*1.1 Definition*

According to the World Health Organization definition, cardiac rehabilitation is "the sum of activities required to influence, favorably, the underlying cause of the disease, as well as to provide the best possible physical, mental, and social conditions, so that patients may, by their own efforts, preserve or resume optimal functioning in their community and through improved health behavior, slow or reverse the progression of a disease" [1,2]. The objectives of exercise-based cardiac rehabilitation are to increase functional capacity level, reduce anginal symptoms and disability, improve quality of life, modify coronary risk factors, and reduce morbidity and mortality rates [3].

Key elements of the comprehensive cardiac rehabilitation program include [3,4]:


Exercise prescription remains a core component of cardiac rehabilitation.

A significant reduction in cardiovascular mortality and hospitalization has recently been validated by Cochrane analysis. The beneficial effects of cardiac rehabilitation reported in a group of patients after myocardial infarction include reductions in [5–7]:


More recent Cochrane review demonstrated a significant reduction in cardiovascular mortality in exercising group compared with controls (10.4% and 7.6% respectively) [8].

The specific mechanisms responsible for these beneficial effects remain disputable, especially considering the insignificant reduction in the recurrence of myocardial infarction after cardiac rehabilitation. It has been postulated that mortality reduction is a result of a reduction in ventricular fibrillation (decreased sympathetic tone and enhanced parasympathetic tone) or due to the mechanism of ischemic preconditioning [9]. As cardiorespiratory fitness improvement is associated with reductions in mortality after a structured, comprehensive cardiac rehabilitation program, the exact mean change in fitness that occurs has been extensively studied. Sandercock's review of 31 studies demonstrated an increase in fitness of 1.55 metabolic equivalents after phase II. Converting this value to peak oxygen uptake, a gain in cardiorespiratory fitness of 5.4 mL/kg/min can be expected. The reported extent of gain was strictly related to the number of sessions completed, with >36 exercise sessions resulting in a greater gain in fitness [10]. As expected, programs with a significantly lower number of sessions resulted in a smaller increase in cardiorespiratory fitness [11].

The effectiveness of cardiac rehabilitation in reducing either cardiovascular mortality or the rate of myocardial re-infarction largely depends on the exercise volume. Data from meta-analyses have demonstrated specific requirements for the best cardiac rehabilitation outcomes as presented in Table 1 [12].


**Table 1.** Optimal conditions for efficacious cardiac rehabilitation.

Source: Authors compilation based on data from [12].

### *1.2. Development*

In the 18th century, Heberden reported that exertion reduced the frequency of anginal pain. He described a patient's health improvement after sawing wood for 30 min every day [13]. The practice of imposing limited mobility on patients with acute coronary events, however, continued over the next 150 years, leading to prolonged hospital stays and declines in functional capacity [14]. In the 1930s, patients remained bedridden for 6 weeks following myocardial infarction due to the pathologic finding that the infarction evolution process from initial ischemic necrosis to the formation of a stable scar lasts over 6 weeks. Chair therapy was introduced in the 1940s, and, from the early 1950s, a strategy of very short daily walks lasting five minutes was advocated, but only after a coronary event and four subsequent weeks of rest [15]. In 1968, a study by Saltin demonstrated the importance of exercise and the detrimental effect of prolonged bed rest [16]. Further studies by Braunwald, Hutter, Askanas, Rudnicki, Boyle, Sonnenblick, Hellerstein, Naughton, and many others established a strong case for the benefits of exercise and supported the use of early mobilization in hospital settings [17]. As a result, a phasic, multi-profile model of cardiac rehabilitation has gradually been implemented, including a return to work and social activities [18]. Early ambulation has become standard and evolved into phase 1 or inpatient cardiac rehabilitation. Activities performed in the coronary intensive care unit are typically limited to 2 METs and include self-care (such as bed bathing, sitting in a chair) and passive and active ranges of motion. The further rapid development of the use of interventional techniques in cardiology, however, has significantly shortened the duration of hospitalization following an acute coronary event to 3–5 days, thus allowing for more intense mobilization processes to be used.

Contemporary cardiac rehabilitation has gradually evolved into a comprehensive secondary prevention, multi-factorial program consisting of exercise training; the management of cardiovascular risk factors; and nutritional, psychological, behavioral, and social support to improve patient outcomes. The target population for cardiac rehabilitation has expanded significantly over the years, but post-myocardial infarction survivors remain a key population. In the guidelines of the European Society of Cardiology and the American Heart Association, cardiac rehabilitation is mandatory—i.e., it has class I recommendations for therapeutic intervention in many cardiac conditions (e.g., acute coronary syndromes, percutaneous coronary interventions and/or myocardial revascularization surgery, stable coronary artery disease, heart failure, cardiac transplant, left ventricular assistance devices or other implanted devices (cardiac pacemakers, cardioverter-defibrillators, cardiac resynchronization devices), cardiac surgery, and high cardiovascular risk-factor profiles) [19]. The different classes of recommendations, with the corresponding level of evidence-based indications needed for the most frequently referred groups of patients, are listed in Table 2.


**Table 2.** Evidence-based indications for cardiac rehabilitation.

Source: Table by authors.

Cardiac rehabilitation is constantly evolving, with new ideas and strategies being implemented all the time. Examples of new strategies are, hybrid telerehabilitation and high-intensity interval training. Recent COVID 19 pandemic highlighted the role of cardiac telerehabilitation as an efficacious, safe, and essential part of cardiac rehabilitation. There is still, however, a need to enhance cardiac rehabilitation enrollment due to the lower cardiac rehabilitation referral and participation rates of women, the elderly, and minorities [20–22]. Furthermore, cardiac rehabilitation is offered in just 55% of countries, as exhibited in Figure 1 [23].

Figure 1. Global cardiac rehabilitation availability. Source: Reprinted from [23]. **Figure 1.** Global cardiac rehabilitation availability. Source: Reprinted from [23].

Thus, the distribution of cardiac rehabilitation services to the highest possible number of eligible patients should be prioritized, as should overcoming many existing barriers—e.g., the insufficient education of patients, poor adherence, lack of availability of structured programs, etc.
