Ventricular arrhythmias peak in wintertime

December 09, 2003

Berlin, Germany - German researchers have shown there is a seasonal variation in the occurrence of ventricular tachycardia (VT) and ventricular fibrillation (VF), with a peak during wintertime. Dr Dirk Muller (Free University, Berlin, Germany) and colleagues report their findings in the December 2003 issue of the American Heart Journal[1].

"We found a clear annual variation of shock episodes, with the highest incidence of VT and VF episodes in January and the lowest incidence in June. These findings are consistent with the results of three recently published studies," Muller and colleagues say.

 
We found a clear annual variation of shock episodes, with the highest incidence of VT and VF episodes in January and the lowest incidence in June.
 

Over a period of 11 years, they registered each appropriate shock episode in patients with implanted cardioverter defibrillators (ICDs) and retrospectively analyzed the annual distribution of out-of-hospital VT/VF terminated by ICD therapy. A total of 308 patients with an ICD were included in the analysis. During the follow-up period of 37+23 months, 233 of the 308 patients (76%) had at least one sustained out-of-hospital episode of VT/VF terminated by an appropriate ICD discharge. Of the shock episodes, 753 were categorized as appropriate and included in the study. The highest number of shock episodes occurred in January (93) and the second highest in November (83). The fewest shock episodes occurred in June (39), revealing a significant variability in the annual distribution of shock episodes, with a peak during winter (p=0.001).

Ischemic events alone not responsible

Discussing the other recently published studies in the field, Muller et al say one in LA showed a frequency of deaths from November through February that was 35% higher than in other periods of the year. Another, from Berlin, also demonstrated a seasonal variation of cardiac deaths with a higher incidence in the winter. These data included sudden death from various causes such as VT/VF, asystole, pulseless electrical activity, or pulmonary embolism.

"However, there is no information about the annual variation of VT and VF episodes independent of myocardial infarction" in that Berlin study, Muller et al say. "Our study...shows a winter peak of arrhythmic events independent of coronary events, due to the fact that none of the 753 shock episodes [we] analyzed were associated with myocardial infarction."

Also, the idea that coronary ischemia may, at least in part, have been responsible for the higher incidence of shock episodes in the winter is not supported by the new findings, say the German doctors. Approximately one third of their study population had no ischemic heart disease, and the annual distribution of shock episodes in these people was similar to that in the subgroup with coronary artery disease. "This suggests that ischemic events are not primarily responsible for the annual distribution of VT/VF episodes."

What are the causes? Could it be lack of daylight? If so, how about phototherapy?

Muller et al go on to discuss a number of possible causes for their findings, including climatic factors such as temperature, air pressure, and duration of daylight. Overindulgence due to the holiday season may also play a role, they note.

 
While temperature may be a contributor to seasonal differences in cardiovascular death rates...it does not solely explain the winter increase.
 

In an accompanying editorial[2], Drs Eric S Williams and Douglas P Zipes (Indiana University School of Medicine, Indianapolis) note that most prior reports of seasonal variation have described patients with known coronary heart disease and that most, but not all, have shown a significant winter increase in cardiac death, especially in the Northern Hemisphere during December and January.

But temperature cannot be the only culprit, they argue, because the winter increase in CV death has been seen in LA, where the seasonal temperature range is limited, and in Kuwait, where winter is the most comfortable season, with an average January temperature of 18C. And although "holiday-related" clustering of events has also been suggested, there is a winter increase in cardiac events in Australia and New Zealand, in June, when there are no big holidays.

"A reasonable conclusion of the available studies is that while temperature may be a contributor to seasonal differences in cardiovascular death ratesparticularly when temperatures are extreme and in patients with underlying ischemic heart diseaseit does not solely explain the winter increase," they state.

 
The short days of winter—independent of temperature—could thereby . . . play a role in susceptible individuals in precipitating cardiovascular events, including ventricular arrhythmias and sudden death.
 

Williams and Zipes go on to discuss alternative or contributing mechanisms for these seasonal differences. "It is possible that entrainment of the circadian clock by the day length (photoperiod) may contribute to a circannual rhythm as well. The short days of winterindependent of temperaturecould thereby affect hormonal, electrophysiologic, and hemodynamic changes that could play a role in susceptible individuals in precipitating cardiovascular events, including ventricular arrhythmias and sudden death."

They say that if this is shown to be the case, it is possible that treatment with light, in a similar way to that proposed for seasonal affective disorder, might be beneficial. "Continued research toward a clearer understanding of the mechanisms and reversibility of cardiovascular triggers could lead to novel treatments and means of prevention," they conclude.

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