ABC | Volume 115, Nº1, July 2020

Original Article Benchimol-Barbosa et al. Dynamic AV-conduction to RR-interval Coupling Arq Bras Cardiol. 2020; 115(1):71-77 making the average slope negative in the Athletes group (Figure 2b). Conversely to these studies, no blocked P -wave was found after a carefully revision of signals. Spontaneous episodes of decremental conduction were frequent in the Athletes group (57.9% of aggregated ECG recording time of Athletes) and rare in the Control group (7.9% of aggregated ECG time recording of Controls). Furthermore, when the AVCT vs. RR-interval regression slope was plotted against MET, it was observed that the higher theMET, the more negative the RR- AVCT slope , showing that spontaneous decremental AV conduction becomes more frequent as physical conditioning status improves (Figure 2a). Noteworthy was that the decremental conduction was more frequently observed when RR-interval was larger than 1022.0 ms. PR-peak interval reduction related to RR interval increase was also observed in Athletes (Figure 1d). A possible explanation for this latter finding is the common occurrence of vagal-induced spontaneous para-sinus pacing activity. It has been shown that the resting ECG of endurance athletes may show distinctive features from demographically equivalent healthy sedentary subjects, bearing similarities to those observed in elderly individuals and/or patients with cardiovascular disease. 18 However, in athletes, AV conduction abnormalities have been related to higher parasympathetic activity, differently from the elderly.19 Contemporary studies have shown that athletic training could induce intrinsic physiological adaptations in the conduction system, contributing to the higher prevalence of AV conduction abnormalities. 11-13 The physiological mechanisms by which endurance training induces those intrinsic changes in the cardiac conduction system are limitedly understood and may be multifactorial, but anatomic changes such as atrial and ventricular dilation has been shown to create a mechanical-to-electrical remodeling necessary to cause intrinsic AV electrophysiological adaptations. 7,11 Limitations of the present study include the sample size of two distinct groups regarding the physical conditioning status. Raw ECG signals were obtained from the ECG database available in the Biomedical Engineering Program (convenience sample). Peak-P to Peak-R interval was employed as a surrogate of conventional PR-interval measurement. Although it has been shown that Peak-P to Peak-R interval appropriately describes PR- interval dynamicity, the duration of the actual PR-interval may be larger than the one observed in the present study. It was observed that both M-AVCT and SD-AVCT were larger among athletes when compared to controls, although statistical significance was not reached. The explanation of this finding may be twofold: i) although AVCT variability was expected to be higher among athletes, no true Mobitz I block was in fact observed after careful signal revision. This indicates that AVCT variability was expected to be increased to a limited extent, and ii) the sample size of the present sample was designed to determine differences related to ventricular activation total energy 6 , thus limiting its statistical power to detect AVCT variation between groups. Subjects were kept on supine rest for 10 minutes before ECG acquisition, aiming at preventing orthostatic memory effect on AV conduction to influence AV conduction to RR-interval coupling dynamicity, in a controlled temperature and acoustically isolated environment. However, it cannot be completely ruled out that some orthostatic memory effect might still be present. In this study, we observed the occurrence of spontaneous PR‑peak interval enlargement as RR-interval decreased, and vice-versa. This phenomenon was interpreted as a manifestation of decremental conduction during the transit of the cardiac activation wave-front through the AV node. However, due to the nature of this study, no invasive electrophysiological test was carried out to further characterize decremental conduction or para-sinus pacing activity. It is still necessary to investigate the potential impact of the present findings on clinical settings, such as a marker for supraventricular tachyarrhythmia, particularly AV nodal reentrant arrhythmia and atrial fibrillation. Conclusion The atrioventricular node undergoes substantial physiological remodeling in elite long-distance runners, characterized by spontaneous AV decremental conduction at supine rest, rarely observed in healthy sedentary subjects under the same resting conditions. The linear regression line slope of PR-peak to RR-interval coupling is a strong and independent explanatory variable of maximal metabolic equivalent achieved during stress test in this population. Acknowledgement We wish to thank Dr. Moacir Marocolo who recruited the study subjects and acquired ECG signals, contributing to the ECG database of the Biomedical Engineering Program. The ECG signals acquisition was carried out with the participation and under the personal guidance of Dr. Paulo Roberto Benchimol-Barbosa. Author contributions Conception and design of the research: Benchimol-Barbosa PR, Nasario-Junior O, Nadal J. Acquisition of data: Benchimol- Barbosa PR. Analysis and interpretation of the data: Benchimol- Barbosa PR, Nasario-Junior O, Nadal J. Statistical analysis: Benchimol-Barbosa PR, Nadal J. Obtaining financing: Nadal J. Writing of themanuscript: Benchimol-Barbosa PR, Nasario-Junior O, Nadal J. Critical revision of the manuscript for intellectual content: Benchimol-Barbosa PR, Nasario-Junior O, Nadal J. Potential Conflict of Interest No potential conflict of interest relevant to this article was reported. Sources of Funding This study was funded by FINEP, CNPq and CAPES. Study Association This study is not associated with any thesis or dissertation work. Ethics approval and consent to participate This study was approved by the Ethics Committee of the Instituto Nacional de Cardiologia under the protocol number 0026/20.02.04. All theprocedures in this studywere inaccordance with the 1975 Helsinki Declaration, updated in 2013. Informed consent was obtained from all participants included in the study. 75