IJCS | Volume 32, Nº4, July/August 2019

397 Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article value. Maximum HR, measured by laboratory and field tests, or estimated by predictive equations, tend to be linearly attenuated with ageing, 6 especially after the age of 35 years. 7 There are many predictive equations for maximum HR in aging. The equation: maximum HR = 208 – 0.7 x age (years) was validated in a Brazilian population. 8 However, the margin of error is quite large, and easily exceeds at any given age and in 5% or more of the individuals. Therefore, when using HR as a tool for the assessment and prescription of exercise, it should be directly measured from an individual rather than predicted by equations. 8 Measured maximum HR can then be used to define, in a better and more precise way, the range of intensity of an exercise training program. 9 However, despite its importance in clinical practice and in exercise prescription and monitoring, there are no standardized guidelines to measure HR. 10 On the other hand, recent technological advances have made HR measurement more accessible and popular due to high availability of monitors, armbands, and even smartphone apps. 2,3 Since 2012, more than 30 new products for HR measurement have been launched, 11 and this number tends to increase. In this regard, this paper presents a brief and updated overview about the use of devices and monitors for HR measurement in exercise and in clinical cardiology. HR in exercise and sports: a brief contextualization Resting condition Resting HR is often used as an indicator of cardiorespiratory or aerobic fitness. Cross sectional studies have shown that cardiorespiratory fitness is inversely related to resting HR in adolescents, 12 adults, 13 and elderly. 14 In untrained women, aerobic training reduced resting HR regardless of age (< 41 or 41-60 years) of participants or duration of intervention they were exposed to (< 3, 4-6 or > 6 months). 15 Although this associationmay be attributed, at least in part, to increased resting cardiac vagal activity, 16 electrophysiologic changes intrinsic to the sinus node may also occur in many physically trained individuals. 17 Cardiovascular drift The balance between cardiac output and oxygen requirement duringexercisemay require a cardiovascular adjustment known as cardiovascular drift, which negatively influences performance. 18 In practical terms, it is important to identify this phenomenon, since HR tends to increase with prolonged effort performed at the same level, particularly if sustained for 30 minutes or longer or under unfavorable thermal conditions. 19 Two hypotheses have been suggested for this phenomenon, which involves an increase in HR associated with a reduction in systolic volume. First, dehydration has been proposed as the mechanism responsible for blood volume reduction, leading to impaired venous return and reduced end-diastolic volume and, consequently reduced systolic volume (Figure 1). In this process, cardiac output would depend on increased HR. 20 More recently, Coyle and Gonzáles-Alonso 21 proposed a inverse pathway, in which HR acceleration would limit the time for determination of the end-diastolic volume, resulting in lower systolic volume. In fact, during prolonged exercise, it is possible that HR increases regardless of variations in exercise intensity. Monitoring of HR during exercise facilitates the control of exercise intensity and maintenance of performance. Maximal exercise During incremental maximal exercise, HR increases in response to gradual decrease in cardiac vagal activity until its complete suppression at peak effort, at the same time that adrenergic stimulation becomes the protagonist of autonomic control of HR. 23 Maximal HR is limited by hypoxia, regardless of age and sex, but this reduction is more evident in individuals with lower cardiorespiratory fitness. 24 Nevertheless, a limitation to physiological increase in HR in response to increased activity is known as chronotropic incompetence. 25 Thismay also result from a late response of HR acceleration, instability or lowered response to exercise intensity. 26 Considering that the increment in HR is the main responsible for the increase in cardiac output and thus for the possibility of performing an aerobic exercise, 25 the chronotropic incompetence turns out to be a restrictive factor to exercise performance, as in some patients with heart failure, 27 and is associated with increased mortality risk. 28 Post-exercise recovery Post-exercise HR recovery, or decrement, is the difference between HR at the end of exercise (maximal or submaximal) and HR at 1 minute or 2-5 minutes after exercise, and is similar in men and women. 29