B. High-risk patients

The definition of a high-risk patient encompasses patients with:


These patients should manifest clinical stability prior to commencing supervised exercise training program to minimize the risk of left ventricular decompensation and complex ventricular arrhythmias. Exercise training for high-risk patients can be delivered as early outpatient or residential programs [4].

Cardiopulmonary exercise tests are recommended for all patients with advanced heart failure to determine exercise intensity in relation to their ventilatory threshold. Diagnostic stratification for patients with heart failure based on CPET-derived parameters has been widely described [15].

Cardiopulmonary exercise testing-derived parameters represent the best basis for exercise prescription. As discussed earlier, exercise intensity zones corresponding to a recovery zone, a light-to-moderate-intensity exercise zone, and a high-intensity exercise zone have been identified.

Cardiopulmonary exercise testing has limited availability; therefore, alternative methods for guiding exercise prescription—i.e., methods based on heart rate and subjective indices such as the Borg scale or talk test have been used. The biggest limitation of the heart-rate-based approach, however, remains the possible impact of chronotropic incompetence or medications that lower the heart rate. Exercise training principles for high-risk patients include the use of low-intensity interval training, moderate-intensity continuous training, or a high-intensity interval approach [12]. The low-intensity interval mode on a bicycle allows precise load changes, and the use of hard and recovery segments of 30 and 60 s duration, respectively, is suggested. The initial intensity of the hard segments should not surpass 50% of the maximum Watts attained during the incremental bicycle test. After a few weeks of well-tolerated training progression, a continuous workload can be implemented. Continuous moderate-intensity exercise is the most popular mode of training executed in cardiac rehabilitation centers, with the intensity set between ventilatory thresholds, or between 50% and 80% of the heart rate reserve. For selected patients with very good physical capacity and good tolerance of steady workloads, a high-intensity interval mode can be offered [55,57].

### 4.6.4. The Authors' Approach

The authors endorse the "ABCD" exercise training model proposed by Rudnicki for different groups of patients stratified by risk. This model provides a meticulous exercise training prescription for four separate training groups of patients as exhibited in Table 32.


**Table 32.** Aerobic exercise training models proposed by Rudnicki.

Abbreviations: HR—heart rate; HRR—heart rate reserve; METS—multiples of resting metabolic equivalent; W—watts. Source: Adapted from [70].

The D model is assigned for patients at the highest risk and with the lowest functional capacity; therefore, individual training is applied with an acceptable heart rate increase of up to 20 bpm above resting heart rate. This model is used by the authors in patients who are unable to perform exercise testing. Typically, patients progress from moderate- to vigorous-intensity aerobic endurance exercise over the course of the program. The authors initially implemented a moderate-intensity interval training protocol (MIIT) on treadmills or cycle ergometers. Further progression to moderate-intensity interval-to-continuous (MIITC), or steady-state exercise (MICE) was implemented a few weeks after the patient's adaptation to the current workload (Figure 6).

**Figure 6.** Training progression model from moderate-intensity interval training (**A**) to moderate-intensity interval-to-continuous training (**B**) conducted at the authors' center. Source: Figure by authors.

A summary of the exercise prescription progression suggested by the European Association of Preventive Cardiology and the modified approach developed by the authors is provided in Table 33.


Abbreviations: EAPC—the European Association of Preventive Cardiology; HIIT—high-intensity interval training; LIIT—low-intensity interval training; MICE—moderate-intensity continuous exercise; MIIT—moderate-intensity interval training; MIITC—moderate-intensity interval-to-continuous. \*—in selected patients. Source: Table by authors.

### *4.7. Training Heart Rate in Practice*

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The adequate prescription of exercise training results in heart rate reduction, both at rest and at any given workload [21]. This physiological principle serves

in a practice as a strong indicator of maximal aerobic fitness improvement. As discussed in detail earlier, an optimal exercise prescription principle should be based on the extrapolation of the percentage of the cardiopulmonary test-derived indices into corresponding heart rate values [25]. The limited availability of CPET, however, results in utilizing an alternative approach for training heart rate calculation. Training heart rate can be determined in a few steps [8]:

1. Maximal heart rate is calculated from a symptom-limited stress test or by age-related formulas (e.g., 220-age, Tanaka, Inbar). Note that the "220-age" formula underestimates the maximal heart rate in patients over the age of 45, as demonstrated in Table 34.


**Table 34.** Maximum heart rate calculation formulas.

Source: Adapted from [8].

2. In cases where no exercise test is performed, subsequent heart rate deduction is required if the patient is on beta-blocker therapy. Beta blockade blunts heart rate response and thus affects the maximal heart rate. There is no consensus as to how much should be deducted from the maximal heart rate in the case of beta blockade [71,72]. The authors of this publication deduct between 10 and 30 bpm depending on the beta-blocker dose [8].

3. The next step is to determine the heart rate reserve using the Karvonen formula [1,12], considering HR max from the symptom-limited test or from the age-based formula after potential beta-blockade correction.

training heart rate = ((HR max − resting HR) × % required) + resting HR

For patients with heart failure, Keteiyan established a separate formula [73]:

119 + (0.5 × resting heart rate) − (0.5 × age) − (0 if test on treadmill/5 if bike).

When utilizing Keteiyan's formula, there is no need for beta-blockers to be considered.

The use of heart rate calculations in practice has been demonstrated below:

**Example 1.** *A 50-year-old male patient underwent a symptom-limited stress test. A resting heart rate of 60 bpm was recorded, and the patient attained a maximal heart rate of 140 bpm. The test was terminated due to fatigue.*

*Heart rate reserve = 140* − *60 = 80 bpm. Planned exercise intensity of 40–50% of heart rate reserve. Thus, 40% of 80 is 32, and 50% of 80 is 40. These values should be added to the resting heart rate (60 + 32 = 92, 60 + 40 = 100), giving a recommended training heart rate range of between 92 bpm and 100 bpm.*

**Example 2.** *A 60-year-old female patient on beta-blocker therapy (low dose of beta-blockers) with a resting heart rate of 70 bpm. An exercise test on a treadmill was terminated prematurely due to pain in the left knee. A bicycle exercise test was unavailable. The planned exercise training intensity was 50–60% of the heart rate reserve. The maximal heart rate calculated by the Inbar equation was 160 bpm (220-age). As a next step, 10 bpm was deducted due to the use of beta-blockers in her therapy. Thus, a maximal heart rate of 150 bpm as calculated (220* − *60 = 160, and 160* − *10 = 150 bpm). Heart rate reserve = maximal predicted heart rate minus resting heart rate—i.e., 150* − *70 = 80 bpm. The planned exercise intensity was 50%–60% of the heart rate reserve; therefore, 80* × *50% = 40, and 80* × *60% = 48. Considering her resting heart rate, a training heart rate range between 110 and 118 bpm should be applied (70 + 40 = 110, 70 + 48 = 118).*

**Example 3.** *A 70-year-old male patient with heart failure and a resting heart rate of 80 bpm. A stress test on a treadmill utilizing the Naughton protocol was terminated early (after 30 s) due to fatigue. The maximal attained heart rate of 95 bpm was documented at the test termination.*

*Planned initial training heart rate of 40% of heart rate reserve. Maximal heart rate calculation according to Keteiyan's formula:*

*maximal heart rate = 119 + (0.5* × *80)* − *(0.5* × *70)* − *0 = 124 bpm*

*Heart rate reserve calculation: 124* − *80 = 44. A planned training heart rate of 97 bpm was calculated (44* × *40% = 17, 80 + 17 = 97).*
