Vigorous exertion, regular exercise training, and the risk of sudden cardiac death due to myocardial infarction in Swedish men.

Background: Although physical activity is associated with cardiovascular health benefits and reduced all-cause mortality, vigorous exertion is also recognized as a trigger for sudden cardiac death (SCD). This study investigated vigorous exertion as a trigger for SCD resulting from myocardial infarction (MI) and the potential modifying effect of habitual vigorous exercise training among Swedish men who subsequently experienced SCD due to MI. Methods: This prospective nested case-crossover study was performed within the Västerbotten Intervention Programme cohort from 1985 to 2006 and included male participants who later experienced SCD caused by MI. The risk of SCD during and within 30 min of vigorous exertion was compared with the risk during periods of non-exertion. Participants were categorized into three groups according to baseline frequency of habitual vigorous exercise training to assess effect modification. Results: We included 192 men with SCD caused by MI, with a mean time from screening to event of 6.5 years. A majority of cases reported physical inactivity, with 161 cases reporting no exercise or <1 exercise event per week. 24 men suffered SCD in relation to vigorous exertion, yielding a relative risk of 43.6 (95% CI: 27.1-70.3) compared to non-exertion. The highest relative risk (107.7 [95% CI: 63.4-182.9]) was found among physically inactive men and was mitigated by a higher frequency of habitual exercise training at baseline. Conclusion: Among Swedish men who experienced SCD caused by MI, vigorous exertion was associated with a transiently increased risk, which was mitigated by higher levels of habitual vigorous exercise training.