Cardiac rehabilitation (CR) is a critical component in the therapeutic care of patients following an acute myocardial infarction or coronary revascularization and in those with chronic heart failure.1,2 Exercise training is a vital element of CR that is associated with improvements in exercise capacity (ie, cardiorespiratory fitness [CRF]) and a myriad of health benefits for patients.3 Guidelines for exercise prescription for healthy adults and during CR incorporate frequency of exercise sessions, intensity, time (duration of each session), type of exercise, weekly volume, and progression.4 Exercise intensity is a key element of the exercise program as it is directly linked not only to the improvement in CRF and clinical outcomes but also to the risk of adverse events during exercise.5 Thus, it is important that exercise intensity be accurately prescribed to ensure patient safety and to optimize the therapeutic benefits.

Exercise-based guidelines recommend that exercise intensity be set relative to CRF using a component of maximal oxygen uptake or maximal heart rate (HRmax). Heart rate (HR) is easily assessable and is therefore commonly used as an indicator of exercise intensity, either as a percentage of HRmax or as the preferred HR reserve (HRR) method, which incorporates resting and HRmax values.4 The accurate determination of HRmax requires a symptom-limited maximal exercise test.6 Unfortunately, <20% of patients entering CR have performed a maximal exercise test and thus, in these circumstances, target exercise intensity must be prescribed without the aid of an objective determination of HRmax.7 In such cases, target HR for exercise intensity can be determined on the basis of age-predicted HRmax equations (such as 220 − age), which are known to carry a high degree of variability.4 Further complicating the use of HR estimations in CR is the common use of β-blocker medications in patients with cardiovascular disease, which are known to influence HRmax and the HR response to exercise. To account for the influence of β-blocker medications, specific equations are available to estimate HRmax that have not been validated for prescribing exercise intensity in CR. Another approach commonly used is resting HR +20 or +30 bpm, which is recommended in exercise guidelines when a maximal exercise test is not available.4 In fact, resting HR +20 or +30 bpm is the most used HR-based method for determining exercise intensity in CR programs.7 Interestingly, the accuracy of these HR estimation approaches for determining target HR for proper exercise intensity is not well described.

In this issue of the Journal of Cardiopulmonary Rehabilitation and Prevention, Keteyian and colleagues8 conducted an elegant study to describe the frequency for which target HR is accurately achieved when exercise intensity is not based on actual HRmax in patients taking β-blocker medications in CR. A cohort of 166 patients from the iATTEND (improving ATTENDance to CR) trial, which was racially diverse (55% Black) and included both men and women, was examined. Inclusion criteria were being enrolled in CR, having completed a symptom-limited maximal exercise test for the determination of HRmax, use of β-blocker medications, and in normal sinus rhythm. Using a target HR intensity of 50-80% HRR determined from measured HRmax as the standard, they compared the frequency by which three commonly used approaches to estimating HR-based exercise intensity in CR programs actually hit the proper target HR. Specifically, they used target HR zones estimated via (a) 60-85% of predicted HRmax, with HRmax determined from 220 – age or a disease-specific equation; (b) 50-80% HRR using predicted HRmax determined from 220 – age or a disease-specific equation; and (c) resting HR +20 and +30 bpm.

As expected, there was wide variability among the estimation methods, ranging from 0-61% of patients falling in the correct target exercise intensity range. Predicting HRmax from the non–disease-specific equation (220 – age) resulted in >80% of patients being prescribed an exercise intensity above the upper limit for exercise guidelines, which has serious potential ramifications for patient safety. On the lower end of the target exercise intensity range, the equations tended to underestimate exercise intensity, likely compromising the therapeutic benefits of the exercise program. Perhaps, most alarming is that the commonly used metric of resting HR +20 bpm resulted in 0% of patients being prescribed an accurate exercise intensity, which was improved to only 40% of patients when resting HR +30 bpm was used.

Where do we go from here? Hopefully, these important findings raise awareness to the inadequacy of common methods used to prescribe exercise intensity in CR programs. Improving CRF should be a primary goal of exercise programs in CR as improvements in CRF are related to long-term prognosis and survival following CR.9–11 Exercise intensity is the primary component of an exercise prescription that is associated with enhancing CRF in healthy adults and for patients in CR.4,5 Thus, the findings that the most common methods of prescribing exercise intensity for patients in CR using β-blocker medications dramatically underestimate exercise intensity suggest that in many instances the exercise prescription is not ideal for improving CRF and optimizing the therapeutic benefits.

These findings also highlight the importance of performing maximal exercise tests, ideally cardiopulmonary exercise testing, prior to CR. In addition to a definitive measure of HRmax, cardiopulmonary exercise testing provides a host of variables, including CRF, that can guide patient care, be used in the development of the exercise prescription, and monitor the effectiveness of the exercise program.6 Current guidelines from the American Association of Cardiovascular and Pulmonary Rehabilitation (AACVPR) recommend exercise testing for patients entering CR.12 Unfortunately, despite these recommendations, exercise testing and cardiopulmonary exercise testing continue to be drastically underutilized in patients with heart disease.7

In the absence of exercise testing, what can be done to improve the accuracy of exercise prescription? Perhaps, most importantly, it should be acknowledged that in patients using β-blocker medications, the common method of resting HR +20 bpm is completely ineffective while using resting HR +30 bpm was slightly better and corresponded to an average exercise intensity of 56% compared with the HRR method. Keteyian and colleagues8 provide a table with estimated target HR based on patient age and resting HR, which might be a more effective approach to ensuring minimum exercise intensity (50% of HRR) is reached. While these values are likely an improvement over current methods, they are estimations and thus will carry significant variability.

As an alternative to using HR-based exercise intensity, many CR programs use subjective indices of exercise intensity as the rate of perceived exertion (RPE), which is included in exercise guidelines.4 A recent survey of CR programs revealed that RPE was the most used method to determine exercise intensity.7 While use of RPE may be convenient in situations were monitoring HR is not feasible, adequate description and understanding of the RPE scale are crucial to its reliability. Ideally, RPE is anchored to objective markers such as workload or HR determined from a maximal exercise test. Furthermore, recent evidence suggests that HR-guided exercise training may be superior to RPE-based exercise for improving CRF in CR.13

In summary, this important work by Keteyian and colleagues8 highlights the need to reevaluate methods for prescribing exercise intensity in CR. As exercise intensity is the most critical aspect of exercise prescription for stimulating improvements in CRF, it is imperative that an adequate intensity be implemented. This is best accomplished by using objective markers of exercise capacity, such as maximal oxygen uptake or HRmax determined by a maximal exercise test, which unfortunately are only sparingly used prior to CR. Thus, for the sake of patient care and to optimize the therapeutic benefits of CR, emphasis should be placed on expanding the use of exercise testing in clinical settings.

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