Objective: To determine whether there are seasonal variations in survival following out of hospital cardiopulmonary arrest.
Design: Prospective cohort study using the Heartstart (Scotland) database.
Setting: All of Scotland.
Patients: 10 890 people who suffered out of hospital cardiopulmonary arrest in the summer or winter between December 1988 and August 1997 inclusive.
Intervention: Univariate comparisons of 5406 arrests occurring in summer with 5484 in winter, in terms of patient characteristics, management, and survival using chi(2) and Mann-Whitney U tests. Multivariate analysis of the association between season and survival following adjustment for case mix.
Main outcomes measures: Survival to discharge from hospital, survival pre-admission, in-hospital survival.
Results: Only 6% of people who arrested in winter survived to discharge, compared to 8% of those who arrested in summer (odds ratio 0.77, p < 0.001). People who arrested in winter had a poorer risk profile in that they were older, more likely to arrest at home, less likely to have a witness, and less likely to receive defibrillation. However, after adjustment for case mix, people who arrested in winter were still 19% less likely to survive compared to those who arrested in summer. Deaths pre-admission were significantly higher in winter (odds ratio 1.18, p < 0.05) but in-hospital deaths were not.
Conclusions: People who suffer cardiopulmonary arrest in winter have a significantly lower likelihood of surviving. This is, in part, caused by the higher frequency of a number of recognised risk factors. However, their prognosis remains poorer even after adjustment for these factors.