INTRODUCTION
The intensity of aerobic exercise training is a key issue in cardiac rehabilitation . Exercise intensity is directly linked to both the amount of improvement in exercise capacity and the risk of adverse events during exercise, and intensity ranges for aerobic training prescription are included in several guidelines and publications regarding secondary prevention and cardiac rehabilitation .1 – 4 Cardiac Rehabilitation (CACR) is to provide professionals with a reappraisal of all aspects related to aerobic exercise intensity assessment and prescription, with specific reference to patients with heart disease. Key issues discussed in this statement include: 1) identification of different exercise intensity domains based on the physiological response to constant-work-rate (CWR) exercise; 2) a review of the methods of direct and indirect determination of exercise intensity for both continuous and interval aerobic training ; 3) discussion of the potential effects that different exercise protocols may have on exercise intensity prescription; and 4) indications for recommended exercise training prescription in specific cardiac patient groups.
Of note, in addition to intensity, two other major components of the weekly volume of aerobic training are duration and frequency of the exercise sessions.5 Domains of exercise intensity: the appropriate basis for exercise prescription below, session duration is intuitively and causally dependent on the chosen exercise intensity, that is, the higher the exercise intensity, the shorter the exercise duration, whereas, regarding training frequency, for the purpose of this paper a frequency of three to four sessions per week will be assumed; it is acknowledged that a higher or lower frequency may require modifications of the exercise intensity prescription. As far as the training modality is concerned, the term ‘continuous’ training used in this position statement is intended as a training modality in which an exercise session can be performed for at least 20 minutes with a mild or moderate sense of fatigue; on the other hand, the term ‘interval’ training refers to shorter exercise sessions that cannot be sustained longer on account of an excessive sense of fatigue. Finally, the terms ‘incremental’ and ‘graded’, as referred to for an exercise test, are used interchangeably throughout the text, and the term ‘exercise test’, whenever quoted, stands for incremental/graded exercise test, unless otherwise specified.
DIRECT EXERCISE INTENSITY ASSESSMENT: PHYSIOLOGICAL BASES
Descriptors of the O2 Transport and Utilization System Response
Aerobic exercise intensity is strictly and causally linked to energy expenditure during effort. In an oxygen-dependent biological system, the latter is mostly described by oxygen uptake (VO2 ) through the O2 energy equivalent, equal on average to 20 kJ or 5 kcal per litre of O2 consumed. As a consequence, peak VO2 and the first and second ventilatory thresholds (i.e. the physiological descriptors of the O2 transport and utilization system response to exercise) are the gold standard references for the evaluation of aerobic metabolism function and, consequently, for aerobic exercise intensity assessment and prescription. For the purposes of this statement, the terms ‘ventilatory’ and ‘lactate’ threshold are considered interchangeable; those interested in the relationship between blood lactate and ventilatory gases modifications are referred to previously published reviews.6 , 7
First Ventilatory Threshold
During light to moderate-intensity incremental exercise, aerobic metabolism satisfies almost all of the body's energy needs and blood lactate is not (or is only marginally) elevated above the resting value. However, with increasing effort intensity, a threshold is reached, termed first ventilatory threshold (1stVT), above which blood lactate and pH start to increase and decrease, respectively.6 – 9 2 in excess of that produced by aerobic metabolism, which makes the VCO2 versus VO2 relationship become steeper.6 – 9 2 versus VO2 relationship, and can be identified as the point of transition in the VCO2 versus VO2 slope from less than 1 to greater than 1 (Figure 1(a) ).6 – 9 2 ratio inverts its trend in the presence of a still decreasing or constant VE/VCO2 , which makes the 1stVT also identifiable as the nadir of the VE/VO2 versus work rate (WR) relationship (Figure 1(b) ).6 – 9 10 , 11 2 or 60–70% of peak heart rate (HR).6 – 11
Figure 1: (a) VCO2 as a function of VO2 during ramp incremental cardiopulmonary exercise test. The point where the VCO2 versus VO2 slope increases its steepness is the first ventilatory threshold. The initial and final phases of exercise (vertical dotted rectangles) are usually excluded from the analysis due to possible hyperventilation during these periods. (b) Ventilatory equivalents for O2 and CO2 as a function of work rate during ramp incremental exercise. The nadir of VE/VO2 identifies the first ventilatory threshold, whereas the nadir of VE/VCO2 identifies the second ventilatory threshold. Open circles are blood lactate concentration. 1stVT: first ventilatory threshold; VE/VO2 : ventilatory equivalent for O2 ; VE/VCO2 : ventilatory equivalent for CO2 ; 2ndVT: second ventilatory threshold; WR: work rate.
Second Ventilatory Threshold
With increasing exercise intensity and lactic acid production above the 1stVT, a point is reached when intracellular bicarbonates are no longer able to adequately counteract exercise-induced metabolic acidosis.6 – 9 2 , and this is termed the second ventilatory threshold (2ndVT) or respiratory compensation point'. Simultaneously, the VE/VCO2 ratio inverts its trend (increase versus initial decrease), and the 2ndVT is identifiable as the nadir of the VE/VCO2 versus WR relationship (Figure 1(b) ).6 – 9 2 and 80–90%peak HR reached during incremental exercise, and it has been proposed to be related to the so-called ‘critical power’ (CP),12 13 – 15 Moderate to high-intensity exercise and High to severe-intensity exercise below). However, it is important to recognize that a mechanistic link between 2ndVT and CP has not as yet been proven.
Peak VO2
Peak VO2 is defined as the VO2 value, averaged over a 20- to 30-s period, achieved at presumed maximal effort during an exercise test, while performing dynamic work involving large muscle groups.8 , 9 2 may or may not be equal to VO2max (intended as the ‘true’ insuperable upper limit for aerobic power), even if the available evidence suggests that these two concepts are substantially equivalent.16 2 values) is crucial for correct aerobic training prescription,17 18
Failure of VO2 and/or HR to increase with further increases in WR;
Peak respiratory exchange ratio (VCO2 /VO2 ) ≥1.10;
Post-exercise blood lactate concentration ≥8mmol/L;
Rating of perceived exertion (RPE) ≥18 in the Borg ‘Category Scale’ or ≥8 in the Borg ‘Category-Ratio Scale’;19
Patient appearing exhausted.
Of these, a plateau in the VO2 versus WR relationship during incremental exercise is considered the gold standard for the determination of maximal effort, whereas the cut-offs for peak respiratory exchange ratio and post-exercise blood lactate concentration values proposed above are somewhat limited by high inter-individual variability.20
Domains of Exercise Intensity: The Appropriate Basis for Exercise Prescription
Aerobic exercise training of cardiac patients has traditionally been performed using the CWR modality. In this regard, it is noteworthy that the metabolic and gas exchange responses to CWR exercise differ depending on exercise intensity. This allows for the identification of four exercise intensity domains: light to moderate, moderate to high, high to severe, and severe to extreme. The physiological characteristics of the four exercise intensity domains are summarized in Table 1 .
Table 1: • Physiological Characteristics of the Different Intensity Domains for Constant-Work-Rate Exercise
Light to Moderate-Intensity Exercise
The light to moderate exercise domain encompasses all WRs engendering steady-state VO2 values below that corresponding to the 1stVT.10 , 11 2 and ventilation steady-states are attained relatively rapidly following the onset of exercise. The steady-state is attained more rapidly in trained subjects and is typically delayed by aging, prolonged inactivity and chronic diseases. The attainment of an early physiological steady-state has the effect of limiting the contribution of non-oxidative metabolism to energy turnover and reducing the depletion (e.g. phosphocreatine, glycogen) or accumulation (e.g. inorganic phosphate, Hp ) of fatigue-related metabolites in the working muscles.10 , 11
Moderate to High-Intensity Exercise
This intensity domain comprises those WRs lying between the 1stVT and the CP (Figure 2 ). CP represents the highest WR still sustainable in conditions of both VO2 and lactate steady-state12 – 14 , 21 – 26 27 2 and 70–80%peak HR as assessed by incremental exercise testing, but with higher steady-state %peak VO2 and %peak HR values during CWR exercise. This is due to the appearance of a ‘slow component’ of the VO2 kinetics after approximately two to three minutes of the start of CWR exercise in this intensity domain, which is not detectable during incremental exercise.28 2 slow component elevates the VO2 above the VO2 that would be expected for a given WR, resulting in a delayed attainment of the VO2 steady-state (by as much as 10–15 minutes or more). In other words, the VO2 and HR steady-state is reached at a level higher than expected according to the below-1stVT VO2 versus WR relationship10 , 11 , 28 Figures 3 and 4 ). The VO2 slow component represents a loss of muscle efficiency,29 2 cost of exercise and, for this reason, exercise in the moderate to high domain cannot be sustained for as long as that in the light to moderate domain. However, continuous exercise sessions of 30-minute duration are obtainable in the moderate to high-intensity domain even in patients with a significantly reduced exercise capacity, such as those with chronic heart failure (CHF).30
Figure 2: Time to exhaustion as a function of power in patients with chronic heart failure and in untrained normal subjects. Critical power is the power asymptote of the hyperbolic relationship linking time to exhaustion and power (work rate) during constant-work-rate exercise in the high to severe- and severe to extreme-intensity domains. Five constant-work-rate exercise tests at 50%, 65% and 80% of the difference between peak work rate and that corresponding to the first ventilatory threshold, and to 100% and 120% of peak work rate were used to fit the relationship. Open circles and filled squares represent patients with chronic heart failure and untrained normals, respectively. Dashed vertical lines are mean critical power values for the two groups. Redrawn from Mezzani et al.
30 High to Severe-Intensity Exercise
This intensity domain comprises all the WRs above CP that cause VO2 to reach its peak value with no steady-state attainment.10 , 11 2 slow component is evident and VO2 may rise with a close to mono-exponential profile that is truncated at peak VO2 . Consequently, the high to severe-intensity domain presents a broad range of exercise WRs at which peak VO2 will be reached. In this domain, blood acid-base balance is severely perturbed,25 31 21 – 23 Figure 2 ) and is typically in the range of 3–20 min. Given both such a short exercise session duration and the impossibility of a steady-state attainment, this intensity domain can only be used for interval, rather than continuous, aerobic training prescription.
The slow component rise in VO2 during moderate to high- and high to severe-intensity CWR exercise means that VO2 changes not only as a function of WR, but also as a function of time in these domains. Consequently, irrespective of the subject's/patient's peak exercise capacity, without precise stipulation of the exercise conditions and timing of measurements, it is difficult to define a given intensity as a percentage of peak VO2 in these intensity domains.
Severe to Extreme-Intensity Exercise
Given the finite kinetics of VO2 , it is inevitable that some WRs are so high that fatigue intervenes before peak VO2 can be achieved. In this domain the tolerable duration of exercise is limited to less than about 3 min.32
Need for Direct Evaluation of Functional Capacity: The Role of Exercise Testing
The administration of an incremental, that is, graded, exercise test to patients entering outpatient cardiac rehabilitation programs has been recommended since the 1970s.33 , 34 cardiac rehabilitation programs strongly recommend exercise testing as a key component of the initial patient assessment, and address the concept of exercise testing as a tool for exercise training evaluation, risk stratification to determine the required level of supervision and monitoring, and individualized exercise prescription.1 – 4 , 35 – 39 cardiac rehabilitation without an exercise test. Reasons stated for the lack of testing include shorter hospital stays, more aggressive revascularization interventions, increased sophistication of diagnostic procedures, extreme deconditioning, orthopaedic limitations, left ventricular dysfunction, knowledge of the coronary anatomy, recent and successful coronary revascularization and uncomplicated myocardial infarction.39
For cardiac rehabilitation patients for whom there are no exercise test results, it is recommended that the exercise prescription could be based on the level of exercise performed during the inpatient phase of rehabilitation and recommended home exercise activities, while monitoring for signs and symptoms of exercise incompetence. In this context, it is important to understand the difference between a diagnostic exercise test and one performed to assess the functional capacity for cardiac rehabilitation purposes. A diagnostic exercise test is often carried out in pharmacologic wash-out as one of the initial steps in determining whether signs or symptoms that are present (e.g. chest pain or dyspnoea) are cardiac in origin. At entry into cardiac rehabilitation , patients already have documented cardiac disease; thus, a test administered at this time on current therapy serves primarily as a functional evaluation to quantify exercise capacity, chronotropic and inotropic responses to exercise, and presence and severity of dysrhythmias, as well as identify signs, symptoms, or other clinical evidence of any residual myocardial ischemia. Using an alternative standardized approach to prescribing exercise intensity (e.g. 20 bpm >resting HR) that is not based on current exercise capacity because exercise test data are not available (see Unavailability of exercise testing below), potentially minimizes the beneficial effects of exercise training and is likely to retard patient progress during cardiac rehabilitation . Thus, it is important to administer a functional sign/symptom-limited exercise test prior to patients beginning cardiac rehabilitation . In this regard, an incremental standard exercise test or cardiopulmonary exercise test with respiratory gas analysis (CPX), which is the gold standard for a direct assessment of the exercise intensity descriptors outlined in Descriptors of the O 2 transport and utilization system response and Domains of exercise intensity: the appropriate basis for exercise prescription above, should be used whenever possible to obtain an exercise prescription tailored on the individual patient's functional capacity and pathophysiological picture.
INDIRECT EXERCISE INTENSITY ASSESSMENT
The Heart Rate Versus VO2 Relationship
HR is widely used for exercise intensity assessment and prescription on the grounds that a linear relationship between HR and both VO2 and WR increase during incremental exercise is known to exist.40 , 41 42 Table 2 )43 2 . On this basis, a ‘target HR range’ is usually proposed in normal subjects ranging between 70 and 85%peak HR.44 2 ,3 , 44 , 45 Table 2 ). However, it must be considered that intensity classifications such as those shown in Table 2 do not reveal to which precise intensity the 1stVT and 2ndVT correspond in an individual patient. Indeed, %peak HR values commonly used for training prescription both in normal subjects and cardiac patients have been found to be associated with levels of metabolic stress higher than those of both the 1stVT and 2ndVT.46 – 49 50 51
Table 2: • ACSM Classification of Exercise Relative Intensity
The Heart Rate Reserve Versus VO2 Reserve Relationship
The concept of HR reserve (HRR) and VO2 reserve (VO2 R), defined as the difference between the basal and peak HR and VO2 values, respectively,52 – 55 52 , 53 2 R (%VO2 R) in both normal individuals undergoing both cycle and treadmill exercise54 – 56 Table 2 ) and in cardiac patients.57 2 reserve' concept fits closely the need for a precise exercise intensity definition since it describes the true amount of energy one can utilize for maximal effort attainment, taking into account the baseline level. As a consequence, %HRR has been adopted by the American College of Sports Medicine as the gold standard for exercise intensity indirect assessment, and ‘training HRR ranges’ of 60–80 %HRR for persons without overt disease44 45 aerobic training intensity.58 47 , 50 , 59 – 61 51 , 62
However, a loss of linearity of both the VO2 versus WR and HR versus WR relationships as peak VO2 is approached has been described in cardiac patients63 64 , 65 2 R values on the basis of %HRR has been demonstrated in CHF patients both on- and off-beta-blockers.66 2 R equivalence may be preserved in CHF patients on optimized beta-blocking therapy.67
The RPE Versus VO2 or HR Relationship
RPE is commonly employed in cardiac rehabilitation , either as a primary indicator of exercise intensity or as an adjunct to HR monitoring.19 , 44 , 68 cardiac rehabilitation staff members when comparing the perceived demands of various exercise devices, for example arm versus leg exercise.
The commonly utilized scales for the RPE include the original ‘Category Scale’ (RPE Borg scale), which rates exercise intensity from 6 to 20, and the ‘Category-Ratio Scale’ (CR10 Borg scale), which utilizes a numerical range from 0 to 10.19 2 (Table 2 ). This approximately corresponds to 2.5–6 on the CR10 Borg scale, which is also loosely associated with a 60–90% range of peak HR and approximately 50–85% of peak VO2 . Several papers have supported the reliability of RPE for effort intensity assessment and prescription in normal individuals and cardiac patients both off- and on-beta-blockers, with a good correspondence between a value of around 13 and the 1stVT.69 – 76 77 78
Intensity and Volume of Exercise Training
Volume of exercise training is associated with overall energy expenditure expressed in kilocalories, for example kilocalories per week.79 44 , 80 – 82
Unfortunately, although the intensity of effort may be appropriate, the stamina required to exercise for 40 minutes and six days per week may be overwhelming. However, as patients improve their fitness level, and become able to expend, say, 7kcal/min, duration and/or frequency can be adjusted.
As the volume of exercise will impact kilocalorie expenditure, it is important to consider the contribution of all three components of the exercise prescription, namely exercise intensity, duration of activity and frequency of exercise sessions.
EXERCISE INTENSITY PRESCRIPTION
Prescribed Exercise Intensity: General Concepts
The idea proposed in this statement is that aerobic exercise prescription in cardiac patients should be based on the choice of a specific exercise intensity domain determined by: i) the patient's clinical and pathophysiological picture, ii) the peculiar physiological response to and the evidence-based benefits of exercise in the different intensity domains and iii) the goals of the rehabilitation program. This involves a shift from a ‘range-based’ to a ‘threshold-based’ aerobic training prescription, based on data obtained by incremental standard exercise test or CPX in the individual patient.
Of note, some caveats must be taken into account when transferring information obtained from incremental exercise testing to CWR exercise, as underlined in Domains of exercise intensity: the appropriate basis for exercise prescription above and in Figures 3 and 4 :
Beyond the 1stVT, a given relative exercise intensity expressed as %peak VO2 will result in an energy expenditure higher than expected (moderate to high-intensity domain) or equal to peak VO2 irrespective of the prescribed relative intensity (high to severe-intensity domain) when performing CWR exercise; as a consequence, %peak VO2 must be used with caution as a reference for training intensity prescription in these domains, since the individual patient's actual energy expenditure during CWR exercise is not easily predictable.
In the CWR moderate to high-intensity domain some relative intensities may not be easily attainable (‘chequered area’ in Figure 3 ).
As shown in Figure 4 , for a given VO2 value, the WRs included in the light to moderate domain are not the same when performing incremental versus CWR exercise. The VO2 versus WR relationship is shifted to the right in the former due to an initial lag in the VO2 increase,83 2 versus WR values are measured after a VO2 steady-state has been reached, thus excluding the initial VO2 on-response delay. As a consequence, when prescribing CWR training in the light to moderate domain on the basis of incremental exercise data, it is necessary to reduce the WR prescription to a lower iso-VO2 value (Figure 4 ). The more prolonged the initial lag of the VO2 response to incremental exercise is, the greater the reduction should be and, as a rule of thumb, should amount to around 10 W for a 10 W/min incremental protocol in the general population of cardiac patients. Experimental confirmations are needed as to this point for moderate to high-intensity CWR exercise.
Figure 3: Relationship between relative exercise intensity as evaluated by ramp incremental cardiopulmonary exercise test and during constant-work-rate exercise sessions at corresponding work rates. The linear relationship between VO2 and work rate is lost for constant-work-rate exercise above the first ventilatory threshold, as determined during the incremental cardiopulmonary exercise test. This means that the VO2 steady-state is reached at a level higher than expected according to the below-first ventilatory threshold VO2 versus work rate relationship (moderate to high-intensity domain) or not attained at all (high to severe-intensity domain). Given the above, for constant-work-rate exercise in the moderate to high-intensity domain some relative intensities may not be easily attainable nor prescribable (chequered area). The relationships shown in the moderate to high-intensity domain are illustrative, and may not be quantitatively confirmed in individual patients. See text for further details. 1stVT: first ventilatory threshold; 2ndVT: second ventilatory threshold; CWR: constant-work-rate; SS: steady-state.
Figure 4: VO
2 as a function of work rate in a single ramp incremental versus several constant-work-rate cardiopulmonary exercise tests. The initial lag of the VO
2 response (grey area) during incremental exercise (full line) shifts the VO
2 versus work rate relationship rightwards with respect to that obtained from several constant-work-rate tests (open diamonds–dotted line). For a given VO
2 value, the first and second
ventilatory thresholds will thus be reached at a lower work rate when exercising at constant-work-rate than during incremental exercise. A VO
2 slow component adds to the on-response during constant-work-rate exercise above the first ventilatory threshold, projecting the steady-state VO
2 to a level higher than expected according to the below-first ventilatory threshold VO
2 versus work rate relationship (dotted line). Critical power, assessed as shown in
Figure 2 , is the highest work rate sustainable in VO
2 steady-state conditions (filled diamond). In the high to severe-intensity domain, VO
2 is, by definition, systematically projected to peak VO
2 irrespective of the applied work rate. See text for further details. 1stVT: first ventilatory threshold; 2ndVT: second ventilatory threshold; CP: critical power; sc: VO
2 slow component; WR: work rate.
Bearing this in mind, the available evidence supporting the prescription of aerobic training in cardiac patients in the different intensity domains is as follows:
Light to moderate-intensity domain. The lowest aerobic training intensity still able to provide a training effect likely depends mostly, in both normal subjects and cardiac patients, on pre-training exercise capacity. In agreement with the lower fitness–lower training stimulus intensity principle,84 aerobic training intensities as low as 40%peak VO2 (corresponding to about 25%VO2 R) have proven to be effective in CHF patients with significantly reduced pre-training peak VO2 .85 , 86 Moderate to high-intensity domain. Strong evidence has accumulated with regards to adverse event-free moderate to high-intensity exercise in cardiac patients with both preserved and reduced left ventricular ejection fraction.87 – 91 Aerobic training in this domain can still be performed in a continuous modality, with reported training sessions of 15–30 min duration.87 – 91 2 compared with normal subjects. For example, it has been reported that during a six-minute walking test (considered to closely mimic habitual walking activities), CHF patients exercise at a percentage of peak VO2 often above that corresponding to their 1stVT.92 – 94 2 conditions above the 1stVT could be crucial to avoid fatigue and termination during activities of daily living. In this regard, recent data show that CHF patients may exercise at the CP (i.e. the upper limit of the moderate to high-intensity domain) for 30 minutes without incurring adverse events;30 High to severe and severe- to extreme-intensity domains. In recent years, training in the high to severe- and even severe to extreme-intensity domain using interval training has proven effective in improving exercise capacity in different cardiac patient populations,95 – 97 2 as low as 13 ml/kg per min96 Interval training prescription below). As indicated in Domains of exercise intensity: the appropriate basis for exercise prescription above, the upper limit of the high to severe-intensity domain is the highest WR that will allow for the attainment of peak VO2 . Of note, times to exhaustion during CWR exercise at 100% peak WR and 135% peak WR have been reported to last on average three minutes and 90 seconds, respectively, not only in both untrained and trained normal subjects but also in CHF patients.30 , 98 Table 3: • Upper Limits of the Different Intensity Domains
Based on the physiological definition of CWR exercise intensity domains (Table 1 ), the upper limits of such domains can be defined by using physiological and/or performance parameters as shown in Table 3 , where correspondence with exercise intensity classes according to the ACSM exercise intensity classification is also reported. In the absence of a direct assessment of aerobic metabolism descriptors by CPX, effort relative intensities can be expressed as %peak HR, %HRR, %peak WR, or using Borg scales. Generally speaking, and with the limitations reported in The heart rate versus VO 2 relationship , The heart rate reserve versus VO 2 reserve relationship and The RPE versus VO 2 or HR relationship above, intensities close to the 1stVT (50%peak VO2 ) should lie around 60%peak HR, 50%peak HRR, 50%peak WR, or 12–13 in the RPE Borg scale, whereas intensities close to the 2ndVT (70–80%peak VO2 ) should lie around 70%peak WR or 15–16 in the RPE Borg scale (Table 3 ).
Interval Training Prescription
Interval training can be defined as repeated bouts of short-duration, high to severe- or severe to extreme-intensity exercise (i.e. 10 seconds to five minutes), separated by brief periods of lower-intensity CWR exercise allowing for active recovery. The term aerobic interval training (AIT) is often used to describe interval training in the high to severe-intensity domain. Although a significant contribution from anaerobic energetic sources to the total energy yield is unavoidable in this domain, most of the energy needed is still produced aerobically (see Domains of exercise intensity: the appropriate basis for exercise prescription above). Currently, the most used AIT model consists of 10min warm-up followed by 4×4-min intervals at 85–95%peak HR, with active recovery phases of 3 min at ∼70%peak HR (Figure 5 ). This AIT model has now been used in several studies, both in healthy subjects and in various cardiac patient populations,95 , 96 , 99 , 100 2 per exercise session actually remarkably similar among patient groups (Figure 6 ).
Figure 5: The 4 × 4 min aerobic interval training model. Intensity is given as percentage of peak heart rate.
AIT has shown significantly greater cardiovascular effects when compared with isocaloric moderate to high- intensity continuous training, both in coronary artery disease (CAD) and CHF patients95 , 96 Figure 6 ). AIT has also been shown to exert favourable effects on left ventricular systolic function. In healthy men, stroke volume has been shown to increase significantly more after high-intensity AIT compared with lower-intensity training of the same energy expenditure .99 96 96 101 102 96 103 104
Figure 6: Peak VO
2 before and after aerobic interval training and moderate to high-intensity continuous training in patients with coronary artery disease and chronic heart failure. Modified from Rognmo et al.
95 and Wisløff et al.
96 CAD: coronary artery disease; CHF: chronic heart failure; AIT: aerobic interval training; MCT: moderate to high-intensity continuous training; *
p < 0.05
vs . MCT.
In contrast to the described 4×4-min AIT model, shorter, sprint-type intervals of all-out severe to extreme- intensity exercise have been shown to induce rapid changes in exercise capacity, improving work performance due to enhanced skeletal muscle energy metabolism with modest effects on peak VO2 .105 , 106 97 , 107 2 to that after more traditional continuous training in CAD patients, but with increased time to exhaustion at 90%VO2 R.108
In a clinical setting, AIT can be performed as uphill walking or running on a treadmill according to the 4×4- min protocol. Patients are supposed to exercise with an intensity corresponding to 85–95%peak HR during the high to severe-intensity intervals, which makes patients breathe heavily without experiencing chest or leg pain. To ensure that the relative intensity is maintained throughout the whole training period, the WR should be adjusted continuously based on the individual HR response. In the recovery periods, patients are supposed to exercise at intensities of ∼70%peak HR. In clinical practice, however, it is sometimes necessary to adjust the HR zones, especially the moderate to high-intensity recovery ones, based on the patient's subjective feelings. In the RPE Borg scale, patients should exercise at an intensity of 15–18 in the high to severe-intensity intervals. Exercise modes other than treadmill walking are possible and AIT using an aerobic exercise group setting has been shown to be feasible in CHF patients.109 110 2 was seen six months after the end of an AIT rehabilitation period.100 111
Arm Exercise Intensity Prescription
Peak HR and VO2 values are significantly lower for arm than for leg exercise, likely due to the reduced muscle mass of the arms. Additionally, at the same absolute WRs, the VO2 for arm work is greater than that of leg work, owing to the reduced mechanical efficiency of arm exercise. Consequently, the cardiovascular responses to standardized WRs during arm ergometry are greater, particularly HR and blood pressure, compared with leg exercise.112 – 116 2 derived from treadmill CPX, by way of example, may result in absolute WRs which are substantially greater than what a patient may be able to comfortably achieve during arm ergometry. The exercise prescription generated from %HRR or %peak HR methods attained from standard treadmill or cycle ergometry exercise testing will often provide a safe guideline for the patient, whereby WR can be adjusted to achieve an appropriate HR response. In this regard, it has been suggested that, at a given submaximal WR, for obtaining a HR similar to that obtained during leg ergometry, a WR equal to about two- thirds of the latter should be used during arm ergometry.117
In addition to the differences in physiological responses to acute arm versus leg exercise, the principle of exercise training specificity suggests that cardiovascular and metabolic adaptation to acute exercise is specific to the type of exercise performed and the muscles involved.118 cardiac rehabilitation exercise training program may need to include arm exercise.118 – 120
Weight Loss-Targeted Exercise Intensity and Daily Physical Activities Intensity Prescription
Obesity or being overweight affects more than half of the adult populations in the developed world and both are associated with an increased risk of many chronic diseases. A large body of evidence demonstrates that even modest weight loss, as low as 3–5% of body weight, by regular physical activity is associated with decreased chronic disease risk.121 122 123 123 124 – 126
These results support findings that 30 minutes of activity daily may be sufficient to lose weight and prevent weight gain.127 , 128 129 , 130 131
The results of the research cited above suggest that incorporating physical activity into daily life improves health outcomes, body weight and visceral adiposity. There appears to be a dose–response related to weight loss from light to high-intensity activity, but both light to moderate- and moderate to high-intensity activity may result in significant weight loss when maintained over time, and especially when combined with appropriate caloric intake for body size and daily energy expenditure . Research suggests that a minimum of 30 minutes of light to moderate activity on a daily basis is the threshold to result in weight loss, but that increasing the duration to 45–60 minutes, or increasing the intensity to moderate to high levels, may further enhance weight loss and cardiorespiratory fitness.
Unavailability of Exercise Testing
There is no contemporary, scientifically validated reason for cardiac rehabilitation programs to substitute the current standard of formal incremental exercise test or CPX, including diagnostic 12-lead ECG appraisal, for other assessments of functional capacity (see Need for direct evaluation of functional capacity: the role of exercise testing above). Therefore, cardiac rehabilitation professionals are strongly encouraged to use every option and opportunity to ensure their patients benefit from standard exercise tests or CPX. However, in cardiac rehabilitation programs where these kinds of exercise tests are not available, there are alternative strategies that may assist programs in both stratifying patients with regard to their risk of exercise-associated adverse events and in developing an exercise prescription. In an effort to try to determine subsequent event risk in cardiac rehabilitation populations without exercise testing results, the six-minute walking test has been proposed as a reasonable alternative to a more formal exercise capacity evaluation.132 – 134 36 135 136 cardiac rehabilitation programs where formal exercise testing cannot be performed or is simply not a readily available service, a risk stratification protocol utilizing the patient's CCS Class, NYHA Class and six-minute walking test has been developed (Table 4 ).37
Table 4: • Exercise-related Risk Stratification
Once a patient's exercise risk has been determined without the aid of a standard exercise test or a CPX, an exercise prescription can be developed using Borg scales and/or subjective tools such as the ‘talk test’.68 , 137 2 to allow for an exercise prescription to be determined;138 , 139 69 – 73 , 140 The RPE versus VO 2 or HR relationship above). In addition, the use of RPE as an acceptable measure of the physiologic response to exercise appears to be valid for patients receiving beta-blockers.76 141 2 R,142
PATIENT GROUPS
Indications for aerobic exercise intensity prescription in specific cardiac patient groups are summarized in Table 5 ; only intensity domain data for which scientific evidence is available in a given cardiac patient group have been included, with grey-shaded areas indicating that there are no available data to warrant a recommendation. Physiological, performance and perceived exertion limits of the different exercise intensity domains are provided in Table 3 , and both directly (i.e. by incremental CPX) and indirectly (i.e. by incremental standard exercise test) assessed physiological and performance limits are shown. As already emphasized in Prescribed exercise intensity: general concepts above, the choice between different exercise intensities in a specific patient will depend on the individual's clinical and pathophysiological status, the evidence-based benefits of exercise in the different intensity domains for that specific patient group and the goals of the rehabilitation program. The information provided in this section are to be considered complementary to those furnished by the recently published EACPR paper ‘Importance of characteristics and modalities of physical activity and exercise in the management of cardiovascular health in individuals with cardiovascular disease (Part III)’.143
Table 5: • Evidence-based Prescribable Aerobic Exercise Intensity in Cardiac Patient Groups
Stable Angina Pectoris
For patients with stable angina pectoris (SAP) secondary to coronary atherosclerosis, the benefits of cardiac rehabilitation are unequivocal and it should be considered standard care for all patients with CAD.144 , 145
The purpose of the warm-up is to increase blood flow to the skeletal muscles, in preparation for exercise, and to facilitate coronary vasodilatation. The anti-ischemic benefits of an adequate warm-up, prior to the initiation of light to moderate/moderate to high exercise have been demonstrated.146 – 148 149 37 150 , 151 152 153
In recent years, research into the most appropriate intensity of exercise training for patients with CAD has focused on the use of AIT (see Interval training prescription above). Although the cardiorespiratory benefits of AIT are well documented in athletes, a recent systematic review of interval training in patients with CAD found only two controlled and five randomized controlled trials, with a total of 213 patients.104 aerobic training . In a study on SAP patients, Guiraud and co-workers found that shorter bouts of severe to extreme-intensity exercise (15 seconds compared with 60 seconds) combined with a passive, rather than active, recovery phase, resulted in improved patient comfort and longer time spent at >80% of peak VO2 .154 44 155 , 156
In summary, it is suggested that patients with SAP exercise three to five times per week, following an adequate warm-up of five to 15 minutes, at moderate to high intensity (in any case below the ischemic threshold) for a period of 20 to 40 minutes (not including warm-up and cool-down), followed by a cool-down period of five to 10 minutes. Most importantly, patients with SAP should engage in the type of exercise activity they find most enjoyable and, therefore, sustainable.
PCI
The same evidence and clinical practice recommendations developed for patients with SAP regarding exercise (seeStable angina pectoris above) are likely applicable to most patients post-PCI. Presently, there is no evidence to suggest that early exercise training and exercise testing post-PCI is either unsafe or adversely affects patient outcomes,157 – 159 160 161 162 163 103 , 157 , 164 – 166 167
Data on the specific effects of exercise intensity on patient outcomes post-PCI are sparse. Munk et al. found that high to severe-intensity interval training helped to reduce six month restenosis in the stented coronary artery segment as assessed by quantitative coronary angiography; this effect was associated with improvements in aerobic capacity and attributed to improved endothelial function and reduced systemic inflammation.103 164 , 166 Aerobic training may result in increased endothelial NO production and/or reduced NO destruction and this may lead to reduced vascular inflammation and reduced restenosis.168 – 170 171 172 , 173
Pacemakers and Implantable CardioverterDefibrillators
Patients implanted with permanent pacemakers (PMs) usually follow the same principles for aerobic training intensity prescription as non-PM-implanted patients, provided an adequate chronotropic response to exercise is warranted by the patient's sinus node and/or the device.174 2 also in this population.175
Patients with implantable cardioverter defibrillators (ICDs) can undergo aerobic exercise training, but care must be taken to avoid receiving inappropriate shocks during exercise. These could occur in the event that the exercise HR increases so that it is within the programmed ventricular tachycardia zone or if exercise-induced supraventricular tachycardia develops. Exercise intensities in the light to moderate and moderate to high domains have been found effective in improving peak VO2 in patients with an ICD. It is important to note that exercise training intensities used in all of these studies resulted in HRs that were 15–20 beats lower than the ICD threshold for detection and termination of ventricular tachycardia.176 – 179 3
Chronic Atrial Fibrillation
Chronic atrial fibrillation (AF) is a very common arrhythmia, characterized by irregularly irregular atrial and ventricular depolarizations. The prevalence of chronic AF is quite high in patients older than 60 years and the arrhythmia may present as ‘lone’ AF or associated with comorbid conditions, such as CHF or valvular heart disease. Patients with chronic AF frequently have incomplete ventricular filling, which leads to an impaired cardiac output response to exercise, very rapid ventricular rates during effort (possibly exceeding predicted maximum) and, ultimately, reduced peak VO2 and VO2 at 1stVT in comparison with patients in sinus rhythm.180 2 values lower than those of CHF patients in sinus rhythm, but with 1stVT occurring at a higher percentage of peak VO2 .181 2 ) and chronotropic response to exercise.182 – 184 aerobic training prescription in some patients, making subjective RPE the most reliable means for exercise intensity assessment and prescription. In this regard, given the high prevalence of chronic AF in elderly patients, randomized controlled studies addressing the most effective type of exercise intensity assessment and prescription in this population are strongly needed.
Coronary Surgical Revascularization
Patients who have undergone traditional open-chest coronary artery by-pass graft (CABG) surgery as well as minimally invasive procedures are a substantial proportion of cardiac rehabilitation exercise programming participants, and this group includes many patients who are age 65 years or older.185 186 aerobic training intensities have proven effective,100 , 187 – 192 2 .193
Because of the possibility of graft closure, program staff should be alert for new patient complaints of angina pectoris or angina-equivalent symptoms or signs, such as exercise intolerance or new ECG signs of myocardial ischemia. Patients should also be educated regarding these possibilities. Recognizing whether the revascularization was complete or incomplete is valuable in this regard as the latter may increase the likelihood of postsurgical signs and symptoms of residual myocardial ischemia during exercise, which may significantly affect the results of the rehabilitation process.194
Valve repair or replacement
The exercise prescription and training of patients with recent valve replacement or repair is very similar to that used with CABG surgery patients.186 , 195 , 196 193 Coronary surgical revascularization above. Exercise intensities in the light to moderate- and moderate to high-intensity domains have been used in patients with recent heart valve replacement or repair and balloon valvuloplasty, demonstrating significant effects on exercise capacity and quality of life.197 – 201 aerobic training in patients with previous aortic valve replacement.202
Patients with valvular heart disease but without valve repair or replacement may also be referred for cardiac rehabilitation . In these patients, critical aortic stenosis is a formal contraindication for exercise training. Patients with less-severe aortic stenosis can exercise but may develop symptoms, for example dyspnoea and significant fatigue, at a given WR. Exercise training intensity should be kept under the threshold that precipitates the onset of symptoms, because these symptoms indicate that their cardiac output is not capable of meeting the demands of that level of exercise.
CHF and Left Ventricular Assist Devices
A reduced ability to perform aerobic exercise is the hallmark of the CHF pathophysiological picture.203 2 transport chain from ambient air to the skeletal muscle, the major consequence of which is a reduced cardiac output and peripheral microcirculatory response relative to exercise-related metabolic needs.204 , 205 206 207 206 208 209 210 2 and VO2 at 1stVT are typically reduced with respect to age-matched normal subjects, and their reduction is proportional to the severity of the syndrome.211 aerobic training .85 , 86 , 96 , 212 Interval Training Prescription above).
Among patients with advanced CHF, that is, by definition with severely reduced exercise capacity and presumed high exercise-related risk, left ventricular assist device implantation is increasingly used as a bridge to transplantation or even as permanent therapy. Patients with left ventricular assist devices can often be managed at outpatient clinics, and an early initiation of exercise training after implantation has been reported to be associated with improvements in exercise capacity.213 2 in this population.214 – 216
Heart Transplantation
Exercise training is recommended for all patients before and after heart transplantation.186 , 217 218 219
Given the complexity of hemodynamic and cardiorespiratory responses during incremental exercise in this population, exercise intensity may best be determined by RPE. At the start of training programs, an RPE of 10–12, that is, light to moderate-intensity in the RPE Borg scale, will generally account for the surgical and disease deconditioning as well as any potential exercise issues associated with steroid myopathy.220 221 , 222 cardiac rehabilitation program.220 – 223 aerobic training .
CONCLUSIONS
In current cardiac rehabilitation practice, the choice of the aerobic training stimulus intensity in individual patients remains largely a matter of clinical judgement. This European, US and Canadian joint position statement provides evidence-based indications for a shift from a ‘range-based’ to a ‘threshold-based’ aerobic exercise intensity prescription, to be combined with thorough clinical evaluation and exercise-related risk assessment. The importance of functional evaluation through exercise testing prior to starting an aerobic training program is strongly emphasized, and an incremental cardiopulmonary exercise test, when available, is proposed as the gold standard for a physiologically comprehensive exercise intensity assessment and prescription. This would allow professionals to match the unique physiological responses of different exercise intensity domains to the individual patient pathophysiological and clinical status, maximizing the benefits obtainable from aerobic exercise training in cardiac rehabilitation .
—Acknowledgments—
This statement was approved by the European Association for Cardiovascular Prevention and Rehabilitation on 28 November 2011, the American Association of Cardiovascular and Pulmonary Rehabilitation Board of Directors on 6 March 2012 and the Canadian Association of Cardiac Rehabilitation on 5 June 2012.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
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