Recruitment : How Much, How Long?

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 Mechanical ventilation may induce per se a lung injury when leading to unphysiological stress and strain, inflammatory response and mechanical lesions. It is widely accepted and proved effective, that low tidal volume ventilation (6 mL/kg ideal body weight) and airway plateau pressures limited to 30 cmH2O may prevent lung injury limiting global stress and strain. This settings are part of a more integrated ventilator strategy known as “lung protective strategy” which includes also the prevention of intra-tidal collapse of pulmonary units by providing a PEEP value sufficient to keep the lung open throughout the respiratory cycle. The adequate PEEP selection and its efficacy in lung injury prevention, however, has not been proved and is still subject of debate. The most recent and largest clinical studies on PEEP application (ALVEOLI, LOV and ExPress Study) were not able to find any difference in outcome between patients ventilated with high vs. low PEEP values. However, in the ExPress and LOV studies it has been clearly shown that patients randomized to higher PEEP had a significantly lower rate of application of rescue therapy, finally leading to a survival benefit in the most severe ARDS patients. It is conceivable, that the best way for setting high or low PEEP levels is on the severity of the pathology and on the potential for lung recruitment evaluated by CT scan. The debate on PEEP selection includes also the basis on which an adequate level may be applied at the bedside. Several methods have been proposed over the years: lung mechanics (setting PEEP 2 cmH2O greater than lower inflection point, analyzing the shape of the inspiratory/expiratory pressure-time curve, considering the changing of the compliance of the respiratory system, testing the gas exchange variations (oxygenation or CO2 decrease). Recently it has been proposed to evaluate the best PEEP according to the esophageal pressure measurement in order to keep the lung open. We found no differences between the various methods, equally inadequate to cope with lung recruitability.