Modelling seasonal and weather dependency of cardiac arrests using the covariate order method.

A data set concerning cardiac arrests treated by the Emergency Medical Service in Trondheim during a nine year period is analysed. The relationship between the occurrence of cardiac arrest and covariates related to weather and season is examined. The covariate order method is used in the analysis of the data. It is explained how this method can be extended to recurrent event data, and the practical usefulness and flexibility of the method is demonstrated in these analyses. In the analyses a significant relationship between outdoor air temperature, or factors closely related to outdoor air temperature, and the occurrence of cardiac arrest is found. The incidence of cardiac arrest decreases with increasing temperature. Further a significant effect of snowfall is also found, with increased intensity of cardiac arrest on days with snowfall. A more borderline significant effect of precipitation is also identified.