Lung Recruitability in Early ARDS

Lung Recruitability in Early ARDS

Lung Recruitment in ARDS

The symptoms of acute respiratory distress syndrome (ARDS) include cyanosis refractory to oxygen therapy, lower lung compliance, radiographic evidence of diffuse bilateral infiltrates, and PaO2/FiO2 ? 200 mmHg (reviewed by LaFollette, Norton, DiRocco, Carney, and Nieman, 2006). Problems arise when normal tidal volumes are used to ventilate ARDS lungs, because much of the lung tissue is refractory to recruitment. As a result, ventilator-induced lung injury (VILI) occurs due to lung distention. Mechanical ventilation of patients with ARDS therefore is associated with a high rate of mortality.

Awareness of the association between a poor prognosis and mechanical ventilation in ARDS is several decades old, yet considerable controversy remains concerning the optimal treatment approach (reviewed by de Matos et al., 2012). There is some consensus that positive end-expiratory pressure (PEEP) should be titrated during recruitment maneuvers, to minimize lung distention. Some clinicians have even argued for the efficacy of the so-called 'open-lung hypothesis', which states that collapsed lung tissue in early ARDS can be reclaimed with acceptable clinical costs. To date this hypothesis has received no empirical support, primarily because no studies have directly tested this theory.

Measuring Lung Recruitment in Early ARDS

Towards the goal of assessing the long-term efficacy of maximum recruitment strategy (MRS), 51 patients suffering from severe early ARDS were managed using MRS and followed for at least three days (de Matos et al., 2012). The mean age of the study group was 50.7 years, with a range from 14 to 80 years of age. Most were suffering from primary ARDS (84%) and sepsis (71%).

MRS maneuvers were conducted while patients were being monitored live by computed tomography (CT) (de Matos et al., 2012, Figure 1). The recruitment phase involved pressure controlled ventilation that increased PEEP from 10 to 45 cmH2O in four steps, with each lasting two minutes. This was followed by a PEEP titration phase that decreased cmH2O from 25 to 10 in four steps. At the end of each end-expiratory pause, the lungs were imaged by CT. The only complications observed were transient decreases in blood pressure in two patients, which were not severe enough to interrupt MRS maneuvers.

The results of this study revealed that the median recruitable lung tissue was 45%, far higher than that revealed by previous studies (de Matos et al., 2012). Surprisingly, no evidence of barotrauma was revealed by CT imaging following MRS and no clinical complications that could be associated with MRS were detected.

These results differ significantly from a previous study by Gattinoni and colleagues (2006), possibly because the methodology and patient population differed. For example, Gattinoni and colleagues used a low PEEP (5 cmH2O) between recruitment phase steps. In addition, the patients may have suffered from more severe ventilation trauma as indicated by a mean ventilation period of 5 days before recruitment into the study. In the present study, patients were recruited into the study within 72 hours of ARDS onset and a PEEP of 10 cmH2O was maintained between the recruitment phase steps. These differences, de Matos and colleagues (2012) suggest, explain why Gattinoni and colleagues found a mean value for potential recruitable lung tissue of only 13 ± 11%.

Notably, the findings of de Matos and colleagues (2012) suggest that baseline CT scans, or the amount of non-aerated tissue, cannot reliably predict an individual patient's response to the MRS maneuver. In other words, the severity of ARDS is not predictive of the efficacy of an MRS intervention as designed by de Matos and colleagues. This explains why no correlation was found between mortality and lung recruitment potential as revealed by MRS maneuvers.