IJCS | Volume 32, Nº4, July/August 2019

404 Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article the improvement of algorithms, it will be possible to correctly detect artifacts and interferences, and thereby the rate of false negative and false positive diagnosis will tend to zero. At that time, these devices will be safely and reliably used in routine clinical practice and become a quality standard. But for now, the detection and recording of apparently abnormal HR values (higher or lower) particularly in high-intensity exercise, deserve careful, weighed, but not alarming consideration. This is evenmore relevant for asymptomatic, apparently healthy athletes and practitioners with a low cardiovascular risk. 66 HR variability As above mentioned, the large storage capacity of HR monitors has facilitated themeasurement of instantaneous HR, i.e., of each cardiac cycle or the RR-interval, and thereby enabling the acquisition of data for analysis of HR variability using mathematical techniques. 67 One of the main contributions of exercise science about this topic occurred at the end of the 20 th century. Lima and Kiss 68 proposed an indirect determination of lactate threshold by analysis of HR variability during maximal incremental exercise test. The physiological assumptions supporting this idea was based on the inverse relationship between parasympathetic autonomic activity and intensity of exercise. 23 The authors used a HR monitor Polar NV Vantage (subsequently updated by the manufacturer as S810i, RS800, V800 and currently Vantage V). HR variabilitywas defined as the mean of six measurements (taken every ten seconds) of the variation in consecutive RR intervals. The HR variability threshold was reachedwhen themean of the sixmeasurements was lower than 3 ms. This point was validated by the curve of blood lactate accumulation and corresponded to the lactate threshold. Paschoa et al. 69 evaluatedHR kinetics during resistance exercise (unilateral leg extension) and observed amarked decrease of HR variability during exercise, even the shorter ones, with a fast recovery after exercise. More recently, Barbosa-Neto et al. 56 showed that such HR variability response does not depend on exercise load. Similar RR-interval curves were found throughout the exercise performed at 50%, 80% and 100% of the load for 10 repetitionmaximum. Exercise cadence was controlled by a metronome, and thus all participants performed the repetitions (10 repetitions) in 15 seconds. Both studies used the Polar S810i monitor and performed the analysis of RR intervals. Two recent systematic reviews showed that these portable devices have an acceptable margin of error 70 and provide reliable and reproducible HR variability measurements. 11 Therefore, they can be used for clinical and research purposes, especially considering its cost- benefit relationship. 70 Four-second exercise test (T4s) Finally, another clinical application of HRmeasurement during exercise is the assessment of cardiac vagal tone. Considering that the rise in HR in the first four seconds of fast movement of the lower or upper limbs, 71 be it active or passive 72 depends solely on removal of vagal tone, Araújo et al. 73,74 proposed the T4s. Briefly, the test consist in pedaling as fast as possible and without resistance a cycle ergometer from the fifth to the eighth second of a 12-second maximal inspiratory apnea. The quantification of the cardiac vagal index is determined by the ratio of two RR-intervals registered in the ECG: the longest RR- interval before exercise and the shortest RR-interval during exercise, which can be cycling 73,75,76 or stationary running. 77 Recent studies have testedwhether HRmonitors could be used to determine this index and shown that HRmonitor can be used as a surrogate for ECG todetermine the cardiac vagal tone in the T4s using a cycle ergometer, 78 but not in the T4s performed in orthostatic position. 79 Conclusions In summary, there is a wide range of monitors and smartphone apps tomeasure HR. The choice of the brand and model should be made based on the purpose of its use. Also, it is fundamental to know how to set up the device for personal, clinical or research purpose. These pieces of equipment may be helpful in the monitoring of the intensity of many types of exercises, and thereby increase the safety and efficacy of a physical exercise or sports training program. In addition, HR monitor devices and apps are in rapid development and would ultimately be useful in the detection of stress-induced cardiac arrhythmias and in the management and follow- up of physically active patients. Author contributions Conception and design of the research: Araujo CG. Acquisition of data: Almeida M, Bottino A, Ramos P, Araujo CG. Analysis and interpretation of the data: Almeida M, Bottino A, Ramos P, Araujo CG. Writing of