More than 200 000 patients are diagnosed with acute respiratory distress syndrome (ARDS) each year accounting for 3.6 million hospital-days of annual admissions in the USA. If heterogeneity or inconsistency among the studies is detected, subgroup analysis will be conducted on factors that may cause heterogeneity. Meta-regression analyses will be performed to identify the most important factors associated with short-term mortality, and subgroup analysis will be used to analyse the following: duration of mechanical ventilation in the prone position per day, patient severity, tidal volume and cause of ARDS. For each selected trial, the risk of bias and quality of evidence will be evaluated using the GRADE system.

Two independent investigators will screen trials obtained by search eligibility and extract data from selected studies to standardised data recording forms. Randomised controlled trials comparing prone and supine positioning in adults with ARDS will be included in the meta-analysis. Method and analysis Relevant studies are collected using PubMed/MEDLINE, Embase, Cochrane Central Register of Controlled Trials and the WHO International Clinical Trials Platform Search Portal.

13 Department of Anesthesiology and Intensive Care Medicine, Kyoto Prefectural University of Medicine, Kyoto, Japan.

12 Department of Surgery, Jichi Medical University, Tochigi, Japan.

11 Biostatistics Unit at Clinical and Translational Research Center, Keio University Hospital, Tokyo, Japan.

10 Department of General Medicine, Tokyo-Kita Medical Center, Tokyo, Japan.9 Department of Anesthesiology and Critical Care Medicine, Jichi Medical University Saitama Medical Center, Saitama, Japan.8 Department of Preventive Medicine and Public Health, Keio University School of Medicine, Tokyo, Japan.7 Department of Intensive Care Medicine, Kameda Medical Center, Chiba, Japan.6 Department of Anesthesiology and Intensive Care Medicine, School of Medicine, International University of Health and Welfare, Chiba, Japan.5 Department of Emergency Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, USA.4 Department of Emergency and Critical Care Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan.3 Department of Anesthesiology and Intensive Care Medicine, Shizuoka General Hospital, Shizuoka, Japan.2 Department of Pulmonary Medicine, Keio University School of Medicine, Tokyo, Japan.1 Department of Pulmonary Medicine, Intensive Care Medicine, Keio University School of Medicine, Tochigi, Japan.

Revman 5 meta-regression