Data extraction and quality assessment
Two independent reviewers (Y.L and D.C) extracted the data on prespecified forms: authors, year of publication, study design, sample size, research center, follow-up time, time of CRT implantation, indication, adjusted covariates, endpoints and aHR value with its 95% confidence interval (CI). When different adjusted models existed in one study, we extracted the HR value with the greatest number of adjusted confounding variables. In instances when incomplete data were obtained, the authors were contacted in writing for permission to obtain further data.
The quality of each study was assessed independently by the same two investigators using the Newcastle-Ottawa scale (NOS), a nine-star scale for nonrandomized study quality assessment in meta-analyses. Briefly, this scale has three broad perspectives as follows: selection, comparability and outcome of studies. Studies awarded more than six stars were considered high-quality studies.