Mechanical ventilation in ARDS: One size does not fit all*

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In this issue of Critical Care Medicine, Dr. Kallet and colleagues (1) report a significant improvement in mortality in patients with adult respiratory distress syndrome (ARDS) and acute lung injury (ALI) who received lung protective ventilation based on the recommendations of the ARDS Network trial of low tidal volume ventilation (ARMA) (6 mL/kg predicted body weight) compared with historical controls receiving routine care (32% vs. 51% mortality, respectively; p < .004) (1, 2). Similar to the ARMA trial, Kallet and colleagues conclude that despite the heterogeneity of underlying lung injury typically present in this patient population, their findings “strongly suggest that the low VT strategy may benefit all patients with ALI or ARDS.” In contrast, other leaders in the field believe that tidal volumes in patients with ALI and ARDS should be titrated based on surrogate markers of lung injury, such as airway pressures or lung strain (3–7). These physicians indicate that although some patients, such as those with poor pulmonary compliance and high airway pressures, would benefit from very low tidal volume, others with less severe lung injury may require larger volumes to maintain ventilation and avoid alveolar collapse.Kallet et al. conclude that ventilation with low tidal volumes in all patients with ALI and ARDS is superior to the routine care employed at the time of their historical control (1). Before adopting these recommendations, it is important to assess the consistency of these results with other available data comparing low tidal volume ventilation with contemporaneous routine practice. The historical control group reported by Kallet et al. received routine care at a participating ARDS Network center during a period of time (1/98 to 12/99) overlapping the ARMA trial (3/96 to 3/99) (1, 2). Therefore, one would expect that survival rates in low tidal volume patients in the ARMA trial would be both (1) improved compared with other patients receiving routine care at ARDS Network centers during the time of the trial (2) and uniformly better than control patients in the ARMA trial independent of their underlying level of lung injury. To investigate these questions, ARMA trial data obtained from the Office of Human Research Protections via the Freedom of Information Act was analyzed with approval from the Internal Review Board at the Clinical Center at the National Institutes of Health (8).The ARDS Network screened more than 6,000 patients for enrollment in the ARMA trial (2, 8). There were 2,587 patients who met enrollment criteria but were ineligible for technical reasons (Fig. 1). This ineligible group of patients received routine care during the course of the trial, and their mortality rate was documented at intervals similar to study participants. Therefore, this group of ineligible patients provides an interesting comparator group reflective of current practice at the time of the trial, similar to the historical control group reported by Kallet et al. Interestingly, these ineligible patients receiving routine care maintained a mortality rate comparable with the low tidal volume arm of the ARMA trial (31.7% vs. 31%, respectively) (2, 8). This is not consistent with the findings of Kallet et al. in which the historical control group mortality was significantly higher than the low tidal volume arm (51% vs. 32% mortality, respectively; p < .004). Given that the mortality rates of the low tidal volume groups were consistent (31% vs. 32%), the discrepancy in results between the ARMA trial data and that of Kallet et al.

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