Table 1 Quality assessment of included studies

Study participation

Low risk of bias was assessed if no patients group was excluded from the study cohort and when in- and exclusion criteria were adequately described.

Moderate risk of bias was assessed if the sample was not adequately described for key characteristics. (age, sex, arrest characteristics).

High risk of bias was assessed when both items were not adequately addressed.

Study attrition

Low risk of bias was assessed if there was no difference between eligible patients registered in a database and the number being analyzed. Also, there should have been no important differences between key characteristics and outcomes in participants who were analyzed the study and those who were not.

Moderate risk of bias was assessed if loss to follow-up was described but was less than 20%.

High risk of bias was assessed when loss to follow-up was not described or was >20%.

Prognostic factor measurement

Moderate risk of bias was assessed if at least the prognostic factor ‘age’ was taken into account.

Low risk of bias was assessed when comorbidity and either functional dependence or comorbidity were reported.

The prognostic factor measure and method should have been adequately valid and reliable to limit misclassification bias. The method and setting of measurement should have been the same for all study participants. When this was not the case, the risk of bias was assessed one step higher.

Outcome measurement

For the outcome ‘survival’, this item was not applicable.

The outcomes ‘quality of life’ and ‘functional status’ of survivors were assessed separately. These outcomes should have been measured using a reliable and adequately valid method, in order to assess a low risk of bias.

Confounding measurement and account

Low risk of bias was assessed when was adjusted for all relevant confounders (shockable rhythm, witnessed arrest, provision of bystander CPR, interval to bystander or EMS CPR start). If was adjusted for only one or two factors, the risk of bias was assessed as moderately.

Measurement of confounders should have been adequately valid and reliable, and method and setting of confounding measurement should be the same for all study participants.

High risk of bias was assessed when no adjustment for confounders had taken place.

Analysis

Low risk of bias was assessed when there was sufficient presentation of data to assess the adequacy of the analysis. When only the significant factors were reported in the multivariate analyses, or when adjustment factors were not reported, the risk of bias was assessed higher.