Heart failure (HF) is a complex clinical syndrome resulting from structural or functional impairment of ventricular filling or blood ejection.1 About 6.5 million adults in the United States live with HF. By 2030, the prevalence of HF is expected to increase by 46% compared with the prevalence observed in 2012.2
Heart failure promotes several hemodynamic changes clinically manifested through signs and symptoms. Some of the most common manifestations of this disease are reduced tolerance to exercise and diminished quality of life.3 To minimize these changes in HF patients, cardiac rehabilitation (CR) programs can be utilized. Cardiac rehabilitation is defined by the World Health Organization as a set of activities that aim to provide patients with heart disease with the best physical, mental, and social conditions, reducing the risk of death and acute events related to their illness.4 An exercise-based CR program is considered safe, in addition to decreasing future cardiovascular events5 and optimizing functional capacity with positive impact on morbidity and mortality in HF patients.6
The exercise program consists mainly of aerobic exercise training performed continuously or in intervals. Continuous aerobic training is still one of the most studied and used types of exercise training in rehabilitation programs of individuals with HF. However, the application of high-intensity interval training (HIIT), which consists of high levels of exercise combined with periods of lesser intensity, has been increasingly used with HF patients.7 HIIT has been shown to induce a higher rate of energy production, requiring different metabolism and muscle fiber recruitment patterns from those triggered by continuous training.8,9 This type of training allows the gradual adaptation of skeletal muscles to greater exercise intensities, in addition to making the training less repetitive.9
Continuous aerobic exercise has been replaced by interval aerobic training in populations ranging from athletes to patients with chronic diseases. However, disagreement continues about HIIT, especially when it comes to its application in individuals with cardiovascular disorders. Furthermore, in considering that the number of people with cardiovascular disease increases every day, it is important to identify the ideal intensities of exercise for each patient.7
With the goal of providing increased information for clinical practice, this systematic review with meta-analysis analyzed whether HIIT promotes a greater increase in exercise capacity, quality of life, and left ventricular ejection fraction (LVEF) than continuous aerobic training in individuals with HF.
A systematic search was carried out for articles published in journals indexed in the PubMed/MEDLINE, Latin America and Caribbean System on Health Sciences Information (LILACS), Scientific Electronic Library Online (SciELO), Physiotherapy Evidence Database (PEDro), Scopus, and Web of Science databases. The descriptors used for the search were in accordance with the MeSH/DeCS terms, which were as follows: “Heart Failure,” “Exercise Tolerance,” and “Exercise Test.” In addition to these, the following key words were used: “Aerobic interval training,” “High-intensity interval exercise,” “High-intensity aerobic interval training,” and “High-intensity interval training.” A search with the same terms in Portuguese was also carried out. The “humans” filter was used to exclude animal studies. The terms were combined using the Boolean operators AND, NOT AND, and OR. No linguistic restriction or year of publication was applied. The search was conducted between April and May 2017. This systematic review is registered on the PROSPERO Web site (number: CRD42017046441).
The articles were initially identified and independently evaluated by 2 reviewers (B.A. and J.L.) using the titles and abstracts, with the purpose of selecting those that met the eligibility criteria. Studies with relevance that resulted in lack of consensus underwent a more precise analysis of the complete text. In the case of disagreement among evaluators during the article selection and analysis process, a third evaluator (D.A.) participated in the decision.
Inclusion criteria included randomized controlled trials using HIIT compared with continuous aerobic exercise in adult patients of both sexes with HF. Exclusion criteria were studies performed during phase 1 of CR and patients with other associated diseases.
The primary outcome used was functional capacity, assessed by a maximal or submaximal exercise test. Secondary outcomes were quality of life assessed through questionnaires and increased LVEF assessed by echocardiogram.
Data describing the characteristics of the population, eligibility criteria, participant flow, intervention details, exercise prescription, and outcome measures, and results were extracted from the included studies. In the case of incomplete or unclear data, the authors were contacted to obtain the missing information.
EVALUATION OF STUDY QUALITY
The Grading of Recommendations Assessment, Development and Evaluation (GRADE)10 scale was used for analyzing the evidence, which aims to identify the constraints, inconsistencies of the results, evidence direction, vagueness, and publication bias present in selected studies. The level of evidence classification is determined using 4 levels: high, moderate, low, and very low level of evidence. Data were extracted from the articles and evaluated according to the Cochrane bias risk tool considering high risk for methodologies incorrectly described; obscure risk when none of the Cochrane criteria were cited; and low risk of bias when the methodological procedure was adequately performed in the study.
Data regarding continuous variables (mean and standard deviation) were extracted from the studies for the construction of the meta-analysis and expressed as average difference using a random-effects model and 95% CI. RevMan software was used to build the meta-analysis (RevMan, Version 5.3; The Cochrane Collaboration).
SELECTION OF STUDIES
A broad search strategy was used resulting in 5258 titles (Figure 1). A total of 7 articles were included in the qualitative synthesis.11–17 Of these, 1 article evaluated the acute effect of HIIT compared with continuous training and was not included in the meta-analysis. In addition, 2 studies11,12 were based on the same data and thus were considered as only 1 article. The studies included adults of both sexes diagnosed with HF, reduced LVEF, and functional classifications II and III according to the New York Heart Association.
All included studies performed assessments that included cardiopulmonary exercise testing. Quality of life outcomes were assessed in 4 of the studies,13,15–17 and systolic function was also reported in 4 studies11,15–17 (Table 1). Only 1 of the studies16 evaluated submaximal functional capacity using the 6-min walk test. Quality of life was measured using the Minnesota Living with Heart Failure Questionnaire and the 36-Item Short Form Health Survey (SF-36) in 3 of the studies.13,15,17 Another study used the MacNew Disease Heart Health-Related Quality of Life Questionnaire.17 In the studies evaluating systolic function, an echocardiography was performed.11,15,17
Among the studies, 3 of them11,16,17 consisted of training for 12 wk, with sessions 3 times/wk. Only 3 studies used a different duration, 1 evaluating the acute effect of HIIT,14 while the other 2 studies13,15 were conducted for 16 wk or 24 wk. The intensity used for HIIT and the interval time were different between the studies. Three protocols used a cycle ergometer,13,15 while the others used a treadmill.11,16,17 The exercise protocols differed regarding the number of sessions, duration of exercise, and monitoring of the patients (Table 1), which could have influenced the magnitude of the results.
QUALITY OF STUDIES AND PUBLICATION BIAS
The characteristics of the study are shown in Table 1 and an analysis of risk of bias is shown in Supplemental Digital Content Figure 1, available at: http://links.lww.com/JCRP/A109. All articles used randomization; however, only 2 of them described the method used for randomization.16,17 Allocation of the research participants was not described in the included studies. One of the articles13 did not include blinding of participants and professionals, while only 2 of the remaining articles16,17 reported doing so. None of the studies reported any incomplete outcomes data.
The meta-analysis for the effect of HIIT and continuous training on maximum oxygen uptake showed evidence of low quality (Table 2) with an effect estimation = 2.36 (95% CI, 0.39 - 4.33), P = .10; I2 = 48% (Figure 2, Panel A) favoring the HIIT group. In comparing both types of training modalities, Iellamo et al11 observed that both HIIT and continuous training resulted in significant improvements in aerobic capacity in patients with HF. However, there were no significant differences between the 2 types of training, which was attributed to the individualized form of training proposed in the study.15 Other studies also reported no significant differences between the 2 types of training with regard to cardiopulmonary parameters.11,13,15 On the contrary, 2 studies found that HIIT resulted in greater gains in functional capacity than continuous training.12,16
QUALITY OF LIFE
For the meta-analysis of the quality of life, scores from the Minnesota Living with Heart Failure Questionnaire were used in 3 studies13,15,16 and these were used in the meta-analysis. Scores from other questionnaires were presented in different ways in each study, thus making a meta-analysis impossible. A moderate quality of evidence (Table 2) was observed for quality of life and results favored the HIIT group, showing an effect estimate = −2.95 (95% CI, −11.06 to 5.16), P = .96; I2 = 0% (Figure 2, Panel B).
LEFT VENTRICULAR EJECTION FRACTION
Echocardiography was performed in 3 clinical trials and LVEF was measured before and after the proposed intervention program.11,15,16 These results also favored the HIIT group, with a moderate quality of evidence (Table 2) and estimated effect = 3.38 (95% CI, −0.15 to 6.91), P = .96; I2 = 0% (Figure 2, Panel C).
Findings from the data analysis provide a low to moderate quality of evidence that supports the use of HIIT in CR programs, providing improvements in functional capacity, quality of life, and LVEF of patients with HF. There was low-quality evidence showing that HIIT is superior to continuous aerobic training for improving peak oxygen uptake (
O2 peak), which reflected an increase in the functional capacity of these individuals and moderate quality of evidence regarding the quality of life and LVEF.
Meta-analyses in the present study were favorable to HIIT for the included variables. These findings are similar to previous meta-analyses18,19 evaluating the effects of HIIT on
O2 peak, LVEF, and other cardiorespiratory variables compared with continuous training. However, just as in this study, the authors had limited conclusions due to the quality of the reviewed clinical trials. The current meta-analysis included more recent articles13,16 not included in the previous analyses and a new method for assessing the quality of evidence using the GRADE scale.
In the study conducted by Normandin et al14 to compare the acute effects of continuous aerobic training with HITT in 20 patients with HF, it was observed that HIIT provided a strong stimulus to the cardiovascular system without adverse effects when compared with continuous aerobic training.14
Among the other studies included in this review, the exercise program of 24-wk duration used by Koufaki et al13 found that HIIT was feasible for patients with HF, but it was not shown to be more effective than continuous training. A similar response was found in 2 other articles,11,15 in which both HIIT and continuous training provided an increase in
O2 peak, but no differences were found between the 2 types of training.
Only 2 of the analyzed studies showed a difference between the types of exercise16,17 and, in those, HIIT promoted a greater increase in
O2 peak. It is possible to infer from these results that training using short duration and high-intensity intervals may improve mitochondrial function,17 helping in the increase in the cellular energy production and consequently the increase of oxygen uptake.
Regarding quality of life, in the study by Ulbrich et al,16 both groups improved after 12 wk of training; however, the improvement was not significantly different in the intergroup evaluation, so neither exercise protocol was found to be superior to the other. On the contrary, Koufaki and colleagues13 found no significant differences in any of the evaluations, neither for intergroup comparison nor for intragroup comparison when comparing baseline with 12 wk after training and after a 24-wk follow-up. The study by Smart and Steele15 reaffirmed that there is no superiority of one type of training over the other when quality of life is evaluated, as both groups improved. Thus, it can be concluded that exercise training results in a clinically important change in measured quality of life; however, these benefits are independent of the type of exercise performed, the prescribed intensity, and the follow-up time.20
When cardiac function decreases in HF, adaptive mechanisms are stimulated to correct ventricular dysfunction. With minor myocardial damage, adaptations can improve the function, while with more severe impairments, such adaptations are not sufficient and continuous stimulation may lead to future deterioration of cardiac function and the onset of symptoms.21 In this context, ejection fraction is considered important data in classifying patients with HF with systolic dysfunction, making it possible to determine the treatment response, as well as the prognosis.1 Among the studies that evaluated cardiac function, a 35% increase in LVEF was found in only 1 study.17 No difference between the groups was observed in the other studies that measured LVEF.11,15,16
The greater age range of participants in some studies, as well as the presence of HF of different etiologies between the groups may have influenced the effects of the exercise interventions. In addition, a lack of clear descriptions of randomization and participant allocation to treatments, as well as the lack of blinding in most studies led to a risk of uncertain bias in most of them. All of these factors led to a reduced level of evidence found in the meta-analyses. This can also be attributed to imprecision and inconsistency, mainly related to inadequate sample sizes and the heterogeneity of the studies.
Both forms of training provide benefits for the patients; however, the quality of evidence still does not allow confirmation that HIIT is superior to conventional training with respect to functional capacity, quality of life, and left ventricular function as measured in these studies. However, studies with improved methodological quality are needed to verify whether HIIT is a better option than conventional training. Patient safety and the recommended exercise intensity and intervals need further analysis.
This systematic review has verified that HIIT promotes greater increases in aerobic capacity than continuous exercise in subjects with HF. The meta-analysis showed that both forms of training provide benefits, but the quality of evidence still does not allow us to state whether HIIT is superior to continuous training.
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exercise; exercise tolerance; heart failure; high-intensity interval training
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