High level physical activity in cardiac rehabilitation: Implications for exercise training and leisure-time pursuits
Introduction
Because exercise at a vigorous intensity appears to confer anti-aging and cardioprotective benefits to a greater extent than more moderate exercise intensities, an increasing proportion of the population, including patients with coronary heart disease (CHD) are now investing an unprecedented number of hours in high-volume and high-intensity endurance training regimens and competition. Nevertheless, an escalating number of case reports and observational studies in individuals with a diseased or susceptible heart now suggest there is a potential for a plateau or even a decline in benefit with more extreme levels of exercise (i.e., in a reverse J-curve pattern), with a heightened risk for acute cardiovascular (CV) disease (CVD) events.
This review details the associated risks, benefits, and limitations of extreme endurance exercise and high-intensity training in cardiac rehabilitation (CR), with specific reference to exercise prescription/proscription, physical activity (PA) dosing and CVD outcomes, moderate continuous versus high-intensity interval training (HIIT), high-volume, high-intensity training and competition (e.g., marathons and triathlons), exercise-induced cardiac maladaptations, and the potential prophylactic use of aspirin and/or beta-blockers prior to vigorous exercise. Additionally, the cardiovascular complications associated with these exercise interventions are discussed, as are recommended strategies to potentially reduce the likelihood of exercise-related acute CVD events.
Section snippets
Cardioprotective benefits of vigorous versus moderate-intensity PA
Moderate-to-vigorous PA, which corresponds to any activity ≥3 metabolic equivalents (METs; 1 MET = 3.5 mL O2/kg/min), has been consistently shown to reduce the health risks associated with chronic diseases.1 However, emerging research strongly suggests that the gradual progression of exercise intensities, from moderate-to-vigorous to HIIT regimens (in selected individuals), may result in even greater CV benefits. Accordingly, vigorous PA (usually defined as ≥60% of functional capacity or 70%89%
CRF and PA as separate risk factors: comparative benefits
CRF appears to be one of the strongest prognostic indicators in persons with and without chronic disease, including CHD. In fact, higher levels of CRF, expressed as mL O2/kg/min or as METs, are associated with a reduced risk of developing hypertension, type 2 diabetes, atrial fibrillation (AF), chronic kidney disease, and major adverse CVD events, including heart failure (HF), acute myocardial infarction (AMI), stroke, and coronary artery bypass grafting.7 Williams reported that increasing
Prescribing age-, sex-, and fitness-adjusted training intensities to maximize cardioprotective benefits
Most middle-aged and older individuals, with and without CHD, initiate exercise programs at ~2 to 3 METs, corresponding to level walking at ~2 to 3 mph. Unfortunately, these individuals often fail to increase the intensity of their exercise over time as their level of CRF improves.12 This failure to advance their training MET levels in proportion to their increased CRF likely prevents them from achieving the maximal possible reduction in their risk of CVD.
Guidelines and recommendations for
Is there an “asymptote of gain” beyond which further increases in CRF confer no additional mortality benefits?
There is debate whether the dose-response relationship between CRF and mortality benefits is, in fact, linear, curvilinear, U-shaped, or J-shaped. In an early landmark study, Blair et al.13 reported an inverse relationship between estimated peak METs during maximal treadmill testing and all-cause mortality in a large cohort (n = 13,344) of apparently healthy middle-aged subjects. Findings from this study, however, suggested the presence of an “asymptote of gain,” beyond which further increases
High-volume, high-intensity endurance training: too much of a good thing?
Because high-volume, high-intensity, exercise training regimens and regular endurance competition can induce cardiac maladaptations such as an increased incidence of AF and accelerated coronary artery calcification (CAC) in some individuals,2,24 there is controversy whether intensive exercise may be harmful to the heart.
Common activities associated with acute CVD events
Strenuous physical exertion, especially when sudden, unaccustomed, or involving high levels of anaerobic metabolism, may increase the risk for AMI and sudden cardiac death (SCD). For example, downhill skiing,33 racquet sports,34 HIIT,35 and competitive sports activities (e.g., basketball)36 are associated with a greater incidence of acute CVD events than other activities. Superimposed neural and psychologic mechanisms associated with the excitement of competition may also lower the threshold
Strategies to reduce exercise-related CVD events
One of the most important defenses against exercise-related CV events in adults is to maintain or enhance CRF via regular PA. This relationship is predicated on the observation that a disproportionate number of complications occur in habitually sedentary subjects with known or occult CHD who were performing unaccustomed vigorous-to-high intensity PA.44 Several strategies to reduce exercise-related CVD events appear prudent although unproven. These include: advocate appropriate warm-up and
Potential HIIT advantages for CVD patients
HIIT in CR is a burgeoning area of interest within clinical exercise physiology. To illustrate the escalating number of reports on this topic, a 2021 PubMed search for the key terms “high-intensity interval training + cardiac rehabilitation” yielded >200 citations, with an exponential increase in 2017 (Fig. 6). Although this PubMed query includes pre-clinical studies and some related publications, there are roughly as many literature reviews and meta-analyses on the topic as randomized
Exercise adherence and the risks associated with HITT CR protocols
Given the current debate regarding the role of HIIT rehabilitation versus MICT, one must also consider whether the potential risks imposed by higher intensity exercise are disproportionate to the potential gains, especially in patients with known CVD. The prescription of high-intensity exercise to non-athletes is inversely related to exercise adherence,1 a fact that may be particularly applicable to CR settings where most patients are unaccustomed to a formal exercise commitment. Moreover, CR
CVD diagnosis and HIIT
The wealth of the data presented within this review primarily addresses HIIT in patients with CHD. There are, however, data to suggest that other forms of CVD and/or related comorbidities may respond favorably to HIIT-based CR. In one study, 41 patients who had undergone cardiac transplantation were assigned to a usual care control group or HIIT-based CR. The 5year follow-up revealed that patients who performed HIIT demonstrated a blunting in age-related V̇O2max decline and less anxiety
Research: evidence gaps and opportunities
Beyond the additional studies needed to further clarify the long-term safety, effectiveness, and outcomes associated with HIIT in varied populations with CVD, information voids or gaps also emerged during our review of the literature. For example, few data are available regarding the best practices for navigating concurrent (HIIT & MICT) versus dedicated classes, attention in the form of staff:patient ratios, and equipment required to implement HIIT versus MICT. In addition to tracking the
Conclusions
Although considerable epidemiologic and clinical evidence suggest that regular moderate-to-vigorous PA, improved CRF, or both, provide direct and indirect physiologic adaptations and pleiotropic benefits for improving CV health and reducing mortality,86,87 the absolute and relative risk of CVD and musculoskeletal complications appear to increase transiently during vigorous PA. The essential feature of a prudent exercise regimen is a gradual progression during which an individual remains below
Declaration of Competing Interest
None.
Acknowledgment
To Brenda White, for her invaluable help with the preparation, formatting, and serial revisions of this manuscript, laboriously checking the accuracy of our citations, with a unique sense of pride and responsibility.
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