Unexpected shift in circadian and septadian variation of sudden cardiac arrest: the Oregon Sudden Unexpected Death Study

Background: Earlier studies have reported both early morning and Monday peaks in occurrence of sudden cardiac arrest (SCA) in the community and appropriate defibrillator shocks in patients with an implantable cardioverter–defibrillator (ICD). However, a more recent analysis of ICD shocks reported absence of these peaks. Objective: The purpose of this study was to perform a contemporary evaluation of the circadian and septadian variation of SCA in the general population. Methods: The analysis was performed from an ongoing, population-based study of SCA in a Northwestern US community of approximately 1 million residents. To maximize accuracy, we focused on consecutive patients who presented with witnessed SCA and were attended by emergency medical services (EMS). The specific time of each SCA event was determined based on the time of the 911 call to EMS. Results: During 2002–2014, we identified 1535 patients age ≥18 years who suffered witnessed SCA, with time of first EMS contact recorded. There was no morning (6 AM to 12 PM) peak, and we observed a nadir in SCA events during 12 AM to 6 AM, with only 13.9% of events occurring during this 6-hour block (P <.0001). There was no peak on Mondays, but a nadir was observed on Sundays that accounted for only 11.3% of SCA events during the week (P =.004). Conclusion: in this contemporary community-based study, we failed to observe the expected morning peak or the Monday peak in SCA, duplicating recent findings in primary prevention defibrillator patients. The significant public health implications of these findings merit further investigation.