Lung Recruitment Strategies in ARDS: MRS Efficacy

Introduction to ARDS and Mechanical Ventilation

The symptoms of acute respiratory distress syndrome (ARDS) include cyanosis refractory to oxygen therapy, lower lung compliance, radiographic evidence of diffuse bilateral infiltrates, and a PaO₂/FiO₂ ratio of 200 mmHg or less (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 is therefore associated with a high rate of mortality.

Awareness of the association between 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 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 had directly tested this theory.

Measuring Lung Recruitment in Early ARDS

Toward 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 Maneuver Protocol and CT Monitoring

MRS maneuvers were conducted while patients were being monitored in real time 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 step lasting two minutes. This was followed by a PEEP titration phase that decreased pressure from 25 to 10 cmH₂O 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 the MRS maneuvers.

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

Key Findings and Comparison with Prior Research

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 of 5 cmH₂O between recruitment phase steps. In addition, those patients may have suffered from more severe ventilation trauma, as indicated by a mean ventilation period of five days before recruitment into the study. In the present study, patients were recruited within 72 hours of ARDS onset and a PEEP of 10 cmH₂O was maintained between 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.

Conclusion

Mechanical ventilation of patients suffering from ARDS has historically been associated with a high mortality rate. Although several studies have investigated lung recruitment strategies, none had achieved significant improvement in lung recruitment without causing additional tissue damage. A recent study by de Matos and colleagues (2012) utilizing MRS may have ended this history of marginal success by showing that potential recruitable lung tissue ranged between 25% and 53% (median 45%). Early and aggressive intervention using MRS therefore appears to represent a method of lung recruitment that successfully limits the damage caused by mechanical ventilation in ARDS patients.