Background

Marathon is generally becoming one of the most popular vigorous endurance training in the world. However, prolonged vigorous endurance training may lead to adverse functional and morphological cardiac adaptations, especially for amateur runners [1]. The incidence of sudden death in marathon runners ranges from 0.54 to 2.1/100,000, with cardiovascular diseases (CVDs) considered as one of the major triggers during long-distance competitions [2,3,4]. Although the risk of sudden cardiac death (SCD) during a sporting event is low, owing to media coverage, people may overestimate the risk of participating in a marathon and reduce their participation in sports.

Preparticipation screening examination might be specifically important to decrease the risk of sudden death during the competitions. The annual incidence of SCD in athletes decreased by 89% in Italy after a nationwide systematic preparticipation screening examination was introduced [5]. Superiority in simple operation, excellent quality and reasonable budget, the 12-lead electrocardiogram (ECG) was recommended as an objective test to increase sensitivity and specificity for preparticipation cardiovascular screening. Except personal history, family history, and physical examination, ECG provided important diagnostic and prognostic information for a variety of CVDs. While, there were still several controversies in preparticipation cardiovascular screening. Opponents criticized the ECG screening for its limitations including high costs, high false-positive rate, potential false negatives, and lack of large sample study [6]. Therefore, research data is still needed to assess the utility of the screening program and to optimize the ECG screening criteria.

Over the past decades, ECG interpretation standards for athletes were established and improved to identify potentially life-threatening pathologic cardiovascular disorders associated with SCD in athletes [7]. The ECG abnormalities were not recognized features of athletic training and always require further assessment to exclude the presence of intrinsic cardiac disease. Numerous studies have reported the prevalence of ECG abnormalities in athletes of different sports [8,9,10]. Several studies have likewise reported ECG abnormalities in marathon runners. Minns et al. reported the prevalence of ECG abnormalities in 87 volunteer runners before and after a half marathon [11]. Herm et al. presented the ECG results of 108 athletes during a marathon race with portable ECG recorders [12]. Although millions of runners participate in thousands of marathons every year, few studies with a large sample size have reported the results of ECGs of marathon runners.

Racial and gender differences in cardiovascular risk exist among athletes. The incidence of SCD is higher in black Americans compared with white Americans [13]. Moreover, abnormal T wave inversion (TWI) is more frequent in black (12%) than in white competitive athletes (< 4%) [14, 15]. Thus, determining the prevalence of ECG abnormalities in other ethnicities would be interesting. However, ECG findings in Hispanic and Asian athletes as well as in those with mixed ethnicities were not well reported [16]. Moreover, it might be beneficial to allocating medical resources reasonably, providing medical services efficiently via determining the prevalence of borderline and abnormal ECG changes. Therefore, this study aims to describe the prevalence of normal, borderline, and abnormal ECG changes in young Chinese marathon runners, which may serve as a reference for preparticipation screening for marathon events.

Methods

Participants

The Hangzhou Marathon first held in 1987 is one of the earliest marathon events for amateur enthusiasts in China. There are approximately 20,000 runners enrolling into the marathon’s competitions every year. Except for a very small number of professional elite athletes participating in the Hangzhou Marathon, most of the participants are ordinary amateur runners to improve health situations, gaining happiness or challenging themselves. The participants were 18,826 contestants from 2015 and 2016 Hangzhou Marathon (half and full categories) competitions. Before the marathon, the amateur runners were required to submit their physical examination report (a 12-lead resting ECG was strongly recommended) through the organiser website for preparticipation screening. The inclusion criteria for the participants in the current study included the following: (1) age was between 18 and 35 years old; (2) submitting 12-lead ECG report (In China, patients would get an ECG report sheet with 12-lead electrocardiogram after taking an ECG exam in their local hospitals); (3) the ECG test was conducted in a qualified county-level hospital or higher within 6 months; (4) accessing to personal information (e.g. gender, age, height, and weight); and (5) only the 2016 data were included if participants both enrolled into the 2015 and 2016 events. All participants read and signed an informed consent form and this study was approved by the Ethics Committee of the First Affiliated Hospital of the Medical School of Zhejiang University.

Procedures

The data analysis of this study was based on the ECGs, submitted by the marathon runners. The ECGs were interpreted by a group of clinical doctors and verified by reviewers with the international recommendation for ECG interpretation in athletes released in 2017 [17]. In cases with inconsistent diagnoses, a third reviewer was responsible for interpreting and making decisions. Personal information, including gender, age, height, and weight, was collected from the physical examination reports.

Data analysis

The prevalence of normal, borderline and abnormal ECG changes was presented, and chi-square tests were applied to compare prevalence rates between the male and female runners. Using the median running time of the full or half marathons as the cutoff point, the participants were divided into good and poor performance groups. Logistic regression models were utilized to determine the association between certain ECG abnormalities and sports performance in marathon. Sports performance and ECG abnormality were considered as the independent factors. Odds ratios of having ECG findings were determined in marathon runners with good performance as compared with their counterparts with poor performance, in which p values less than 0.05. All analyses were done with SPSS 20.0 (SPSS Inc.).

Results

A total of 13,079 adults, including 4896 full marathon runners and 8183 half marathon runners, meet the inclusion criteria in this study. 64.6% of the participants were young adults no older than 30 years (Table 1). More than three-quarters of the participants were men. Although the study population was composed of marathon runners, more than 20% of the male runners were overweight. Besides, most runners are still entry-level marathon enthusiasts, who had not experienced long-term intensive training. About 75% of runners took more than 4 h to complete the full marathon and nearly 80% of runners need over 2 h to finish the half marathon.

Table 1 Characteristics of the study population
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According to the 2017 international recommendation for ECG interpretation, the prevalence of training-related changes was shown in Table 2. Sinus bradycardia and sinus arrhythmia were found in approximately 14.67 and 8.09% of the participants, respectively. The prevalence of left ventricular high voltage was 3.12%, and the male runners exhibited a significantly higher incidence (4.04%) compared with the female runners (0.25%). 1.25% of runners were found to have early repolarisation. No more than 1% of the runners demonstrated first-degree atrioventricular block (AVB), Mobitz type 1 (Wenckebach) second-degree AVB, ectopic atrial or junctional rhythm, and incomplete right bundle branch block (IRBBB). It is noteworthy that, the prevalence of sinus bradycardia and sinus arrhythmia in normal weight runners is 15.8 and 6.0% respectively, which are significantly higher than the prevalence in overweight runners (14.3 and 4.0%). The results of logistic regression illustrated that the good-performance group presented higher rates for sinus bradycardia, left ventricular high voltage, right ventricular high voltage, and AVB after the age, gender, and weight status were adjusted in the models (Table 3).

Table 2 Prevalence of training related ECG changes in the male and female marathon runners
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Table 3 Odds ratios of having normal or abnormal ECG findings in marathon runners with good performance using the runners with poor performance as reference
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The prevalence of borderline ECG findings is shown in Table 4. The incidence of right axis deviation in the marathon runners was 1.78%, which was slightly higher than the incidence of left axis deviation (0.88%).0.70, and 0.54% of runners were found to have left and right atrial enlargement respectively. A total of 18 men and only 1 woman were found to have the complete right bundle branch block (CRBBB).

Table 4 Prevalence of borderline ECG findings in the male and female marathon runners
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The prevalence of abnormal ECG findings was very low, and none was greater than 1‰ (Table 5). Among the amateur runners, 0.03% (2 men and 2 women) were found to have TWI. It was noteworthy that 6 (0.06%) male and 6 (0.19%) female runners were found to have ST-segment depression. Also, several runners were found to have a premature ventricular contraction, pathologic Q waves, atrial fibrillation, and prolonged QT interval.

Table 5 Prevalence of abnormal ECG findings in the male and female marathon runners
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Discussion

Currently, the prevalence of training-related, borderline, and abnormal ECG changes in a large sample of amateur marathon runners were reported to serve as a reference for SCD prevention during marathons. It was noteworthy that the prevalence of some ECG changes in the current study were extremely low. Only 0.97 and 0.15% of the runners were found to have IRBBB and CRBBB, respectively, which was lower than the prevalence reported in previous studies (IRBBB: 7–20%, CRBBB: 2–3%) [18,19,20]. 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 [10, 14, 21, 22]. Low prevalence was also found in ST-segment depression, TWI, and pathologic Q waves. TWI was found in 22.9% of 329 adolescent black athletes and 4.5% of 903 adolescent white athletes [23]. Another study using 1000 athletes showed that the prevalence of TWI was 4% [10]. 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 was attributed to multiple reasons, such as participants’ training levels. Lots of ECG changes including AVB, RBBB, and ST-segment elevation are associated with long-term endurance training [24, 25] and could be influenced by the number of years of training in a respective sport [20]. In the current study, more than 75% of participants took more than 4 h to complete full a marathon and nearly 80% of runners need more than 2 h to finish the half marathon. Therefore, most participants in this study were 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 [26], the physical active participants may have a significant lower prevalence of abnormal ECG findings than sedentary Chinese adults and professional athletes.

The data of this study confirmed that the prevalence of borderline changes and abnormal changes in amateur marathon runners are lower than the prevalence in the ordinary Chinese population [27, 28]. For instance, it was found that the prevalence of CRBBB (men: 0.18%, women: 0.03%) and atrial fibrillation (men: 0.01%, women: 0%) is significantly lower than the prevalence in ordinary Chinese (CRBBB: men: 0.51%, women: 0.35%; atrial fibrillation: men: 0.14%, women: 0.08%) reported by Yu and her colleagues [28]. The data also confirmed that the prevalence of training related ECG changes including sinus bradycardia (men: 15.60%, women: 11.79%) and sinus arrhythmia (men: 7.98%, women: 8.42%) are much more common in amateur marathon runners as compared with ordinary Chinese with similar age (sinus bradycardia: men: 3.99%, women: 1.62%; atrial fibrillation: men: 0.94%, women: 1.78%).

Numerous studies indicated that ECG abnormalities vary across ethnicity and these variations may have implications for further diagnostic testing [7, 29]. Electrocardiographic repolarisation changes and echocardiographic left ventricular hypertrophy have been demonstrated to be more common in black athletes [15, 30]. However, most of these studies were conducted among the black and white athletes, lacking enough ECG data from Chinese athletes. The prevalence of TWI reported by Feng et al. was 1.7% in young Chinese athletes and none of the college students was found to have TWI [31]. While 4.5% of West-Asian young athletes and 15.9% of black athletes were found to have TWI [32]. The prevalence of ECG abnormalities was found relatively lower among the Chinese population when compared with South Asians groups [33]. Moreover, pathologic Q waves may also be infrequently in Chinese. Ng et al. reported that only 20 out of 18,476 young male military conscripts had abnormal Q waves [33]. Another study demonstrated that the prevalence of 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. [27]. These research shreds of evidence suggested Chinese populations may have a relatively low prevalence of ECG abnormalities.

Also, the 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 there were no obvious changes regarding prevalence of most normal and borderline ECG in the current 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 2533 athletes was 1.46 and 1.11%, respectively [34]. 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 [21, 22, 35]. It confirmed that long-term marathon training caused physiological changes in cardiac function, reflecting in ECG training-related changes. Sinus bradycardia, sinus arrhythmia, and training-related ECG changes were associated with the increased vagal tone, being common in long-distance runners [36]. The prevalence of sinus bradycardia in sedentary healthy Chinese adults is 5.99–10.51% [22, 37], which was lower than that in the marathon runners (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%) [22, 37].

This study has several limitations. Firstly, the process of data collection was not standardized. Although Chinese hospitals implement very similar operation standard of ECG examination [38], the ECG examination was conducted in different hospitals and the reports were submitted by the participants. Moreover, due to the self-submitted ECG data, the prevalence of ECG abnormalities could be underestimated. Thirdly, all the participants were Chinese in this study. As ethnicity is an important determinant of cardiac adaptation to exercise [23, 39], the results may not be generalised to other ethnicities. Fourthly, further cardiovascular examinations were not conducted in participants with positive ECG results, so the true disease rate is unclear. Moreover, the medium marathon completion time was applied as the cut-off point to divide the runners into good and poor groups because the marathon completion time in the participants was not normally distributed and most of the participants are amateurs with low performance. In addition, another limitation is the lack of a control group in the study design.

This study had several notable strengths. The size and demographic diversity of sample are significant. Despite these limitations, our study provided the new perspective in understanding the cardiac status of amateur marathon runners, offering the platform for future investigation in exploring the influence of prolonged marathon training on the heart, and the prevention of SCD during marathon events.

Conclusion

Training-related ECG changes, including sinus bradycardia, sinus arrhythmia, and left ventricular high voltage, were common in amateur marathon runners. Besides, left and right axes deviation, as well as atrial enlargement, were common borderline ECG changes. Most abnormal ECG changes, including ST-segment depression, TWI, premature ventricular contraction, pathologic Q waves and prolonged QT interval, were infrequently found in amateur marathon runners. The data also suggested Chinese amateur marathon runners may have a relatively lower prevalence of ECG abnormalities than black and white runners.