Symptom-limited (maximal) exercise testing at the start of cardiac rehabilitation (CR) has been a clinical standard for many years, primarily as a method of risk assessment for patients with coronary heart disease (CHD) before they initiated progressive exercise training.1 Exercise testing was particularly important to guide safe exercise prescription and progression during the time when neither procedures nor medications ensured stability (ie, a time when many patients were referred for CR to advance exercise amid residual ischemia and/or sequelae of completed myocardial infarctions—congestion, arrhythmia, and anxiety). However, contemporary care for CHD entails coronary revascularization as well as aggressive adjunctive therapies that have significantly modified the concept of risk and the significance allotted to exercise testing. In fact, contemporary revascularized patients with CHD are often encouraged to initiate low-intensity exercise soon after discharge and to advance frequency and intensity of exercise as tolerated. These shifts challenge the earlier paradigm that exercise testing is required before CR and even the concept that CR is a necessary element of care.2
Despite temporal shifts, contemporary CR is flourishing as a multifaceted program that promotes long-term health for a wide spectrum of cardiovascular (CV) patients.3 Within this broadened conception of CR, exercise testing also remains clinically relevant. Despite advances in treatment of CHD, residual ischemia is common. Anatomic challenges may result in incomplete revascularization, but even when revascularization seems ideal, ischemia may persist from endothelial dysfunction and/or microvascular disease.4 Beyond ischemia, exercise testing also provides key insights pertaining to patients with heart failure, valvular surgery, and cardiac transplantation, all of whom are now eligible for CR coverage by the Centers for Medicare and Medicaid Services (CMS).5,6 Exercise testing constitutes a standardized approach to assess dynamic responses (ie, hemodynamics, arrhythmias, and symptoms that are relevant for all CV patients). Furthermore, patients are more likely to have compounding diseases for which exercise testing provides unique integrated perspectives (eg, concurrent atrial fibrillation and/or aortic stenosis).5 Similarly, CR patients now constitute a broader range of ages and complexities (including multimorbidities, polypharmacy, frailty, and obesity) for which functional assessment of aerobic capacity, strength, and balance provides valuable insights regarding cumulative health (ie, perspectives pertinent to broader risks, such as risks of falling) to better inform therapeutic decisions, including choices of medications and interventions and/or to serve as a basis for an exercise training prescription.
Utility of exercise testing to gauge peak cardiorespiratory fitness (CRF) has also evolved as a codominant priority in CR.7 Cardiorespiratory fitness is a powerful index of health and a strong predictor of mortality and morbidity. Using graded symptom-limited exercise testing (GXT), CRF is usually calculated as estimated metabolic equivalents (METs), with 1 MET representing resting energy expenditure. Using cardiopulmonary exercise testing (CPET), CRF is measured directly as peak oxygen uptake (o2).5 Pre- versus post-CR assessments of CRF can be used to assess the impact of CR, and to guide care and promote long-term positive lifestyle patterns.
This review addresses the role and optimal use of exercise testing in contemporary CR and discusses the merits and limitations of symptom-limited GTX, CPET, and submaximal walking tests in this setting. The utility of additional functional assessment of gait speed, strength, and balance in selected subgroups is also discussed.
CONCEPTUAL VALUE VERSUS THE CONUNDRUMS OF CLINICAL PRACTICE
Despite its enduring conceptual benefits for contemporary CR, conflicting priorities still commonly encumber exercise testing. Rapid referral and enrollment of eligible patients into CR have been demonstrated to facilitate its successful usage, adherence, and benefit.8,9 Many novel strategies have been suggested (eg, direct referral, telehealth, and homecare), each with similar emphasis on efficient and uncomplicated transitions from acute care to CR. Slowing this flow to perform an exercise test is potentially disadvantageous if patients lose interest and motivation in attending CR. Exercise testing for CR can become particularly impeding because many referring cardiologists complete the tests for their patients in their own offices, creating additional barriers and delays to CR enrollment. Adding to the complexity, providers often order exercise tests on the basis of their own practice preferences that are not necessarily well-matched to CR management priorities. For example, many providers order pharmacological perfusion stress tests, especially for patients with limited functional capacity, a common finding among those referred for CR. However, such tests fail to provide information on exercise-related physiologic responses and functional capacity that facilitate exercise prescription and enhance prognostic stratification.10 Thus, nonexercise stress tests often have the ironic impact of generating excessive costs and delays in enrollment, but with relatively little clinical value added to CR.
Many have asserted that increased utilization of allied providers (exercise physiologists, physical therapists, nurses) rather than physicians to supervise maximum exercise stress tests may help facilitate exercise testing that is more efficient and possibly less expensive. This concept is reinforced by various American Heart Association statements attesting to the capacities of many allied providers to perform exercise testing.11 Nonetheless, for patients with known cardiac disease, current standards still assert the primary leadership and availability of a cardiologist as a fundamental standard of maximal stress testing.11 Thus, whether a maximum exercise test is physically administered by an exercise physiologist or other nonphysician, it still remains linked to the availability of a physician, thereby creating logistic complexities.
RETHINKING STANDARDS OF CARE: THE ROLE FOR “SUBMAXIMAL” WALK TESTS
Although the concept of exercise testing within CR programs continues to be inherently relevant but frequently encumbered, alternate forms of exercise testing have evolved to better achieve goals of care. One such strategy is the substitution of submaximal exercise testing for maximal tests because they can be done by CR staff with little or no additional costs beyond standard CR fees, and without scheduling intricacies. Thus, regardless of whether patients may have had exercise tests by their referring providers, many CR programs now routinely perform submaximal exercise testing as part of the intake. In particular, the 6-minute walking test (6MWT) is frequently used for this application (see below for further information).
Exercise assessment provides particular value at CR intake as it clarifies clinical responses relevant for safety and exercise prescription (Table 1) as well as broader prognostication.12 Serial assessments of physical function before, during, and after a CR program add value as they clarify relative improvements as a result of the intervention, guide refinement of exercise prescription as functional performance improves, and often provide patients with useful perspectives in regard to recreational goals, work duties, sexual activity, symptoms, as well as improvements in prognosis on the basis of CRF. Functional gains can be measured as differences in maximal or submaximal performance.
Table 1 • -
Measures and Corresponding Estimated Exercise Intensitya
2 reserve, %
|WR at ventilatory threshold
|GXT (or CPET)
|Peak HR, %
|Peak WR (approximate), %
|Walking speed (mean), %38
Abbreviations: CPET, cardiopulmonary exercise test; GXT, graded exercise test; HR, heart rate; HRR, heart rate reserve; 6MWT, 6-minute walk test; RPE, rating of perceived exertion; o2
, oxygen uptake; WR, work rate.
Adapted from Mezzani et al.12
reserve and HRR are the differences between resting and peak o2
and HR, respectively.
cIntensity estimated by 6MWT is more variable.
dMay represent light or moderate intensity because of variability in 6MWT intensity.
Given the encumbrances of exercise testing, the notion of completing serial maximal exercise tests is often unrealistic. This is further confounded by the fact that few insurers are willing to pay for serial maximal exercise testing. Whereas it was once common to have a pre- and postexercise test in relation to CR, this has become quite rare. In contrast, the 6MWT as a submaximal exercise evaluation is conducive to serial assessments and provides a convenient gauge of improvement in meters walked or changes in perceived exertion.13 In fact, many assert the more pragmatic value of the 6MWT compared with maximal exercise testing as a measure of fitness as it may better correspond to activities of daily living and indicate a more pertinent type of fitness.14 Moreover, the 6MWT can be measured serially over years, and perhaps by patients at home, as ways to track fitness regularly as an important vital sign of health.15
INFLUENCE OF CHANGES IN THE CR POPULATION
Heart failure is the most recent condition approved by the CMS for CR.16,17 Eligibility criteria of patients with heart failure (HF) for CR mirror the inclusion criteria used within HF-ACTION, a seminal trial that demonstrated safety and efficacy of exercise training for systolic HF.18 Patients enrolled in the trial were chronic stable outpatients with HF with reduced left ventricular ejection fraction. Expert consensus and CMS policy call for a period of clinical stability before CR enrollment.17,19,20
This contingency that CR enrollment occurs after a period of stability distinguishes patients with HF from most other CR participants, who typically enroll after an acute event (eg, acute myocardial infarction) or procedure (eg, percutaneous coronary intervention, coronary artery bypass graft surgery). Some may assume that the extended time requirement for eligibility implies that patients with HF are more stable for exercise than a patient who just left the hospital after, for example, a myocardial infarction; however, this belies the fundamental nature of HF. In fact, HF entails insidious pathophysiology with progressive cardiac remodeling, inflammation, and other elements of disease that propel evolving risks.21 Therefore, exercise assessments for patients with HF may have particular value in guiding safe and effective exercise prescription amid progressive pathophysiologic changes. Heart failure pathophysiology also frequently includes abnormal heart rate responses to exercise (eg, chronotropic incompetence), pulmonary hypertension, comorbidities such as chronic obstructive lung disease, and other complexities (eg, hypotensive exercise responses) that can be best delineated by dynamic exercise assessments.
Aging, Multimorbidity, and Frailty
The growing challenges of advanced age, frailty, and multisystem comorbidities among cardiac patients have broadened perspectives and rationale regarding physical function assessment for CR. Assessment of strength, balance, cognition, and other dimensions of function has increasing relevance both in the characterization of baseline capacity and as meaningful metrics by which to gauge safety, devise treatment, and assess efficacy of care.22–24
The importance of frailty in patients with CV disease is being increasingly recognized.22 Cardiovascular disease is associated with increased rates of frailty25,26 as are advanced age and multimorbidity, both common in cardiac patients. With increasing patient complexity and multimorbidity, longer survival and increasing treatment options (eg, transcatheter aortic valve replacement, internal cardioverter defibrillator, and improved stent technology) for older patients with CV disease, ongoing efforts to improve CR referral and uptake, and the overall aging of the population in general, the number of frail patients presenting to CR can be expected to grow. Consequently, a basic understanding of frailty, associated limitations in physical function, and potential benefits of exercise, diet, and key supplements (eg, vitamin D and protein supplements) becomes increasingly important to those delivering CR.27,28
EXERCISE TESTING OPTIONS
Symptom-Limited (Maximal) Exercise Test
Symptom-limited exercise testing can be performed with either GXT or CPET. Each provides a valuable assessment of cardiopulmonary exercise performance that can be used to guide safe exercise prescription. Completion of a maximal exercise test may also provide a patient with greater confidence in his or her ability to participate in an exercise training program, thus improving physical self-efficacy and adherence. Selection of the appropriate exercise testing protocol for functional capacity assessment is a key consideration for either GXT or CPET.
Most exercise laboratories in the United States are familiar with the Bruce treadmill protocol and often use it as a default choice. However, the Bruce protocol contains large and unequal work increments between stages, resulting in a nonlinear relationship between physiologic responses and work rate, which is even more pronounced among patients with HF.29 The modified Bruce protocol is a variation on the original Bruce protocol that is better tolerated by many patients, but it has the same shortcoming as the original protocol in respect to the large and unequal work increments. In comparison, other protocol options such as the Naughton or Balke protocol reduce the workload changes between stages in a way that is more useful for typical CR patients, particularly those with low CRF.
Regardless of the specific protocol chosen, the goal is to select one that is best tailored to the individual's baseline fitness to yield a fatigue- or symptom-limited exercise duration, which is ideally about 10 minutes.5 Shorter durations may produce a nonlinear relationship between physiologic responses and work rate, whereas durations over 12 minutes may cause subjects to terminate exercise because of muscle fatigue or orthopedic factors.
Both treadmill and cycle ergometer protocols are usually options, but treadmills are more common in the United States, where fewer adults are accustomed to riding bicycles. Cycle ergometry protocols require individuals to pedal at a constant cadence against increasing resistance, which requires a high degree of motivation that may not be achievable for many patients.29 Treadmill protocols increase progressively in speed and incline, and usually provoke a 10% to 20% higher metabolic exertion than cycle ergometer protocols, reflecting greater muscle involvement and motivation (one cannot slow down on a treadmill without falling, whereas exercise cadence can slow on a cycle ergometer). Thus, it is important to use the same modality pre- and post-CR intervention to assess serial change in CRF.
Graded Exercise Test
Current guidelines from the European Society of Cardiology and American Heart Association for CR programs strongly recommend a GXT, also commonly referred to as an exercise tolerance test (ETT) as the initial CR patient assessment.5,30 A GXT before starting CR provides assessment of ischemia (signs, symptoms, changes on electrocardiogram), arrhythmia, chronotropic responses, hemodynamics, and other safety parameters, as well as objective assessment of a patient's functional capacity, which is derived from treadmill or cycle protocols.5 A GXT also helps characterize the subjective rated perceived exertion (RPE),31 with an RPE of 13 to 14 often identified as a target intensity of training. The GTX is often applied as the basis of exercise prescription, usually relative to the peak heart rate but also in relation to an ischemic threshold, perceived exertion, or symptoms (Table 1). Exercise prescription is then tailored to the goals and clinical circumstances for each patient.
Limitations of GXT
In standard GXT assessments, CRF is routinely reported as the number of METs achieved. METs are estimated using equations that incorporate a patient's exercise performance (on the basis of the speed, incline, and sex). Such calculations possess inherent inaccuracies as some key physiologic parameters (eg, weight) or differences in walking technique (eg, holding handrails) are not incorporated into the assessments. The GXT is also limited by its inability to assess other useful aspects of exercise performance, such as the anaerobic threshold, as well as insights regarding other factors limiting exercise performance (eg, pulmonary or skeletal muscle pathologies). In these respects, CPET is relatively advantageous.
Cardiopulmonary Exercise Test
Cardiopulmonary exercise testing entails a cycle or treadmill exercise modality that essentially is the same as a GXT in respect to the exercise protocol, as well as to eliciting related signs, symptoms, electrocardiographic changes, arrhythmia, heart rate responses, hemodynamics, and other safety parameters. However, for a CPET, patients also wear a facemask or mouthpiece to facilitate measurement of ventilatory gases. Oxygen inhaled (o2), carbon dioxide exhaled (co2), and overall volume of air movement (minute ventilation [VE]) are analyzed to provide elegant integrated assessments of the underlying heart, lung, vasculature, and skeletal muscle physiology.32 Cardiopulmonary ventilatory assessments occur in parallel with GXT assessments. The gas exchange components (gas analyzers and computers) are increasingly compact and mobile, and produce easy-to-read, well-organized real-time data outputs.
The primary utility of CPET is that measured peak o2 provides a more reliable and accurate assessment of CRF than estimated METs. It is usually reported in relation to body weight or to lean body mass, to link cardiorespiratory physiology and body size. By measuring ventilation directly, a CPET overcomes many of the limitations that potentially reduce the accuracy of GXT.32 For example, CPET assessments of co2 account for the differences in performance that may occur when some patients clench the handrails while walking on a treadmill to maximize walking time. Another ventilatory index, the respiratory exchange ratio (RER), calculated as co2 divided by o2, can be used to estimate effort. An RER >1.1 is typically regarded as substantiation of high effort; thus, RER is used to assess whether serial assessments of o2 are associated with similar exertion.
Whereas CPET has a particularly long track record for patients with systolic HF6,32,33 as a means to assess prognosis and gauge therapeutic interventions, it has similar utility for other CV (CHD, hypertrophic cardiomyopathy, pulmonary hypertension) and non-CV disease (eg, chronic obstructive lung disease) populations.32,34 Peak o2 normalized to body weight is typically applied as a robust measure of CRF; other indices are used to expose distinctive patterns implicating cardiac versus pulmonary versus skeletal muscle limitation to exercise performance. The minute ventilation to exhaled carbon dioxide ratio (VE/co2) is routinely assessed as an index of breathing efficiency, a higher ratio indicating reduced efficiency because of pulmonary shunting or increased dead space ventilation. It is typically assessed in combination with peak VO2 to enhance diagnostic and prognostic insights of HF, pulmonary hypertension, and chronic obstructive lung disease.33 An assortment of other CPET indices, most derived from Vo2, co2, and VE, can be applied to enhance assessment in a wide variety of common clinical contexts.32,33
Cardiopulmonary exercise testing has proven to be quite safe for patients with HF as well as those with other forms of CV disease. In over 4411 symptom-limited CPET tests conducted during the HF-ACTION trial, there were no deaths, and the major CV event rate was very low (0.45 per 1000 tests),35 with similarly low event rates observed in community-based samples.36,37
The advantages, application, and disadvantages of GXT as well as the other forms of exercise testing included in this review are summarized in Table 2. For exercise prescription, CPET provides distinct advantages; tracking o2 changes during progressive increases in exercise intensity provides perspective on the shift from aerobic to anaerobic metabolism, an index usually called the ventilatory anaerobic threshold (VAT).32 Training at or slightly above the anaerobic threshold intensity provides a physiological basis of training intensity that is usually more dependable than prescriptions on the basis of perceived exertion or heart rate.12,32
Table 2 • -
Comparison of Exercise Testing
||Application to CR
Response to exercise (HR, BP, arrhythmia, ischemia, symptoms with or without oxygenation) at a maximal workload
Estimate of maximum (symptom-limited) exercise capacity (METs)
Physician oversight and/or conduct by highly trained personnel (eg, exercise physiologists)
Potential to delay initiation of CR
Same as GXT. In addition:
Reliable (“gold standard”) assessment of aerobic capacity (peak
Objective measure of effort (RER)
Identification of mechanism of aerobic limitations (heart vs lung vs periphery)
Enhanced prognostic information (HF)
Assessment of disease-related factors impacting exercise performance (HF)
Any patient enrolling in CR
Unexplained dyspnea or exercise intolerance (clarifies the underlying etiology)
When more precise exercise-intensity assessment is desired
Requires specialized equipment and training to conduct and interpret
Greater time for patients and providers
Potential to delay CR initiation
Response to exercise (HR, BP, arrhythmia, symptoms with or without oxygenation), typically at a submaximal workload
Easy to integrate into CR practice
Serial testing more feasible
May better reflect daily activities
Challenges with standardization and reproducibility
Represents a different exercise stimulus (submaximal vs maximal) in different populations
Confounded by use of assistive device for ambulation
|Physical function and frailty
Assess additional dimensions of physical function (strength, balance, mobility)
Insight into fall risk
Insight into disability
Easy to integrate into CR practice
Serial testing feasible
Identify functional limitations beyond aerobic capacity
Alternative endurance modalities and supervision to minimize fall risk or injuries
Referral for further assessment and treatment (geriatrics, PT/OT)
Poor functional capacity
Only a frailty screen; does not replace comprehensive assessment
Does not provide assessment of CRF
Benefit of targeting deficits in CR not well established
Abbreviations: BP, blood pressure; CHD, coronary heart disease; CR, cardiac rehabilitation
; CRF, cardiorespiratory fitness; GXT, graded exercise test; HF, heart failure; HR, heart rate; METs, metabolic equivalents; 6MWT, 6-minute walk test; OT, occupational therapy; PT, physical therapy; RER, respiratory exchange ratio; o2
, oxygen uptake.
Limitations of CPET
The key limitations of CPET relate to the expertise of those performing the tests and the costs for the equipment. In a time of cost constraint and a demand for increased efficiencies of care, the cumulative expenditure for staffing and infrastructure required for CPET is often a deterrent for its routine use in CR. Although CPET carts have become simpler to use over time, in most cases they need to be calibrated on a regular basis, and the personnel administering CPET need to be sufficiently skilled to navigate predictable quirks related to its metabolic sensors, computers, air leaks, or other complexities of testing.13,32 Therefore, exercise physiologists, physical therapists, nurses, or physicians who are administering the tests must have advanced training. Costs for this proficiency are compounded by added time requirements for CPET. Tests usually entail more time than standard GXT to apply mouthpieces or masks, calibrate the machine, and ensure accurate measurements. Similarly, data analysis is also more time-consuming.
A robust literature points to the value of the 6MWT as a valuable submaximal performance measures in respect to logistics, efficacy, cost, repeatability, and other important considerations.13,14,22,38,39 The 6MWT is often utilized to estimate functional exercise capacity before enrollment in CR. Its advantages include being self-paced, with better acceptance and tolerance by cardiac patients compared with GTX or CPET. It is also easy to administer, does not require costly equipment, and can be performed at a CR center without the requirement that a physician be present. The 6MWT mimics activities of daily living13 and can be carried out by many elderly and severely limited patients who cannot or who are reluctant to complete a GXT or CPET.39
Submaximal walking tests may be useful in revascularized, well-medicated patients with CHD as well as patients with other CV disease in relation to their utility to induce clinically relevant symptomatic responses, sometimes at a lower workload than during a maximal exercise test. The literature demonstrating the value of the 6MWT to gauge overall health for CHD and other CV diseases is abundant and substantive.13,38,40–44 The evidence supporting the utility of the 6MWT for assessing HF prognosis is robust.6,14,33,45 For example, in 2054 outpatients from HF-ACTION who performed both 6MWT and CPET at baseline on the same day, the tests provided similar prognostic information with respect to all-cause mortality and hospitalization.14
The primary outcome of the 6MWT, the 6-minute walk distance (6MWD), has been successfully used to prescribe exercise training intensity as well as to gauge CR outcomes.40,46 The value for the 6MWT in CR can be optimized by tracking the ECG (using telemetry), RPE, blood pressure, and oxygen saturation during the test. This provides assessments of peak heart rate, perceived exertion, and hemodynamics to assess safety and, coupled with the 6MWD, guides exercise prescription (Table 1).12,38 Although such approaches are not well validated, multiple studies have found that the 6MWD was reliable in successive tests and correlated reasonably well with METs and o2 as measured on GXT and CPET protocols, respectively.12,13,38,42,47 A related application of the 6MWT is that it can be repeated throughout the course of a CR program to tailor the exercise regimen and after CR is completed as a metric of CRF.13,38 Data collected from serial 6MWTs are sensitive to changes in clinical status, enabling early detection of reduced exercise capacity requiring further evaluation.
Limitations of the 6MWT
The major disadvantage of a 6MWT is difficulty with standardization and reproducibility. An American Thoracic Society statement, published in 2002, includes a standard procedure for conducting the walking test to help provide useful and comparable information.42 The 6MWT should be performed on a flat 30- to 50-m long indoor walking track with which the patient has been familiarized. Extrinsic factors such as a short track (<20 m), a circular track, and absence of a practice test can influence the test result. Reproducibility may also be somewhat limited, with variability between 2% and 8% being observed when the 6MWT is performed on separate days.13 Performance of 2 separate tests at baseline has been suggested. Although this may not always be possible in the setting of CR, it is at least recommended to perform serial tests at the same time of the day by the same tester.38
In comparison to symptom-limited GTX, another criticism of the 6MWT is that it constitutes a different level of effort for different populations. For a patient with advanced HF and/or someone severely deconditioned, a 6MWT may constitute the equivalent to a maximal symptom-limited exercise test.45–49 However, to a middle-aged man with stable CHD, it may comprise only a modest effort. In summary, the 6MWT is convenient, but does not assess maximal CRF or detect ischemia, arrhythmias, or other pathological responses to exercise elicited by maximal tests.
Frailty and Functional Performance Assessments
Conceptually, frailty represents decreased physiologic reserve across multiple organ systems, leading to increased vulnerability to adverse events.50 The prevalence of frailty in patients with CV disease is high, including approximately 20% of older patients with chronic stable HF51 and 25% of older patients hospitalized with significant CHD.52 When present, frailty consistently predicts higher rates of morbidity and mortality.22
Weakness, slowness of gait, and generalized fatigue are all common features of frailty and are incorporated into the most commonly used frailty assessments.22,53 Impairments in balance, functional mobility, and endurance may also be evident.54,55 Such impairments typically impact overall performance on the traditional endurance-based assessments (ie, GXT, CPET, and 6MWT) commonly used in CR and have obvious safety implications.
Gait speed is one of the simplest frailty-related functional assessments, while also being an effective frailty screening tool and retaining nearly all the prognostic power of more elaborate, multicomponent assessments.22,50,56,57 Patients are timed as they walk a short distance (usually 4 m) at their usual pace. Among elders, a gait speed >1.0 m/s identifies a relatively robust population, and a value <0.8 m/s is commonly used as a marker of frailty, with lower speeds indicating progressively greater impairment and poorer prognosis.56,57 Slow gait speed is likely to impact performance on walking-based endurance tests and may be associated with impairments in balance and strength,58 which can be further assessed with other simple functional tests.
The Short Physical Performance Battery is a brief (10-12 minutes) 3-component test combining gait speed with assessments of functional strength (timed repeated chair rise) and standing balance.59 Performance on each component is scored from 0 to 4 for a total score of 0 to 12. A score of 6 or less has been proposed as a frailty cut-off22 although limitations in physical functional may be present with higher scores (eg, <10).60 In addition to the composite score, performance in each component can give insights into specific functional limitations. The Timed Up and Go (TUG) is another brief and simple test of functional mobility.61 Patients are timed as they stand from a chair, walk a short distance (3 meters), turn around, and sit down. The TUG has been used to identify frailty and predict risk of falls and functional decline. However, cut-off values (>10-30 seconds) and performance vary considerably depending on study population and outcome measure.62–64
Even if there is little concern regarding frailty, consideration should be given to formal strength assessment among CR participants. Muscle weakness is common in patients with heart disease, especially in HF, where skeletal myopathy is a prominent feature of the disease process and contributes significantly to decreased exercise capacity.65,66 Common comorbid conditions, such as chronic obstructive pulmonary disease or diabetes mellitus, may also adversely impact skeletal muscle. Isokinetic knee strength is a frequently used measure, but requires specialized equipment not widely available at most CR facilities. Dynamometry can be incorporated at relatively little expense to quickly and reliably assess hand grip strength67 or other muscle groups, such as knee extensors.6,68
Frailty-related functional assessments can help identify safety concerns related to falls or injuries, particularly in populations at risk (eg, older, multiple comorbidities) that may not otherwise be identified. Closer supervision and modification of CR training modalities may help address safety concerns identified during these assessments. Older patients at particularly high risk for falls or injuries may benefit from rehabilitation in an alternative setting such as one-on-one training with a physical therapist before participation in group-based CR.69
Limitations of Brief Frailty Assessments
Frailty assessment is rarely simple or straightforward, and physical frailty often does not occur in isolation. Rather, frailty typically involves multiple aspects of a patient's global functioning, extending beyond physical performance to include domains such as cognition or mood, where changes can be subtle and easily missed in routine clinical practice.50,54 At times, referral to a specialist (eg, physical therapist and geriatrician) for a more comprehensive assessment may be appropriate. Although some aspects of frailty may also be responsive to CR and can be evaluated serially to track progress,70,71 the utility of interventions to specifically target frailty or strength deficits is still unproven and it remains an area of active investigation.27,28,72,73
Exercise testing and functional assessment for CR are in a state of evolution. Although original indications for such testing might appear less relevant in the current era of aggressive revascularization and medical therapy for CHD, increasing patient complexities and a wide array of information derived from them strongly support their need. One take-home message is that dogmatic “one size fits all” approaches to care are poorly suited to the wide range of patients in contemporary CR. Exercise testing for a revascularized middle-aged CHD patient may have little similarity to such testing for an older patient with HF with significant comorbidities and clinical complexity. Therefore, patient-centered goals of care should be used as a driving standard by which to consider and select exercise testing options.
A second key take-home message is that exercise testing provides far more than evaluation for myocardial ischemia. Rather, it facilitates a broad spectrum of diagnostic, prognostic, and strategic perspectives to optimize safety and efficacy of management. In fact, most CR patients can benefit from such assessment to optimize the initial exercise prescription irrespective of any concerns regarding ischemia.
A third key point is that given the broad utility of exercise assessment, the range of useful exercise testing options is also much greater than a GXT. Cardiopulmonary exercise testing provides aerobic capacity assessment that is generally superior to standard GXT, both with respect to diagnostic and prognostic assessments, but entails higher costs and management intricacies. Submaximal walking tests also provide diagnostic and prognostic utility and have the distinct advantages of lower costs and greater convenience than GXT or CPET. Measures of strength and frailty are useful in selected settings, particularly as they facilitate insights that are additive to those from other functional assessments and thereby enhance understandings regarding health, disease, physical function, and other vital dimensions of care.
Dr Forman is supported in part from NIA grant P30 AG024827 and VA RR&D F0834-R. Dr Reeves is supported in part from NIA grant R01AG045551.
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