations. It must be pointed out, however, that variations of gas exchange are inadequate to assess lung recruitment as shown by comparing gas exchange with CT scan It is in fact well known that a decrease in cardiac output (as always occurs during a recruitment maneuver) is associated with an increase in systemic arterial oxygenation. Therefore, in our opinion, lung CT scanning, to date, remains the gold standard for assessment of anatomical lung recruitment