IJCS | Volume 32, Nº4, July/August 2019

403 Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article Figure 5 - Theoretical model of the sports training process. Adapted from Impellizzeri et al. 57 became easy to obtain instantaneous HR by measuring the duration of each cardiac cycle or the RR-interval. Therefore, these devices allowed the collection of data for analysis of both cardiac rhythmand changes of HR in short intervals and HR variability by mathematical techniques. Detection of cardiac arrhythmias Cardiac arrhythmias may occur without symptoms and thus, may not be perceived by the patients. This is common in clinically relevant arrhythmias. Thus, with the advent of digital technology, which facilitates real- time recording and storage of a great amount of data, and allows a fast, efficient transmission and even distance and real-timemonitoring, there has been a growing interest in the use of monitors (some of them in the formof watches) and mobile apps for HR reading. This subject was initially discussed in case reports, 60,61 small studies, 62,63 and data series, and more recently has been studied in epidemiological investigations including huge samples, 64 that have benefited from machine learning and big data strategies. However, it is not the aim of this brief review to present a deeper discussion about this interesting theme. It is worth noting that, due to a fast and encouraging progress, the possibility to correctly diagnose arrhythmias using monitors and smartphone apps has become more and more a reality in Cardiology. 65 Also, very soon, with