title: Seasonal variations of ventricular arrhythmia clusters in defibrillator recipients.
abstract: BACKGROUND
  Clustering ventricular arrhythmias are the consequence of acute ventricular electrical instability and represent a challenge in the management of the growing number of patients with an implantable cardioverter-defibrillator (ICD). Triggering factors can rarely be identified.
  
  
  OBJECTIVE
  Several studies have revealed seasonal variations in the frequency of cardiovascular events and life-threatening arrhythmias, and we sought to establish whether seasonal factors may exacerbate ventricular electrical instability leading to arrhythmia clusters and electrical storm.
  
  
  METHODS
  Two hundred and fourteen consecutive defibrillator recipients were followed-up during 3.3 +/- 2.2 years. Arrhythmia cluster was defined as the occurrence of three or more arrhythmic events triggering appropriate defibrillator therapies within 2 weeks. Time intervals between two clusters were calculated for each month and each season, and were compared using Kruskal-Wallis test and Wilcoxon-Mann-Whitney test with Bonferroni adjustment.
  
  
  RESULTS
  During a follow-up of 698 patient years, 98 arrhythmia clusters were observed in 51 patients; clustering ventricular arrhythmias were associated with temporal variables; they occurred more frequently in the winter and spring months than during the summer and fall. Accordingly, the time intervals between two clusters were significantly shorter during winter and spring (median and 95% CI): winter 16 (5-19), spring 11.5 (7-25), summer 34.5 (15-55), fall 50.5 (19-65), P = 0.0041.
  
  
  CONCLUSIONS
  There are important seasonal variations in the incidence of arrhythmia clusters in ICD recipients. Whether these variations are related to environmental factors, change in physical activity, or psychological factors requires further study.
authors.affiliation: Swiss Cardiovascular Center Berne, University Hospital, Berne, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Stuber
authors.forename: Christa
authors.initials: C
authors.lastname: Eigenmann
authors.forename: Etienne
authors.initials: E
authors.lastname: Delacrétaz
mesh_terms: D017147:Defibrillators, Implantable / Q000706:statistics & numerical data*; D004554:Electric Countershock / Q000706:statistics & numerical data*; D004868:Equipment Failure; D005260:Female; D006801:Humans; D015994:Incidence; D008030:Lightning; D008297:Male; D008875:Middle Aged; D010507:Periodicity; D018570:Risk Assessment / Q000379:methods*; D012307:Risk Factors; D012621:Seasons; D013557:Switzerland / Q000453:epidemiology; D017180:Tachycardia, Ventricular / Q000453:epidemiology* / Q000517:prevention & control*