Exercise as it relates to Disease/The effects of interval training on cardiovascular health in adults with coronary heart disease
This is a critical analysis of the journal article "Aerobic interval training versus continuous moderate exercise after coronary artery bypass surgery: A randomised study of cardiovascular effects and quality of life"  (Trine T. Moholdt, et al. 2009)
What is the background to this research?
Coronary artery disease (CAD) can be defined as the narrowing of coronary arteries due to the build-up of fatty material called plaque. This plaque clogs up the arteries and reduces blood flow to the heart.  In some cases surgical procedures are required to help blood flow correctly to the heart. Coronary artery bypass surgery, also known as coronary artery bypass grafting (CABG) is a procedure that uses blood vessels from another part of the body and connects them to blood vessels above and below the narrowed or blocked artery, bypassing the narrowed or blocked coronary arteries, allowing for blood to flow to the heart more efficiently. 
The goal of CABG and exercise interventions is to reduce mortality rates. Patients enrolled in rehabilitation programs after CABG surgery have total and cardiac mortality reduced by 20%-26% compared to standard medical care patients. Exercise capacity is however, the best predictor of survival in both healthy individuals and those who suffer from cardiovascular disease. The best intensity, frequency, duration and type of exercise is still unknown. Aerobic interval training (AIT) however, has been shown to be more effective in improving cardiovascular function than moderate continuous training (MCT). 
Where is the research from?
The research was completed by a large team from the Department of Circulation and Medical Imaging at the Norwegian University of Science and Technology, in conjunction with multiple different hospitals and medical centres from across the country.
The article was published in the American Heart Journal, which is a highly regarded medical journal consisting of peer-reviewed articles primarily focussed on cardiovascular disease. The journal aims to provide primary investigation, scholarly reviews and opinion based pieces about the practise of cardiovascular medicine. 
What kind of research was this?
The study was a randomised controlled study, in which one group participated in Aerobic interval training, and the other completed moderate continuous training. The study aimed to investigate the effects of AIT and MCT on peak oxygen uptake (VO2peak) and quality of life after CABG surgery.
What did the research involve?
The study involved 59 CABG patients; 48 male, 11 female, who were randomly assigned to either AIT or MCT, stratified by gender. The randomisation code was completed using a computer number generation by a different unit at the University. Patients who suffered from heart failure, inability to exercise or drug abuse were excluded from the study.
The aerobic training program (AIT) consisted treadmill walking, 5 days a week for 4 weeks duration. It involved an 8-minute warm-up, followed by 4 sets of 4-minute intervals with the patient at 90% maximum HR and active pauses of 3-minute walking at 70% maximum HR. The session was completed with a 5-minute cool-down.
The moderate training group (MCT) walked continuously at 70% maximum HR for 46-minutes, to meet isoenergetic training protocols. 
The intensity was controlled by heart rate monitors, and patients reported their rate of perceived exertion using the Borg CR10 scale. In addition to the MCT or AIT, participants also participated in training sessions held by the rehabilitation centre. These consisted of a variety of exercise modes and intensities, lasting between 45-60minutes duration. Both groups completed the same rehabilitation centre training, and all sessions had intensities measured by HR monitors.
If patients were discharged from rehabilitation centre, they received written exercise plans, which consisted of a 3-4 week training program with exercise at the same intensity and duration as their AIT or MCT group.
What were the basic results?
The study aimed to primarily measure VO2peak, as well as measuring left ventricular function, HR recovery, resting HR, work economy, quality of life and blood markers of cardiovascular disease as secondary outcome measures. These were measured a 3 time points; at baseline, after 4-weeks of rehabilitation and 6-months after being discharged from centre. Secondary measures were only taken at baseline and after 4 weeks.
Baseline characteristics from the two groups were well balanced, with no major complications and only one patient being excluded due to technical error.
|AIT GROUP||MCT GROUP|
|BASELINE||4 WEEKS||6 MONTHS||BASELINE||4 WEEKS||6 MONTHS|
|Heart Rate Recovery||19.6+6.8||22.5+7.6||25.5+8.6||20.3+9.4||25.4+8.4||24.6+7.7|
|Quality of Life: Emotional Domain||5.7+0.7||6.2+0.5||6.1+0.6||5.5+1.1||6.0+0.7||5.9+0.7|
|Quality of Life: Physical Domain||5.3+0.7||6.2+0.4||6.2+0.7||5.4+1.0||6.0+0.6||6.1+0.6|
|Quality of Life: Social Domain||5.6+0.6||6.5+0.4||6.5+0.6||5.4+1.3||6.3+0.7||6.3+0.6|
The most significant changes from both groups were in VO2peak, HR recovery and quality of life. The AIT increased VO2peak from week 4-6 months and was also superior in increasing HR throughout the duration of the program and through to the 6-month follow up. This is ideal as low HR recovery is an independent predictor of mortality in cardiovascular disease patients. Quality of life also improved significantly in both groups in all three domains and therefore, was deemed to have improved patient's overall perceived quality of life. This can be seen in the table above.
During the 6-month follow up only the AIT group increased their VO2peak through their own at home exercise program, while MCT maintained the same capacity and did not improve. The improvement in the AIT group solely is believed to be caused by the higher intensity the exercise was, and how this transferred into patients at-home exercise routine. This aerobic capacity maintained for 2 years after the initial program was finished and could be due to the fact that patients in the AIT group had their activity pattern change, and were able to make a routine out of it.
What conclusions can we take from this research?
This study was a small, specific study, focussing on coronary artery bypass grafting patients from a small number of institutions in Norway. With only 58 participants including 11 female patients. The study size is too small for the results, evidence and conclusions made to be applicable to the population as a whole, which would require longer duration and more participants.
Interval training has been found to improve cardiovascular fitness more than moderate continuous training does.  This research does however, come fundamentally from healthy individuals, and can be difficult to compare to CABG patients due to multiple confounding variables such as body composition, resting heart rate and blood pressure responses, selected risk factors, and cardiac performance. Due to the lack of control, improvements can only be made form baseline measurements.
Overall, the results from the study showed that aerobic interval training increased patients' VO2peak more than moderate continuous training did, although the low change from baseline could be due to the short duration of the exercise program, and results could change if duration was longer. The addition of rehabilitation centre training sessions in conjunction with the AIT/MCT sessions could have also influenced the VO2peak due to the training sessions difference in intensity and duration.
Aerobic interval training has been found to increase both HR recovery and VO2peak and may be the way to help improve cardiovascular function in CABG patient. Both aerobic training and moderate continuous training has been found to increase quality of life.
It may be as simple as just implementing regular exercise into patients routine for positive changes to result.
As cardiovascular diseases are becoming more common, more research and studies need to be completed for conclusions to be made in regard to the ideal duration and intensity of exercise to yield the best results.
Below are extra studies and resources which provide extra information and may be of interest:
- Benefits and Risks of High-Intensity Interval Training in Patients With Coronary Artery Disease
- Effectiveness of High-Intensity Interval Training for the Rehabilitation of Patients With Coronary Artery Disease
- High-intensity interval training in patients with coronary heart disease: Prescription models and perspectives
- Trine T. Moholdt, et al. (2009) Aerobic interval training versus continuous moderate exercise after coronary artery bypass surgery: A randomised study of cardiovascular effects and quality of life. American Heart Journal; 158:(6): 1031-1037
- Darren E.R Warburton, et al. (2005) Effectiveness of High-Intensity interval training for the rehabilitation of patients with coronary artery disease. The American Journal of Cardiology 2005;95(9): 1080-1084
- Serruys, P. W., et al. (2009). Percutaneous coronary intervention versus coronary-artery bypass grafting for severe coronary artery disease. New England Journal of Medicine; 360(100):961-972.
- ACC/AHA PRACTICE GUIDELINESACC/AHA Guidelines for Exercise Testing. (1997) Journal of the American College of Cardiology. 30(1): 260-315. https://doi.org/10.1016/S0735-1097(97)00150-2.
- Rognmo, Ø. et al. (2004) ‘High intensity aerobic interval exercise is superior to moderate intensity exercise for increasing aerobic capacity in patients with coronary artery disease’, European Journal of Cardiovascular Prevention & Rehabilitation, 11(3), pp. 216–222. doi: 10.1097/01.hjr.0000131677.96762.0c.
- John C. Quindry, Barry A. Franklin, Matthew Chapman, Reed Humphrey, Susan Mathis. (2019) Benefits and Risks of High-Intensity Interval Training in Patients With Coronary Artery Disease. The American Journal of Cardiology. 123 (8):1370-1377