The use of low tidal volumes is a standard of care supported by a wide body of evidence that has demonstrated the efficacy of this intervention for saving the lives of patients with acute respiratory distress syndrome (ARDS) and mild ARDS (formerly acute lung injury). The ideal ventilator strategy for intensive care unit patients without ARDS remains less clear, though some studies have suggested higher morbidity and mortality rates in this population. In the October issue of The Journal of the American Medical Association, Ary Serpa Neto and colleagues conducted a meta-analysis to determine whether conventional (higher) or protective (lower) tidal volumes would be associated with lung injury, mortality, pulmonary infection, and atelectasis in patients without lung injury at the onset of mechanical ventilation.
A variety of databases were utilized to locate 2,122 articles for possible inclusion. Articles were required to evaluate two types of ventilation in patients without ARDS. Protective lung ventilation was defined as a tidal volume set for 5 mL/kg to 8 mL/kg ideal body weight (one study included 9 mL/kg compared to 12 mL/kg), and conventional ventilation was defined as a tidal volume of 9mL/kg to 12 mL/kg ideal body weight. Randomized trials, cohort studies, cross-sectional studies, and before/after studies were included. The Grades of Recommendation Assessment, Development and Evaluation (GRADE) approach was used to summarize the quality of evidence for each outcome. Pooled estimates (relative risk) were calculated, while sensitivity analyses were carried out for different subgroups of studies.
Twenty articles were identified for inclusion. All but five studies were randomized controlled studies. In fifteen studies, the reason for intubation was surgery; in the other five, the reason was mixed. The mean tidal volume in the protective group was 6.5 (standard deviation [SD] 1.1) and 10.6 (SD 1.1) in the conventional group. In the protective ventilation group, 4.2% of patients developed lung injury versus 12.7% of patients in the conventional group (risk ratio [RR], 0.33; 95% confidence interval [CI], 0.23-0.47, number needed to treat, 11). When only randomized studies were analyzed, patients in the protective ventilation group had a lower relative risk of developing acute lung injury (RR, 0.26; 95% CI, 0.10-0.66, number needed to treat, 10). Both overall mortality and hospital stay (but not ICU length of stay) was lower in the protective ventilation group.
Strengths of this study include application of the GRADE validity criteria, sensitivity analyses, and assessments for publication bias. Although it is unlikely that most relevant studies were missed with the search strategy, the strategy did not utilize additional databases, such as EMBASE, or a search of the gray literature. While homogeneity was relatively low in the meta-analysis, the included studies consisted of disparate populations, treatment locations (e.g., operating room versus intensive care unit), and tidal volumes. Finally, patients had varying lengths of follow-up, ranging from as short as 3 hours to as long as 672 hours. Despite these and other limitations, the results regarding the primary endpoint (acute lung injury) yielded large and consistent estimates of treatment effect, with narrow confidence intervals, favoring a protective lung strategy for several outcomes of interest. The results from this trial should motivate further prospective clinical trials investigating the effect of lower tidal volumes in patients without ARDS.
Concise Critical Appraisal is a regular feature authored by SCCM member Samuel M. Galvagno Jr., DO, PhD. Each installment highlights journal articles most relevant to the critical care practitioner.