Aims: To investigate temporal variations in qualities of bystander CPR and their association with 1-Y neurologically favourable survival.
Methods: During the period of April 2010 to December 2017, the qualities of bystander CPR were evaluated by EMS in 3,755 (88%) of 4,253 cases of EMS-unwitnessed OHCA that received bystander CPR in Ishikawa prefecture. Nighttime was defined as 9:00 PM–7:00 AM.  Nighttime/non-nighttime differences in quality of bystander CPR and the rate of neurologically favourable survival were analysed in association with classifications of OHCA location and bystander CPR performer. Multivariable logistic regression analysis was applied to elucidate factors associated with survival.
Results: When analysed for all non-EMS-witnessed OHCAs, proportion of good-quality bystander CPR or rate of survival did not significantly differ between nighttime and non-nighttime. However, nighttime decreases in CPR quality and survival were significant in OHCAs that occurred at locations other than home (good-quality CPR; 89% vs 80%, survival rate; 5.3% vs 0.5%, p<0.01 respectively) and in OHCAs received CPR by non-family lay bystander (87% vs 78%, 5.4% vs 0.9%, p<0.01 respectively). Good-quality of bystander CPR was more frequently confirmed when CPR performer had training experience, BCPR was initiated without dispatcher-assisted instruction and multiple rescuers presented (p<0.01). In multivariable logistic regression analysis including other critical factors, shockable initial rhythm (adjusted odds ratio; 95% confidence interval, 13.2; 8.3–21.1), good-quality bystander CPR (3.0; 1.3–7.0) and bystander-witnessed OHCA (4.6; 2.8–7.5) were associated with survival but nighttime/non-nighttime (0.5; 0.2–1.2) was not.  
Conclusions: Qualities of bystander CPR is likely to be associated with low chance of neurologically favourable 1-Y survival of OHCAs that occur at out-of-home locations and those receiving CPR by non-family lay bystanders during nighttime. Recruitment of trained, multiple rescuers may be needed to improve the survival of out-of-home OHCAs during nighttime.

Are qualities of bystander CPR associated with low chance of neurologically favourable 1-Y survival of EMS-unwitnessed OHCAs during nighttime?