1. Introduction
Heart disease (HD), as a relatively common disease of the circulatory system, is a general term [
1] that refers to cardiac injury or abnormal heart function. Heart disease can be congenital (CHD) [
2] or acquired. Common symptoms of HD include chest tightness, chest pain, palpitations, shortness of breath, fatigue, dizziness, and sweating, with chest pain being the most typical symptom [
3]. Heart-disease-associated morbidity and mortality rates have increased in recent years, affecting 30–40% of patients over 60 years of age [
4]. This is mainly due to the patients ignoring their health and external environment for a long period of time. This leads to high levels of circulating triglycerides [
5], which can contribute to atherosclerosis—the most common cause of heart disease. Heart disease can lead to sudden cardiac death, representing a serious threat to human health [
6].
Cardiac rehabilitation (CR) aims to improve the daily living and quality of life of patients with HD through a comprehensive rehabilitation strategy to prevent the recurrence of cardiovascular disease [
7]; it includes rehabilitation assessment, regular medication, exercise therapy, diet therapy, behavioral therapy, and health education [
8]. Cardiac rehabilitation is generally aimed towards patients with stable angina, atherosclerosis, heart transplants [
9], and/or cardiac infarctions [
10]. Exercise therapy is a very important part of CR. Physical function and the ability to perform daily work and life activities can be improved by exercise [
11]. Exercise also improves vascular endothelial function [
12] and resistance to disease, increases the excitability of the nervous system, enhances the activity of cardiomyocytes, and reduces cardiovascular risk factors and mortality [
13].
Moderate intensity continuous training (MICT) is the preferred exercise modality for CR because of its high safety and effectiveness [
14]. However, high-intensity interval training (HIIT) has recently been found to be more effective than MICT in the CR of patients with cardiovascular disease [
15]. High-intensity interval training is a training method in which the practitioner exercises with extreme intensity (i.e., equal to or greater than the anaerobic threshold or maximum lactate steady-state load intensity) for a few seconds to a few minutes with sustained effort interrupted by a short, inadequate rest interval so that the body is in an incomplete recovery state [
16]. High-intensity interval training has been shown to be safe and is used in the rehabilitation of patients with chronic diseases, as well as for physical fitness by the general population [
17].
Bibliometric analysis is a key process for further understanding of a field through functions such as clustering, which can help to determine research topics and sort through large amounts of literature data [
18]. CiteSpace is a citation visualization analysis software platform developed in the context of scientometrics and data visualization [
19] that analyzes literature citations in specific regions and presents the structure, laws, and distribution of scientific knowledge through visual graphs [
20]. CiteSpace is now widely used to study disciplinary dynamics and hotspots and can clarify the research status of a field by analyzing the number of articles published, institutional collaboration networks, author partnerships, and keyword co-occurrence and clustering. Therefore, this study aimed to use CiteSpace to explore hot research topics and new directions regarding the role of HIIT in CR, and to describe the dynamic development of the field.
4. Discussion
Cardiac rehabilitation is designed to improve the function and structure of the heart in patients with heart disease and to optimize the patient’s physical and mental state [
23]. Long-term HIIT intervention produces good results, increasing the patients’ ability to care for themselves and leading to an improved quality of life. CiteSpace provides analysis by visual means to present the structure, pattern, and distribution of scientific knowledge and summarize the relationships between authors, countries, journals, and institutions through visual mapping, which is now widely used to study disciplinary dynamics and hot spots.
Trends in the impact of HIIT on CR and the level of research interest in this area can be reflected by the number of papers published over a given time period. Our data show that the benefits of HIIT are being increasingly reported, particularly regarding its role and safety in CR. More relevant research findings are bound to emerge, as shown by the trendline of the index.
Among the top 10 journals with the highest number of publications, Circulation from the USA ranked first in IF, CiteScore, and citation frequency. The category of the journal is medicine, its subcategory is cardiology and cardiovascular medicine, and its rank in this category is 1/336. This indicates that the academic papers published in this journal are of high quality and have a strong influence. In terms of countries of publication, four of the top nine journals were from the USA, indicating that the USA has invested more and attached more importance to this area of study.
Collaboration mapping allows researchers to describe the contributions of individual scholars to the field and their collaborative relationships with one another, as well as to visually identify the more prolific authors who have published more papers [
24]. Among them, Juneau, M and Nigam, A from Canada ranked first in terms of total citations, indicating that their scholarly work has been recognized by numerous scholars. Juneau, M and Nigam, A focused on the effects of HIIT intervention in patients with heart disease—particularly those with heart failure and reduced ejection fraction [
25,
26,
27]. A comparison or combination of different forms of exercise was also performed to find the most suitable form of exercise for cardiac patients [
28,
29,
30]. A comparison of HIIT and MICT revealed that the HIIT intervention was more effective, safer, and better tolerated by patients [
31]. Additionally, experiments with HIIT exercise prescriptions of different intensities were conducted, and a progressive individualized model was proposed to be refined and used according to the patient’s specific situation [
32]. Several cooperative networks exist to study HIIT in CR, permitting higher levels of communication and cooperation. However, no cooperative relationships were seen between different cooperative groups, suggesting that they focused on internal cooperation.
Among the top 10 most cited articles, the “Cardiovascular risk of high versus moderate-intensity aerobic exercise in coronary heart disease patients” had the highest total number of citations, indicating that this article is of high academic value and reflects research hotspots and trends to a certain extent. This article compares the effects of HIIT and MICT on cardiovascular rehabilitation. The results showed that both HIIT and MICT reduced the incidence of cardiovascular events, but HIIT provided greater cardiorespiratory protection and was more suitable for patients with coronary artery disease [
33].
The results showed that Norway, Canada, and the United States were dominant in the field of HIIT in CR. These countries constituted a connection point for exchange and learning between other countries, and this improved academic research and exchange between countries. China only had cooperative relations with the United States, having little exchange with other countries. There was still a big gap between Norway, Canada, and other countries. Future research on HIIT in CR in China could be improved by increased exchanges with researchers from other countries.
The research institute can directly see the leading position and the backbone of the research field of HIIT’s effect on cardiac rehabilitation [
24]. In line with previous analyses of authors and countries, the main research institutions were the Norwegian University of Science and Technology, University of Oslo, University of Queensland, and some other universities. These universities are mainly located in countries such as Norway and Canada, further indicating that these regions are among the world leaders in research on the effects of HIIT on cardiac rehabilitation. Most of these research institutions are universities. There were no Chinese universities listed, suggesting a significant gap between China and other countries in this field.
Keywords provide a high-level summary of the research topic and research content and can reveal the core content of an article; they can also reflect both the basic direction of the research content and relationships between individual keywords [
34]. Keyword co-occurrence refers to the occurrence of different keywords in the same document and is used to explore hotspots in the research field [
35]. The size of the keyword nodes can be observed in the graph, which represents the frequency of keyword occurrence. The higher the keyword frequency in a certain field, the stronger the indication that it is a research hotspot, although not all high frequency keywords have high centrality. Centrality is used to determine the importance of the literature, and higher centrality indicates that the keyword has more influence on the whole field. Those keywords with a centrality greater than 0.1 are considered key nodes and potential research hotspots. One study found that implementing a long-term CR program led to sustained muscle strength gains in men with cardiovascular disease, reducing the decline in health caused by aging and the decline in muscle mass associated with age [
36]. Wang Bozhong et al. [
37] found that HIIT training after a myocardial infarction in rats could protect cardiomyocytes and blood vessels, reduce inflammation of the heart and peripheral circulation, reduce cardiac fibrosis, and improve cardiac function. Abdelhalem et al. conducted a study of an HIIT model that included aerobic exercise, in order to reduce the prevalence and mortality of patients with coronary heart disease, and were able to increase the aerobic capacity of patients and improve the VO
2max of cardiac patients [
38]. The main indicators of CR’s effectiveness were exercise capacity, cardiorespiratory fitness, and quality of life. The effectiveness of HIIT was identified via the patient’s physiological response. Studies have shown that HIIT can significantly increase peak oxygen consumption (VO
2peak) after interventions for coronary artery disease, thereby reducing the risk of death and improving the cardiorespiratory fitness of patients with coronary artery disease. High-intensity interval training can also achieve similar results to MICT in a shorter period of time [
39]. VO
2peak and maximal oxygen consumption (VO
2max) represent not only improved circulatory function, but also increased exercise capacity [
40,
41]. Ellingsen et al. [
42] performed a 12-week trial of regular HIIT and showed that it could reverse the left ventricular remodeling of patients with heart failure and improve their aerobic capacity [
43]. High-intensity interval training significantly improves cardiac ejection fraction (i.e., increased end-diastolic ventricular volume and decreased end-systolic ventricular volume in patients in heart failure) and can significantly improve systolic-diastolic function [
44]. High-intensity interval training is a short and efficient exercise method with a proven safety profile and few adverse effects in heart failure studies [
45,
46]. After the HIIT intervention, the overall physical health of the patients improved, their physical functions were enhanced, and the long-term intervention produced good results, increasing the patients’ ability to take care of themselves and leading to an improved quality of life. Contraindications to training should be identified and evaluated in advance, permitting the selection of appropriate exercises, strict exercise supervision, and termination of training if necessary [
47].
Researchers were more focused on cardiovascular diseases such as coronary artery disease and myocardial infarction, so as to understand the adverse effects of these diseases and the benefits of the combination of drug and exercise therapy. Even when medication is ineffective, HIIT can provide symptom relief, yielding some improvement in exercise capacity and quality of life [
48]. Exercise compliance in patients with a cardiovascular disease is a necessary foundation for good long-term recovery, but poor compliance has also been an important problem. The HIIT program can be combined with different exercise modalities to make it easier for patients to stick with the program by adjusting the intensity and enhancing the fun of the exercise, thereby imparting improved body composition, cardiopulmonary function, and glucolipid metabolism. As scholars continue to study HIIT, there has been an increased focus on postoperative rehabilitation tools for patients suffering from coronary artery disease, and myocardial infarction. This shift in research is in line with realistic developmental characteristics [
49]. Patients with cardiovascular disease can exhibit symptoms such as autonomic nervous system disorders that lead to an imbalance between sympathetic and naval nerves, resulting in reduced myocardial contractility and increased cardiac load. Implementing HIIT in cardiac rehabilitation programs can improve patients’ cardiovascular health and autonomic nervous system function [
50]. Moderate-intensity continuous training, as a traditional form of aerobic exercise, was more commonly applied as endurance training for people, which was beneficial for the rehabilitation of patients with heart disease. Meanwhile, HIIT, in contrast to MICT, can increase aspects such as peak oxygen uptake, heart rate, and cardiac output, all of which contribute to the improvement of the VO
2peak and VO
2max of patients [
51]. Physical activity can improve exercise capacity in people with heart disease. HIIT can be used to reduce the risk factors for cardiovascular disease, such as body fat content and glucolipid metabolism [
52]. High-intensity interval training enhances the cardiac function of people with cardiovascular disease, as evidenced by increased cardiac output, beat-to-beat output, and ejection fraction [
53]. However, the patient’s physical indicators and physiological responses should be closely monitored during and after HIIT [
54]. Exercise intensity should be increased gradually to allow the patient to gradually adapt, ensuring patient safety and avoiding secondary morbidity [
55].
The most emergent key term in recent years was “heart transplantation”, indicating that HIIT in heart transplantation will be the latest research hotspot in the field. The burst keywords in recent years were “HIIT” and “oxygen uptake”, indicating that these aspects may also be included among the latest research hotspots in the field. High-intensity interval training had a better impact than “continuous moderate exercise”, improving patients’ VO
2peak and aerobic capacity while reducing their cardiovascular mortality. High-intensity interval training programs that are longer in duration and higher in intensity can improve cardiovascular function and increase patients’ tolerance to exercise [
56]. High-intensity interval training programs need to be tailored to the individual patient’s abilities and exercise habits in order to maximize compliance and safety while minimizing physiological discomfort [
57].