Participants

Patients admitted to a phase 2 cardiac rehabilitation program in the Midwestern United States were approached to volunteer for this study during the period of September 2016 to February 2017. Inclusion criterion included the following: at least 45 years old, left ventricular ejection fraction ≥40%, more than 3 weeks postmyocardial infarction or percutaneous intervention, and at least 4 weeks postcoronary artery bypass graft surgery. Patients must have attended 4 of the first 6 rehabilitation sessions, which was classified as the run-in period, and have been free of any comorbidity, such as neuropathy or lower leg injury, that would limit them from undergoing treadmill exercise.

Procedures

Participants were recruited from patients admitted into the phase 2 cardiac rehabilitation program at the University of Toledo Medical Center. Each patient participated in the study for 12 weeks, 3 times per week. Patients were also encouraged to be more active on their own, but this was not measured. The first 2 weeks were the run-in period, which consisted of standard phase 2 cardiac rehabilitation therapy, 12MWT, and a cardiopulmonary exercise test (CPX). Eligible patients were then randomized into either HIIT or MCT exercise groups. Participants then completed 10 weeks of the assigned MCT or HIIT exercise training under supervision of a clinical exercise physiologist who monitored heart rate (HR) by electrocardiogram telemetry to ensure proper intensity adherence. If HR was not in the correct range, the workload was modified by the clinical exercise physiologist.

Patients in the MCT group completed a 5-min active warm up (slow walking on a track) and 35 min of cardiorespiratory training at 60% to 80% of their HR reserve (HRR). This intensity was selected because it matched what is usually prescribed during cardiac rehabilitation at this facility. Exercise was primarily performed on a treadmill (~80% of the time), followed by 5 min of active cool down (slow walking on a track).

Patients in the HIIT group completed a 5-min active warm up, 36 min of cardiorespiratory training primarily on treadmills (but patients were free to choose from several different modalities), and 5 min of active cool down on a track. The cardiorespiratory training for the HIIT exercise group included 6 intervals, which consisted of a 3-min period of higher intensity work intervals at an intensity of 80% to 90% HRR followed by a 3-min active recovery period at an intensity of 60% to 70% HRR.

The MCT and HIIT exercise groups completed a total of 45 and 46 min of cardiovascular training during each exercise session, including warm up and cool down, respectively. All patients ended each rehabilitation session by participating in 15 min of resistance training using weights and stretching, for a combined total of 60 and 61 min per cardiac rehabilitation session for the MCT and HIIT exercise groups, respectively. All participants attended group education sessions and received individual counseling from a registered dietitian, a clinical exercise physiologist, and a registered nurse. A follow-up 12MWT and CPX test were completed during the last 2 weeks of the study. To be considered “completed,” patients must have attended at least 90% of sessions and finished all posttesting.

Measures

Demographic data were collected on all patients, including age (years), height (m), weight (kg), and body mass index (BMI). In addition, ejection fraction (%) and cardiovascular diagnoses were collected from medical records. Resting HR (RHR), SBP, and DBP were measured in the seated position at the beginning of each session prior to any testing or cardiorespiratory training.

Exercise capacity was evaluated using a CPX test via the Modified-Balke Treadmill Protocol as described elsewhere (21). Measurement of gas exchange was used to evaluate the patient's exercise capacity via a metabolic cart (MGC Diagnostics, Minneapolis, Minnesota). BP was measured during rest and every 2 min during the exercise test and at the end of the 10-min recovery period. HR was measured continuously via the electrocardiogram from rest until the end of the 10-min postexercise observation.

Functional capacity was evaluated through the use of the 12MWT as described previously (27). Though the 6-min walk test (6MWT) is more commonly used, the 12MWT has the advantage of being a better method to evaluate fitness in patients with higher exercise capacity (28). The 12MWT has similar reproducibility as the 6MWT (29) and is reliable and valid (30).

Statistics

Data were analyzed using IBM's SPSS (v21, IBM, Armonk, New York). Patient baseline characteristics were compared using an independent t test. Differences between groups following training were also analyzed using an independent t test, 2 tailed. For all the analyses a P < 0.05 was accepted as significant.

Baseline and Outcome Measures

Patient characteristics were used to evaluate if the 2 exercise groups were similar and are included in Table 2. The only significant difference in patient characteristics between the 2 exercise groups was height (P = 0.001). However, of those completing the study, only 2 were female, both of whom were in the HIIT group. In addition, all baseline outcome variables, including peak VO₂, 12MWT distance, RHR, and BP, were not significantly different between the 2 exercise groups (Table 3). There were no adverse events during exercise testing or training.

TABLE 2. Comparison of HIIT and MCT baseline patient characteristics.^a

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TABLE 3. Comparison of HIIT and MCT testing variables at baseline.^a

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Results of the CPX test indicate that the MCT and HIIT exercise groups improved their exercise capacity by an average of 2.25 and 2.94 mL · kg⁻¹ · min⁻¹, respectively. There was no difference in the improvement of peak VO₂ between the groups (P = 0.173). In addition, results of the post- 12MWT indicate that the MCT and HIIT exercise groups improved their 12MWT distance by 88 and 171 m, respectively. There was no significant difference in the distance walked during the 12MWT between the 2 exercise groups after exercise training (P = 0.096).

Changes in RHR and BP

Following training, the MCT exercise group did not change RHR, with a slight increase in RHR of 1 b · min⁻¹. The HIIT exercise group had a decrease in RHR of 6 b · min⁻¹. There was a difference in the change in RHR between the exercise groups (P = 0.033).

Participants in both groups had a reduction in resting SBP following training, but there was no significant difference between the groups (Table 4). Interestingly, the MCT group had an increase in DBP of 1.6 mm Hg, but the HIIT exercise group decreased their DBP by 3.1 mm Hg (Table 4). These changes were not statistically different.

TABLE 4. Changes in outcome variables.^a

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Changes in Peak VO₂

Peak VO₂ is known to be a strong predictor of overall mortality (22). Thus, it is important to improve peak VO₂ in those who have cardiovascular disease. Peak VO₂ can be improved through MCT and HIIT, but optimal training intensity and duration to improve peak VO₂ using HIIT has yet to be determined. Multiple studies suggest that the use of HIIT with phase 2 cardiac rehabilitation patients may lead to greater improvements in peak VO₂ when compared with MCT (6–8). Findings from our study suggest that both MCT and HIIT exercise training protocols increase peak VO₂. Although there was no statistically significant difference in peak VO₂ between the 2 groups, there were greater absolute improvements following HIIT compared with MCT. While both groups improved, in the HIIT group, the absolute improvement in peak VO₂ (2.92 mL · kg⁻¹ · min⁻¹) is nearly 1 metabolic equivalent (MET). In previous work, an increase of 1 MET was associated with a 13% decrease in risk of all mortality and coronary heart disease (32).

Improvements in peak VO₂ in the HIIT groups from the works of Freyssin et al. (4) and Keteyian et al. (12) are very similar to those of the present study. In all 3 studies, the HIIT exercise groups improved their peak VO₂ by approximately 3 mL · kg⁻¹ · min⁻¹. However, improvements in peak VO₂ in the MCT group were not similar across all 3 studies. In the Freyssin et al. and Keteyian et al. studies, the MCT group only improved their peak VO₂ by 0.2 and 0.7 mL · kg⁻¹ · min⁻¹, respectively (4,12). Both of these were lower than the present study. Differences in results from previous studies as compared with the current study may be attributed to differences in the intensity and duration while performing MCT.

Changes in 12MWT Distance

Distance walked during a 12MWT is an objective measure of functional capacity and is a more appropriate test than the 6MWT in cardiac rehabilitation patients with higher fitness (28). Previous studies have determined that the 12MWT is a valid and reliable test to measure functional capacity in those with coronary heart disease (27). Our study is the first to compare the change in 12MWT distance following MCT or HIIT. However, similar to our findings, Freyssin et al. used the 6MWT to compare the effects of MCT versus HIIT and reported that both groups improved their distance walked similarly (4).

Changes in RHR and BP Responses

Typically, with aerobic exercise training there is a decrease in RHR, with an associated reduction in the risk for cardiovascular disease (33). However, the MCT group's RHR increased by a minor nonsignificant amount, the opposite of what was expected. Even though these results suggest that MCT does not improve RHR, previous studies have found that 8 weeks of MCT reduced RHR by 2 to 4 b · min⁻¹ (12,34). There are many factors that affect RHR, such as medications and psychological factors, including experiencing anxiety or stress. These factors could have contributed to the unexpected lack of change in RHR in the MCT group.

Resting BP is also often lower following aerobic training primarily due to decreased activity of the sympathetic nervous system and the improvement in peripheral vascular resistance (35). Both resting SBP and DBP were evaluated in the study, but with no significant difference in the changes between the groups. BP has been reported to improve similarly between groups performing MCT and HIIT exercise (36). Similarly to HR, BP response is sensitive to medications (e.g., beta blockers), environmental factors (e.g., temperature), physiology (e.g., state of hydration), and psychological issues (e.g., anxiety and stress). It is unclear in this case why we did not observe overall reductions in BP.

Limitations and Future Research

In this study, exercise duration was equal between the MCT and HIIT exercise groups. However, energy expenditure most likely was different. The HIIT group exercised at a higher intensity during their work interval (80%–90% of HRR) compared with the MCT group (60%–80% of HRR). During the HIIT group's recovery interval, they were working within the intensity range that the MCT group was given to exercise within for the duration of the study. Due to the HIIT group being required to work at a higher intensity, they most likely expended more energy compared with the MCT group. Future research should evaluate whether there is a difference in functional capacity improvement when energy expenditure between the groups is equal.

Physical activity performed outside of rehabilitation was not assessed. This may have led to a wide variety in the amount of physical activity completed between the 2 exercise groups and as a result could have affected the outcomes variables response.

There were patients who did not complete the program for a variety of reasons, including returning to work, nonadherence, noncardiac-related health complications, and deciding to no longer participate. It is very common for phase 2 cardiac rehabilitation patients to be nonadherent to the exercise program, but the dropout rate in our study was lower than typically occurs in cardiac rehabilitation programs. However, the dropout that did occur reduced the sample size to a smaller level than previous studies and may have led to our study being underpowered to detect differences in the outcome measures.

Patients in the HIIT group were significantly taller than those in the MCT group. It is unknown why this occurred. However, this could have had an impact in walking speed and therefore the 12MWT. There were also predominately males in the study, and future studies should attempt a better balance of males and females.

Finally, we attempted to recruit a representative sample from cardiac rehabilitation, which meant that, within our inclusion criteria, there were some much older than others. Some older adults can respond differently to the stimulus of exercise training. However, there was no difference in age between the groups in our study. We also did not gather data about body composition (only BMI) or caloric intake on these subjects, which would be helpful to do in future research.