Seven studies (3,404 participants, range 96 to 1,212) were included: one (1,212 participants) randomised controlled trial (RCT) and six (2,192 participants) observational studies. Mean follow-up ranged from at least seven months to 58 months
The RCT was at moderate risk of selection and attrition bias and low risk of detection and performance biases. One observational study was at high risk of selection bias and others were at moderate risk. One study was at high risk of attrition bias and others at low risk. One study was at moderate risk of performance bias and the others were at low risk. All were at low risk of detection bias. Tests showed no evidence of publication bias.
At longest follow up, compared to CRT, CRT-D was associated with a reduction in all-cause mortality (OR 0.59, 95% CI 0.43 to 0.81; Ι²=63.9%; six studies), sudden cardiac death (OR 0.20, 95% CI 0.07 to 0.59; Ι²=64.7%; four studies) and heart failure-related mortality (OR 0.60, 95% CI 0.47 to 0.76; Ι²=0%; four studies). There was a reduction in all-cause mortality after one year (Ι²=64.9%; three studies) but not during the first year (Ι²=6.7%; six studies). There was a reduction in sudden cardiac death after one year (Ι²=75.3%; three studies). There was insufficient data for further analyses related to length of follow-up.
Sensitivity analyses that excluded the RCT showed similar results to the main analyses except for heart failure-related mortality which showed no statistically significant difference between treatment groups.
Figures indicated that results for the RCT alone showed no statistically significant difference between treatment groups for all-cause mortality at one year and at longest follow up.