Circadian variation in witnessed out of hospital cardiac arrest

Objectives-To examine the effect on circadian variation of out of hospital cardiac arrest according to the underlying aetiology and presenting rhythm of arrest, and to explore strategies that might help to improve survival ou tcome using circadian variation. Design-Population based retrospective study. Setting-County of Nottinghamshire with a total population of. 993 914 and a n area of 2183 km(2). Subjects-Between 1 January 1991 and 3 December 1994, all witnessed cardiac arrests attended by the Nottinghamshire Ambulance Service, of which 1196 pa tients had a cardiac cause for their arrest (ICD, 9th revision, codes 390-4 14 and 420-429) and 339 had a non-cardiac cause. Results-The circadian variation of the cardiac cases was not significantly different from that of non-cardiac cases (p = 0.587), even when adjusted fo r age, sex, or presenting rhythm of arrest. For cardiac cases, the circadia n variation of those who presented with ventricular fibrillation was signif icantly different from those presenting with a rhythm other than ventricula r fibrillation (p = 0.005), but was similar to the circadian variation of b ystander cardiopulmonary resuscitation (p = 0.306) and survivors (p = 0.542 ). Ambulance response time was also found to have a circadian variation. Conclusions-There is a common circadian variation of out of hospital cardia c arrest, irrespective of underlying aetiology, when the presenting rhythm is other than ventricular fibrillation. This is different from the circadia n variation of cases of cardiac aetiology presenting with ventricular fibri llation. The circadian variation of ventricular fibrillation, and consequen tly survival, may be affected by the availability of bystander cardiopulmon ary resuscitation and the speed of ambulance response.