Cardiorespiratory fitness is related to the risk of sudden cardiac death: a population-based follow-up study

Objectives: Our aim was to examine the relation of cardiorespiratory fitness with sudden cardiac death (SCD) in a population-based sample of men.

Background: Very limited information is available about the role of cardiorespiratory fitness in the prediction of SCD.

Methods: This population study was based on 2,368 men 42 to 60 years of age. Cardiorespiratory fitness was defined by using respiratory gas exchange analyzer and maximal workload during cycle ergometer exercise test.

Results: During the 17-year follow-up, there were 146 SCDs. As a continuous variable, 1 metabolic equivalent (MET) increment in cardiorespiratory fitness was related to a decrease of 22% in the risk of SCD (relative risk: 0.78, 95% confidence interval: 0.71 to 0.84, p<0.001). In addition to cardiorespiratory fitness, ischemic ST-segment depression during exercise testing, smoking, systolic blood pressure, prevalent coronary heart disease, family history of coronary heart disease, and type 2 diabetes mellitus were related to the risk of SCD. The Harrell C-index for the total model discrimination was 0.767, while cardiorespiratory fitness provides modest improvement (from 0.760 to 0.767) in the risk prediction when added with all other risk factors. The integrated discrimination improvement was 0.0087 (p=0.018, relative integrated discrimination improvement 0.11) when cardiorespiratory fitness was added in the model. However, the net reclassification index (-0.018) was not statistically significantly improved (p=0.703).

Conclusions: Cardiorespiratory fitness is a predictor of SCD in addition to that predicted by conventional risk factors. There was a slight improvement in the level of discrimination, although the net reclassification index did not change while using cardiorespiratory fitness with conventional risk factors.