Volume

Exercise volume is the product of frequency, intensity, and time (Volume = Frequency × Intensity × Time). Energy expenditure can be estimated according to the suggested equations [19]: x (kcal) = 3.5 × MET × body mass × time (min)/200 or according to the formula:

x (kcal) = MET × body mass × time (h).

Example: An 80 kg patient is walking briskly for 30 min at a speed equal to 5 MET; therefore, his/her energy expenditure equals 5 (MET) × 80 (kg) × 0.5 (h) = 200 kcal.

### Progression

Exercise progression rate depends on a patient's clinical status, fitness level, and training response [1]. It is reasonable to increase training duration by 5–10 min over 1–2 weeks [12]. Progression should take place gradually and only if tolerance to the current training parameters has been attained. Typically, the training duration is increased prior to the load or frequency being increased [8,21]. Table 25 exhibits the aerobic exercise prescription recommended by the AACVPR.


**Table 25.** AACVPR aerobic training recommendations [1].

SBP—systolic blood pressure. Source: Adapted from [1].

### *4.4. Aerobic Training Intensity*

## 4.4.1. Indices of Exercise Intensity

The exercise intensity prescription is recommended based on assessment with the cardiopulmonary exercise test (CPET), which is the gold standard for assessing exercise capacity. Alternative objective methods for prescribing exercise intensity based on heart rate may be affected by medications lowering heart rate, such as *beta-blockers* or chronotropic incompetence, defined as the inability to increase the heart rate adequately during exercise to match the cardiac output to metabolic demands. Subjective methods for determining exercise intensity include the rate of perceived exertion and the talk test; these methods should only be considered as an adjunct to the objective methods mentioned above [4].

Objective Indices

A. Indices of Peak Effort: 1. % of Peak Oxygen Uptake (VO2peak): The gold standard for assessing cardiovascular fitness. Training maximal oxygen uptake = % of maximal oxygen uptake. 2. % of Peak Work Rate:

1—Estimating peak work rate during incremental cycle ergometry.

Training work rate (watts) = % of peak work rate.

2—Estimating peak work rate during incremental treadmill:

% of metabolic equivalent (MET) reserve %.

Training MET reserve = % of (peak MET − resting MET) + resting MET (resting MET = 1).

3. % of Peak Heart Rate:

This method is not recommended for patients undergoing treatment with beta-blockers.

Training heart rate = % of maximal heart rate.

4. % of Heart Rate Reserve:

Heart rate reserve (HRR) based on the Karvonen formula (HRR in %)

Training target heart rate (THR) = % of (maximum heart rate − resting heart rate) + resting heart rate. The THR intensity percentage usually ranges from 40% to 80%. Peak values of these indices represent the highest values attained during the last 20–30 s of a symptom-limited cardiopulmonary stress test. Typically, a so-called peak respiratory exchange gas ratio (RER) > 1.1, a plateau of oxygen uptake, and/or heart rate with increasing effort are used as determinants of maximal or near-maximal effort [22,23]. Aerobic exercise intensity indices are presented in Table 26.


**Table 26.** Aerobic exercise intensities [12].

Source: Adapted from [12].

However, the major limitation of this is that not all patients with cardiovascular diseases achieve maximal or near-maximal effort during CPET. Moreover, a major concern has recently emerged with regard to a discrepancy between exercise domains based on peak exercise indices and individual responses to exercise, as described in a chapter regarding intensity domains [24].

B. Ventilatory Thresholds

The first ventilatory threshold represents a transition point from aerobic metabolism to lactate rise in the blood, with steady-state and blood lactate levels of 1.5–2.0 mmol/L [21,25]. The increase in blood lactate accumulation elicits fast breathing to remove the extra carbon dioxide produced by the buffering of acid metabolites. Therefore, before VT1 intensity, relatively small amounts of lactate are produced.

The second ventilatory threshold, or respiratory compensation threshold or "critical power", reflects the exercise intensity at which rapid lactate increase occurs, with a blood level of 3–5 mmol/L. As a result, the rise in carbon dioxide output (VCO2) is disproportionate to the carbon dioxide output.

The two most-popular methods of VT1 determination are the relationship of the nadir of VE/VO<sup>2</sup> to the work rate—i.e., the lowest point in the curve before an increase in VE/VO2—and the V-slope method. VT2 represents the nadir of the VE/VCO<sup>2</sup> to work rate relationship—i.e., the lowest point in the curve before the VE/VCO<sup>2</sup> increases [21]. These thresholds are extrapolated to the corresponding heart rates and work rates, determining exercise intensity domains. It has been postulated that exercise intensity is low at heart rates and work rates below VT1, moderate to intensive at heart rates and work rates between VT1 and VT2, and high at heart rates and work rates above VT2 [4,21,25]. Exercise prescription based on ventilatory thresholds improves the peak oxygen uptake more effectively than if based on the % of peak oxygen uptake in healthy individuals [26,27]. These data, however, should be confirmed in patients with cardiovascular disease. A major limitation of ventilatory threshold-based exercise prescription remains the lack of ergo-spirometry in many cardiac rehabilitation facilities. Other restrictions—e.g., substantial inter-and intra-observer variability—are reported using the V-slope method [28].

Another disadvantage of the extrapolation of ventilatory thresholds is that it cannot be translated directly into constant-load exercise training. This can be explained by the so-called lag-time—i.e., the initial oxygen uptake on-response delay between the onset of the ramp and the onset of linear increase in oxygen uptake [29]. Therefore, it has been proposed that the constant-load exercise prescription should be 10 watts lower than one executed by the 10 W/min incremental protocol at the beginning of cardiac rehabilitation [21].

Subjective methods (1):

1. The Rating of Perceived Exertion (RPE): Borg Category Scale, with recommended values of 11–16 from the Borg 6–20 scale or Borg Category Ratio Scale.

The Borg Scale [30]:

7: very, very light; 9: very light; 11: fairly light; 13: somewhat hard; 15: hard;

17: very hard; 19: very, very hard.

The alternative scale of perceived exertion is the 0–10 Borg Category Ratio Scale, which is more intuitive and allows for better patient cooperation than the 6–20 scale. 0–10 Borg Category Ratio Scale:

0: nothing at all; 1: extremely weak, just noticeable; 2: very weak; 2.5–3: weak; 4–5: moderate; 6–7: strong; 8–10: very strong.

RPE scales reflect the subjective feeling of aerobic exercise intensity a person experiences during exercise [30]. Despite their feasibility, many studies have demonstrated the insufficient correlation of RPE scales with % of peak oxygen uptake, lactate level, and respiratory rate. RPE may also be influenced by psychological and environmental conditions [31]. In clinical practice, ratings of perceived exertion are predominantly used in the case of patients without a reliable heart rate, i.e., patients with atrial fibrillation, who have undergone heart transplantation, or with chronotropic incompetence [4].

Interestingly, it has been postulated as a useful tool for maximal symptom-limited stress test termination cut-off in healthy individuals. Sirco et al. assessed the exercise test endpoints that coincide best with ECG changes in a healthy population (85% of maximal age-based heart rate, RPE, and METS). The rating of perceived exertion appeared to be the most significant endpoint, with an average value of 17 at peak exercise [32].

### 2. The Talk Test

The talk test has gained popularity as a simple subjective tool for monitoring appropriate exercise prescription. As a safe method, it has been widely utilized, predominantly in home-based cardiac rehabilitation [33]. In clinical practice, the talk test facilitates maintaining an exercise intensity at which conversation is still comfortable. Several studies have tested the effect of the talk test on the breathing rate; several have reported that a rapid increase in breathing rate beyond the second ventilatory threshold causes difficulty in talking during exercise; however, these studies are inconsistent. In addition, it has been documented that talk tests have a weak correlation with ventilatory thresholds [34]. In contrast, the stronger relation of the talk test to physiological and perceptual variables analogous to the lactate threshold than to the ventilatory threshold has been demonstrated [35]. Furthermore, as the talk test is linked to an increased breathing rate related to the second ventilatory threshold, it cannot be used to determine the first ventilatory threshold; thereby, it is not utilized in guiding low-intensity exercise. Consequently, besides RPE, the talk test should be used as an adjunct to guide exercise intensity in patients with cardiovascular diseases in activities such as their activities of daily living [6].

### 4.4.2. Aerobic Intensity Domains

### Range-Based Approach

The range-based exercise prescription principle is based on extrapolating the percentage of the peak oxygen uptake into a corresponding percentage of the peak heart rate. The suggested training heart rate "zones" for healthy individuals' range between 70 and 85% of the peak heart rate, and for patients with cardiovascular diseases, training intensities range between 40 and 80% of the peak oxygen uptake [36, 37]. The well-known Karvonen method, which utilizes a percentage of the heart rate reserve, with heart rate reserve equal to 60%, has been demonstrated to correspond with the first ventilatory threshold [38]. The Karvonen method gained popularity worldwide and was adopted by the American College of Sports Medicine as the gold standard for exercise intensity. The evaluation of the recommended training HRR zone using the Karvonen method can provide an indirect assessment of the training HRR zone of 60–80% of the heart rate reserve for healthy individuals and 40–70% of the heart rate reserve for patients with cardiovascular diseases.

### Threshold-Based Approach

In 2013, Mezzani et al. promoted a threshold-based approach because exercise intensity can be determined more accurately in relation to the first and the second ventilatory thresholds than when it is expressed as a percentage of the peak exercise capacity [21]. This approach represented a shift from range-based to threshold-based aerobic training prescription. According to the study of Mezzani et al., the first ventilatory threshold, which is reached at around 50–60% of peak VO<sup>2</sup> or 60–70% of the peak heart rate, is a point between the light-to-moderate-intensity and moderate-to-high-intensity effort domains [39]. The second ventilatory threshold, usually attained at around 70–80% of peak VO<sup>2</sup> and 80–90% of peak HR during incremental exercise, marks the upper intensity limit for prolonged aerobic exercise [40]. Both ventilatory thresholds allow for the identification of four exercise intensity domains: light to moderate, moderate to high, high to severe, and severe to extreme.

According to this concept, there are four domains of exercise intensity:

1. The first ventilatory threshold reflects very light exercise as presented in Table 27. Exercising in this domain is generally well tolerated and sustainable for a long period (>30–40 min).


**Table 27**. Very light exercise intensity parameters.

Source: Adapted from [21].

2. Between the first and the second ventilatory thresholds reflecting light to moderate exercise (with both aerobic and anaerobic energy supply) as exhibited in Tables 28 and 29:

**Table 28.** Light exercise intensity parameters.


Source: Adapted from [21].

### **Table 29.** Moderate exercise intensity parameters.


Source: Adapted from [21].

3. The second ventilatory threshold reflects heavy exercise, as presented in Table 30:


Source: Adapted from [21].

In this intensity domain, only interval aerobic training can be used for exercise prescription [41].

4. The next domain reflects severe-to-extreme-intensity exercise, with a tolerable exercise duration of less than 3 min.

Many recent studies have revealed inconsistencies between exercise intensity prescriptions based on the ventilatory thresholds and indicators derived from peak exercise parameters in cardiac patients [24,42,43]. Hence, position statements on aerobic exercise intensity have evolved over the last few years, and some concepts have been modified subsequently [25]. Hansen et al. compared the exercise training parameters measured at the first (VT1) and second (VT2) ventilatory thresholds with exercise intensity domains following the existing cardiac rehabilitation guidelines (% of peak oxygen uptake (% of peak VO2), % of peak heart rate (% of peak HR), % of peak watts (% of peak W), and % of heart rate reserve (% of HRR)). A total of 272 cardiovascular disease patients performed a maximal cardiopulmonary exercise test on a bike (peak respiratory gas exchange ratio > 1.09). The VT1 and VT2 were determined and extrapolated to % of peak VO2, % of peak HR, % of HRR, and % peak W. Surprisingly, the results revealed a significant discrepancy between individuals' response to exercise and the guideline-based exercise intensity domains. VT1 was noted at 62 ± 10% of peak VO2, 75 ± 10% of peak HR, 42 ± 14% of HRR, and 47 ± 11% peak W, which corresponded, in fact, to the high-intensity-exercise domain (for % peak VO<sup>2</sup> and % of peak HR) or the low-intensity-exercise domain (for % of peak W and % of HRR). Inconsistency related to the VT2 was also noted at 84 ± 9% of peak VO2, 88 ± 8% of peak HR, 74 ± 15% of HRR, and 76 ± 11% of peak W, corresponding to the high-intensity-exercise domain (for % of HRR and % of peak W) or the very-hard-exercise domain (for % of peak HR and % of peak VO2). The use of % of peak W in only 63% and 72% of all patients for VT1 and VT2, respectively, corresponded to the same guideline-based exercise intensity domain, whereas it only corresponded in 48% and 52% of patients when using the % of HRR and % of peak HR, respectively. In particular, peak VO<sup>2</sup> was related to significantly different guideline-based exercise intensity domains [24].

### 4.4.3. Current Guidelines

Published statements on aerobic exercise intensity have recently been modified regarding previously reported inconsistencies [25]. The current recommendations emphasize optimizing total energy expenditure rather than one specific training feature (e.g., exercise intensity). Nevertheless, determining the exercise intensity in patients with cardiovascular diseases remains important for making exercise programs more time-efficient and achieving short-term clinical benefits. A personalized patient-centered approach should be utilized (with self-selected rather than imposed intensities regarding long-term adherence). Moreover, peak indices,

such as peak oxygen uptake or heart rate, should be carefully applied. If CPET is performed, the assessment of the first and second ventilatory thresholds should be carried out for the determination of the aerobic exercise intensity in most patients with cardiovascular disease.

The talk test and Borg RPE scale should only be used as adjuncts to objective aerobic exercise intensity determination. Progression should be made with the targeted exercise session duration achieved before the exercise intensity is increased. Although cardiopulmonary exercise testing represents the gold standard in functional capacity assessment and exercise prescription, many cardiac rehabilitation centers still lack access to cardiopulmonary testing equipment. Thereby, for the EAPC, the minimum requirement is a cycle ergometry test, with the determination of the exercise intensity based on the % of peak workload or peak heart rate (considering all the described limitations), while the ultimate requirement would be to execute a CPET with the subsequent exercise intensity domain determined based on ventilatory thresholds [44]. Subsequent exercise intensity adjustment after 3 months based on CPET or ergometry is recommended [25].

Different exercise intensity domains for different groups of patients with cardiovascular disease have been recently suggested [21].

The Table 31 shows the initial exercise prescription by the AACVPR for cases without performed exercise test [1].


**Table 31.** Initial exercise prescription without exercise test.

Abbreviations: AACVPR—the American Association of Cardiovascular and Pulmonary Rehabilitation; METS—multiples of resting metabolic equivalent; RPE—rating of perceived exertion. Source: Adapted from [1].

### *4.5. High-Intensity Interval Training*

### 4.5.1. Concept of High-Intensity Interval Training

High-intensity interval training (HIIT) consists of alternating periods of intensive aerobic exercise with periods of passive or active recovery [45]. Recovery phases are usually of low intensity (below the first ventilatory threshold). HIIT was used by athletes for several decades [46,47] before it was applied in patients with coronary artery disease and chronic heart failure in the 1990s in Germany by Katharina Meyer [48,49]. Many studies show that significant physiological differences exist between exercising at a continuous moderate intensity versus HIIT. The greater utilization of carbohydrates during HIIT in comparison with MICT ultimately causes a greater increase in the mitochondrial content of the skeletal muscles. A substantial increase in the total time at high intensity will cause the skeletal muscles to be exposed more to intense exercise training. As expected, it has been demonstrated that HIIT enables exercise time to be maintained for longer periods in comparison with moderate-intensity continuous modes; hence, it has emerged as a promising alternative training method [50]. Moreover, it is postulated that patients may feel more confident performing HIIT, and they may find it an attractive form of training, as the protocol is more diverse than it is during a constant workload. In addition, in a study conducted by Wisloff, reverse left ventricular remodeling after HIIT was found [51].

### 4.5.2. HIIT Protocols

In practice, the prescription of HIIT is complex, allowing for an unlimited number of potential exercise/recovery interval combinations, with the operation of up to nine variables (work interval intensity and duration, recovery intensity and duration, exercise modality, number of repetitions, number of series, and between-series recovery duration).

The recovery phase is crucial and has a powerful impact on performance [50, 52]. The most applied HIIT model comprises 10 min of warm-up followed by four hard segments lasting for 4 min each at an intensity above the second lactate threshold (typically at 90% of peak HR), divided by 3 min recovery segments [53]. Passive recovery segments have an intensity below the first lactate threshold, and the intensity of active recovery segments is set beyond the first lactate threshold—i.e., at 70% of peak heart rate.

Guiraud et al. compared the use of different HIIT protocols for patients with CAD:


All training models included 8 min of warm-up. As a result, the longest time to exhaustion was seen in model A and was significantly longer than in models B and D. In other words, short (15 s) bouts of high-intensity exercise with a passive recovery phase have emerged as the most effective. Moreover, model A showed superiority in terms of perceived exertion, patient comfort, and time spent above 80% of maximal oxygen uptake. Thus, passive recovery models seem to allow for the better utilization of energetic substrates [54].

The same group of researchers suggested HIIT as a strategy for CAD patients with preserved left ventricular ejection fraction and exercise tolerance > 5 METs, as follows:

Two introductory sessions at 60% of peak power output, subsequent progression to 80% of peak power output, and further progression to 100% of peak power output if well-tolerated. In the case of patients with reduced left ventricular ejection fraction, these researchers recommend beginning in continuous mode for at least 2 weeks (or 8–10 sessions), then progressing training to HIIT, as described above. Fifteen-second phases at 100% of maximal aerobic power interspersed with short phases of passive recovery have been well tolerated in patients with coronary artery disease [50,54]. In addition, the complete disappearance of clinical and ECG signs of ischemia has been observed, with no recurrence seen. This finding may mimic the phenomenon of ischemic preconditioning [55]. The use of successive phases of high-intensity exercise interspersed with periods of rest may favorably affect the myocardium. Recent studies conducted in animal models have demonstrated that intermittent ischemia provoked by HIIT results in the formation of collateral coronary vessels [56]. Many studies have demonstrated that HIIT can be an attractive alternative for patients with CAD and HF [57,58]. A popular HIIT protocol for heart failure individuals was introduced by Meyer, with the progression of the training occurring through the shortening of active phases with a concomitant intensity increase up to 80% of the maximal short-term exercise capacity. Exercise intensity has been characterized as the percentage of so-called maximal short-term exercise capacity (MSEC), while MSEC has been determined by utilizing the steep ramp cycle ergometer test. The most popular protocol incorporates 30 s exercise phases at 50% of MSEC and 60 s phases of active recovery (at 10 watts). The gradual shortening of exercise phases with concomitant increases in intensity (to 15 s at 70% of the MSEC, then to 10 s at 80% of the MSEC) has been used without changes in the recovery period [59].

### 4.5.3. HIIT versus Moderate-Intensity Continuous Exercise

In the last decade, debate has emerged as to whether HIIT is more effective than moderate-intensity continuous exercise (MICE) regarding improvements in functional capacity. In answer to this, multiple studies have been performed in cohorts of patients with coronary artery disease and in heart failure patients with

a reduced or preserved left ventricular ejection fraction [60–62]. A meta-analysis evaluating 24 studies with over 1000 participants demonstrated a more significant improvement, of 1.4 mL/kg/min, in peak oxygen uptake after the use of HIIT compared to MICE. In an attempt to confirm these beneficial effects of HIIT, two large multicenter studies comparing HIIT versus MICE in patients with coronary artery disease (the SAINTEX-CAD study) and in patients with heart failure with reduced left ventricular ejection fraction (SMARTEX-HF) have been conducted. More than 200 patients with reduced left ventricular ejection fraction were included in the SMARTEX-HF study, and the SAINTEX-CAD study encompassed 200 patients with coronary artery disease and normal left ventricular ejection fraction. In contrast to earlier findings, SAINTEX-CAD and SMARTEX-HF demonstrated no superiority of HIIT versus MICE in terms of improving peak oxygen uptake [63,64]. The effect of HIIT has also been investigated in heart failure patients with preserved left ventricular ejection fraction [65]. HIIT has been found to induce a greater improvement in aerobic capacity in this group compared with MICE. These data, however, should be interpreted with caution due to the small study group used (19 patients). Further large studies appear to be necessary to confirm the beneficial effect of HIIT in this group. In summary, HIIT appears to be safe and non-inferior versus MICE in patients with coronary artery disease and in heart failure patients and incorporating HIIT may be beneficial for fostering long-term adherence to physical activity, as its interval nature appears to make it more attractive to patients. Larger trials are warranted to confirm optimal HIIT models and the groups of patients that should be targeted.

The idea of a combined approach—i.e., beginning with moderate-intensity continuous training, followed by a high-intensity interval approach—has been successfully implemented as presented in Figure 5 [66].

#### *4.6. Aerobic Training Protocols*

### 4.6.1. Introduction

555

Aerobic or cardiorespiratory training is rhythmic in nature and involves large muscle groups. There are two types of aerobic training: continuous and interval [1]. Currently available cardiac rehabilitation software provides for the following training modes [21,67]:

1. Continuous, load-controlled training: After a few minutes of warm-up, the load is constant, followed by a cool-down. Training intensity requires manual adjustment.

2. Interval load-controlled training: This involves blocks of active/hard and recovery phases. Training intensity requires manual adjustment.

3. Continuous, heart-rate-controlled training: This is the most advanced option. After setting the training heart rate range, the system automatically adjusts the exercise intensity to keep the programmed heart rate within this range.

Constant-workload exercise, up to 45–60 min, typically at moderate or moderate-to-high intensity, is currently the most widely recommended aerobic exercise modality.

Interval-mode exercise at low, moderate, or moderate-to-high intensity is usually conducted on leg cycle ergometers; typically, the intensity of the first few hard/active segments is reduced, allowing for an adequate warm-up. With a gradual increase in intensity over a few weeks, patients with good adaptation can be switched to steady-state exercise and subsequently to high-intensity interval training [21].

4.6.2. Parameters of Training Protocols

A. Leg cycle ergometer:

1. Continuous watt-controlled training.

Used in patients with good functional capacity. Stress testing using a cycle ergometer is preferred. Training intensity: 30% (40%)–80% of peak work rate/heart rate reserve.

2. Interval watt-controlled training.

Used in patients with low functional capacity (as low-intensity interval training) or patients with moderate-to-high- or high functional capacity (as moderate- or high-intensity interval training). Prior to training, a stress test on a cycle ergometer is recommended.

Training intensity:


Phase duration:


3. Continuous, heart-rate-controlled training.

Preferable for patients with good functional capacity. Training intensity: 30%–70% of heart rate reserve.

B. Treadmill:

1. Continuous MET-controlled training.

Suitable for patients with good or very good functional capacity. Training intensity is typically up to 70% of MET reserve, with a resting MET equal to 1.

2. Interval MET-controlled training.

Suitable for patients with moderate or high functional capacity. Training intensity:


Active phase durations of 2–4 min and recovery phase durations of 1–3 min are recommended.

3. Continuous heart-rate-controlled training.

Used in patients with good functional capacity. Training intensity: up to 70% of heart rate reserve.
