Purpose
Early cardiopulmonary resuscitation (CPR) with a considerable quality is essential for making incidence of shockable rhythm on out-of-hospital cardiac arrests (OHCAs). The present study was conducted to test our hypothesis that dispatcher assisted CPR improve the incidence of shockable rhythm for the patients in the residence by family bystander.

Methods
A total of Utstein-style 123,421 OHCAs patients in 2015 in Japan’s nationwide registry were enrolled. We excluded non-witnessed, public access defibrillation, the patients for whom the time exceeded the 99 percentile of collapse to EMS contact interval and the collapsed after EMT contact data. The primary end point was adjusted incidence of shockable rhythm as initial cardiac rhythm. 
The CPR with dispatcher assistance were categorized into three groups as a dispatcher assisted CPR (DA-CPR), bystander-initiated CPR (BI-CPR) and No CPR. Patients were categorized into 3 groups according to their ages as 15-64 yo, 65-74 yo and 75-84 yo. The bystander is also categorized 3 groups as families, friends and others. 
From the survival plot of incidence of shockable rhythm, the proportional hazard between DA-CPR and BI-CPR exists greater than 6 min of response time (119 call to first EMT contact). The Cox’s proportional hazards regression model was used to test the significance of shockable rhythm adjusting for age, response time, gender, and dispatcher assistance.

Results 
The total of 9,838 cases with family bystander has occurred in the residence. Adjusted hazard ratio and 95% confidence interval for using BI-CPR as reference on the DA-CPR; 1.39 (1.03-1.94) on 15-64 yo. , 0.98 (0.72-1.41) on 65-74 yo. and 0.82 (0.60-1.04). 

Conclusion
The dispatcher assistance significantly improves the incidence of shockable rhythm on 15-64 yo. after 6 min of response time. Our observational study should be considered to make a better instruction for emphasizing the family by dispatcher assistance.

6 - Dispatcher assistance makes significantly better incidence of shockable rhythm for out-of-hospital cardiac arrest patients in the residence with family bystander.