Background: As in myocardial infarction and transient ischaemia, out-of-hospital sudden cardiac death has an increased morning incidence. However, sudden death occurring in hospital is evenly distributed over the 24 h period suggesting that there might be subgroups of patients with atypical circadian patterns of sudden death. Patients who received an implantable defibrillator constitute an ideal group for studies of circadian patterns of sudden death since this generation of devices are able to store the exact time when defibrillation occurred.
Methods: The distribution of sudden death aborted by the implantable defibrillator was analysed during the 24 h period for 87 presumed appropriate shocks delivered in a group of 22 patients, 18 men and four women, 58.7 +/- 11.9 years old and with a mean left ventricular ejection fraction of 39.4 +/- 17.6%.
Results: Each patient received an average of 4.42 +/- 3.04 shocks during a mean follow-up of 9.4 +/- 5.6 months. Apart from a clear tendency for shocks to occur during the morning hours (42% of total shocks), five of 16 patients who received multiple shocks also showed a trend to repeat the shocks around the same period during the day.
Conclusion: Our results support the accepted view that changes in autonomic tone in the early morning play a role in the circadian variations of sudden death. Sudden death not only occurs more frequently in the morning hours, but it also clusters in certain periods for individual patients.