IJCS | Volume 32, Nº4, July/August 2019

400 Figure 3 - Basic structural components of a radio telemetry heart rate monitor. Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article the display, until more sophisticated ones, that include GPS and performance indexes, such as pace, average pace throughout the route, cadence, among others. GPS devices obtain components that give external training loads greater importance, as well as wider diversity of measurements. Altimeters, gyroscopes, magnetometers, accelerometers, and inclinometers provide real-time information regarding direction, amount of G-force, and vectors of force, allowing a precise control of variables that help to make better decisions. Detection of HR by HR monitors occurs primarily by two types of technology, optical sensor (PPG) and ECG. While ECG-based HR monitors work with a chest strap (Figure 3), PPG-based monitors are wrist devices, although optical sensors have been adapted to products worn in other parts of the body, such as headphones, arm straps and even headbands. 43 Precision and accuracy of heart ratemeasurements using HR monitors and apps Cadmus-Bertram et al. 44 highlighted the need of health professionals and general population to know the accuracy of HR monitors and apps for correct use of the devices. The authors recognized the difficulties inherent to the validation of these tools. In addition, with the rapid development of technology, there are many recent publications on HR devices that may have already been outperformed by others, 41 notably in terms of updating of algorithms that are not well explained, i.e. it is unclear how HR measurement shown on the display was actually determined (sample frequency). Besides, from an operational standpoint, it is not feasible to perform a comprehensive validation of all these devices considering all their possible applications. In this context, Cadmus-Bertram et al. 45 reported that PPG- based HR monitors are accurate to measure HR at rest, but not during exercise. Also, Singh and Sittig 46 reported discrepancies in the measurements of both resting and post-exercise HR (walking and running) obtained by a HR monitor as compared with a control measurement, indicating that the device being tested lost signal when HR was over 140 bpm. Although this is not a concern for most users of HR monitors and apps, and these devices do provide good and precise measurements for daily life application, clinical cardiologists should be aware that the measurements displayed by these devices may not be accurate. Similar results were observed by Boudreaux et al., 47 who compared eight different models/brands of HR monitors with a gold-standard method, a six-lead ECG measurement in resting conditions, during aerobic exercise of different intensity and resistance exercise. Although all monitors tested had good validity at rest, only three showed good accuracy (r ≥ 0,75) – the Apple Watch Series 2, the Polar H7 and the Bose SoundSport Pulse, highlighting that the last two did not have a wristwatch. The higher the exercise intensity, the lower the accuracy of the measurement, with a tendency of an underestimation of HR, in both aerobic and circuit resistance exercise. In addition, some of the monitors did not measure the HR in real time, with a 3-5 second delay. This is evident since HR tends to increase slightly