Aims
Disaster may psychologically affect the social behaviour of citizens. This study aimed to investigate whether and how the East Japan Great Earthquake and Tsunami on 11 March 2011 influenced bystander cardiopulmonary resuscitation (BCPR) in out-of-hospital cardiac arrest (OHCA).

Methods
From nationwide OHCA registry, we extracted 108,311 bystander-witnessed OHCA cases without prehospital physician-performed advance life support, which were recorded between 11 March, 2010 and 10 March, 2013. The alterations in BCPR rate after the disaster in Tsunami-affected prefectures and others were analysed by univariate and multivariable analyses. 

Results
Multivariable regression analysis following 4-weeky trend analysis after the day of disaster disclosed that BCPR rate in Tsunami-affected prefectures considerably decreased during 20-weeks period after disaster in 2011, compared with the average of 2010 and 2012 (2010&2012): BCPR rates in disaster year 2011 and 2010&2012 were 50.0% (627/1253) and 54.7% (1228/2884), respectively: adjusted OR (95% CI), 0.84 (0.72–0.9). This decrease in BCPR rate in Tsunami-affected prefectures was more prominent in the family- and friends-witnessed subgroup receiving dispatcher-assisted CPR instruction: 54.7% (580/835) and 62.1% (269/433) , respectively: 0.73 (0.57–0.93). A lower rate of dispatcher-assisted CPR instruction (2011 vs 2010&2012, 45.2% vs 48.8%) and higher proportion of presumed cardiac aetiology (60.3% vs 53.7%) were also observed during the same period of 2011 in Tsunami-affected prefectures. However, one-month survival (8.5% vs 10.1%, P = 0.13) or neurologically favourable outcome (3.9% vs 4.7%, P = 0.31) of OHCA during 20-weeks after the day of disaster did not statistically differ between 2011 and 2010&2012. In other Tsunami-unaffected prefectures, the changes in BCPR, dispatcher-assisted CPR, backgrounds and outcomes of OHCA were very small during the study period.

Conclusions
A great disaster is likely to affect the BCPR rate partly by diminishing the willingness of family and friend bystanders to follow the CPR instruction provided by dispatchers.

2 - Effect of 2011 Great East Japan Earthquake and Tsunami on bystander-performed cardiopulmonary resuscitation: A nation-wide database analysis in Tsunami-affected and the other prefectures