A marathon is a vigorous long-term exercise that challenges human limits and imposes a heavy cardiac load. In response to prolonged exercise, physiological and anatomical adaptations can develop in healthy marathon runners. In the current study, the prevalence of ‘training-related’, borderline and abnormal ECG changes in a large sample of amateur marathon runners was reported to serve as a reference for SCD prevention during marathons.
It is noteworthy that the prevalence of some ECG changes in the current study are quite low. Only 0.97% and 0.15% of the runners were found to have incomplete and complete RBBB, respectively, which is obviously lower than the prevalence reported in previous studies (IRBBB: 7–20%, CRBBB: 2–3%) (25–27). The prevalence of AVB in the athletes in this study (0.34%) was also significantly lower than that reported in previous studies (approximately 4.5–7.5%) and similar to that in sedentary healthy adults in other studies (17, 21, 28, 29).Low prevalence were also found in ST segment depression, TWI and pathologic Q waves. TWI was found in 22.9% of 329 adolescent black athletes and in 4.5% of 903 adolescent white athletes (30). Another study using 1,000 athletes showed that the prevalence of TWI was 4% (17). However, in the current study, no more than 0.1% of the amateur marathon runners exhibited ST segment depression, TWI, premature ventricular contraction, pathologic Q waves and prolonged QT interval.
The large difference in prevalence rates can be attributed to multiple reasons, one of which could be participants’ training levels. Lots of ECG changes including AVB, RBBB and ST segment elevation are associated with long-term endurance training (31, 32) and could be influenced by the number of years of training in a respective sport (27). In the current study, most runners are still entry-level marathon enthusiasts. More than 75% of participants took more than 4 hours to complete full marathon and nearly 80% runners need more than 2 hours to finish half marathon. Therefore, most participants in this study are physical active adults and different from professional athletes who receive long-term intensive training. Due to the U-shaped relationship between physical activity and cardiac abnormality morbidity (33), the physical active participants may have significant lower prevalence of abnormal ECG findings than sedentary Chinese adults and professional athletes.
Numerous studies indicated that ECG abnormalities vary across ethnicity and these variations may have implications for further diagnostic testing(34, 35). Electrocardiographic repolarisation changes and echocardiographic left ventricular hypertrophy have been demonstrated to be more common in black athletes (22, 36). However, most of these studies are based on black and white athletes, and limited data on ECG data of Chinese athletes were reported. The prevalence of TWI reported by Feng et al. was 1.7% in young Chinese athlete and none of the college students was found to have TWI(37). While 4.5% of West-Asian young athletes and 15.9% of black athletes were found to have TWI (38). Another study reported the prevalence of ECG abnormalities in South-East Asians, which also showed that Chinese have lower prevalence of T wave abnormalities than South Asians (39). Moreover, pathologic Q waves may also be infrequently in Chinese. Ng et al reported that only 20 out of 18476 young male military conscripts had abnormal Q wave (39). Another study demonstrated that the prevalence pathologic Q waves was 1.8% in ordinary Chinese (35–54 years old) which was also significantly lower than the data in the U.S. (40).These research evidences suggested Chinese populations may have a relatively low prevalence of ECG abnormalities.
In addition, possibility of selection bias may also be an important reason for the low prevalence of ECG abnormalities found in this study. The data of this study came from the self-submitted ECG report by the marathon runners. Therefore, the prevalence of ECG abnormalities could be underestimated, as some marathon runners maybe not willing to submit their ECG report if they had serious cardiac problems.
It is noteworthy that the prevalence of most normal and borderline ECG in the current study are similar to the data in previous studies. Left and right axes deviation is classified by the new criteria as a borderline training-related ECG change. Gati et al. reported that the prevalence of left and right axes deviation in 2,533 athletes was 1.46% and 1.11%, respectively (41). In the current study, 1.56% and 0.82% of the marathon runners were found to have right and left axes deviation respectively, which is similar to the results reported by Gati et al. Furthermore, the data suggested that no significant differences existed in the incidence of left and right axes deviation between amateur marathon runners and sedentary healthy adults (28, 29, 42).The data of the current study confirmed that long-term marathon training can cause physiological changes in the heart of runners, which was reflected in ECG training-related changes. Sinus bradycardia and sinus arrhythmia, which are associated with increased vagal tone, as well as training-related ECG changes are common in long-distance runners (43). The prevalence of sinus bradycardia in sedentary healthy Chinese adults is 5.99–10.51%(29, 44), which was lower than that in the marathon runners in this study (14.67%). The prevalence of sinus arrhythmia in the participants was 8.09%, which is similar to that in sedentary healthy adults (5.38–8.14%) (29, 44).
Several limitations should be acknowledged when the results are interpreted. Firstly, the prevalence of ECG abnormalities could be underestimated due to the self-submitted ECG data. The results from this population could be translated to others with extreme caution. Secondly, all the participants of this study were Chinese. As ethnicity is an important determinant of cardiac adaptation to exercise (30, 45), the results may not be generalised to other ethnicities. Thirdly, further cardiovascular examinations were not conducted in participants with positive ECG results so the true disease rate is unclear. The strengths of this study include the large sample size and the special study population. Despite these limitations, this study is beneficial in understanding the cardiac status of amateur marathon runners, the influence of prolonged marathon training on the heart and the prevention of SCD during marathon events.