Is the HIIT or MISS Trial A Hit or a Miss?

This was also published in PracticeUpdate on Tuesday, February 21, 2023

Endurance athletes know that interval training rapidly increases exercise capacity and improves endurance performance. Intervals are a form of training in which high intensity exercise bouts, lasting several minutes, are performed repetitively, separated by periods of less intense exertion. Such interval training has historically been avoided in cardiac rehabilitation because of concern that the high intensity segments are dangerous for cardiac patients. That attitude has changed and the last 10–15 years have seen a plethora of studies examining the role of high intensity interval training (HIIT) in cardiac rehabilitation programs. These studies were prompted by the knowledge that interval training rapidly increases maximal oxygen uptake (VO2 max), the gold standard, physiologic measurement of exercise performance, and the observation that higher VO2 max values are associated with improved clinical outcomes in cardiac patients.

The HIIT or MISS UK Trial randomized cardiac patients from 6 United Kingdom cardiac rehabilitation programs to HIIT (n=187) or moderate intensity steady-state (MISS) (n=195) exercise training for 8 weeks. (1) HIIT consisted of 10 x 1 minute exercise bouts at >85% of measured maximal heart rate. The exercise bouts were short because prior studies had not shown much difference when cardiac patients performed 4 minute bouts separated by 4 mins of rest (4x4’s) perhaps because the patients could not maintain that intensity for the full 4 minutes. MISS consisted of 20–40 minutes of exercise at 60–80% of VO2 max. VO2 max was measured after the 8 weeks of rehabilitation and again at 12 months. Subjects were encouraged to continue exercise training on their own between the 8 week and 12 month evaluations. Unfortunately, COVID related issues reduced the number of participants examined at 2 and 12 months to only 290 and 147 subjects, respectively.

Not surprising the increase in VO2 max at 8 weeks was significantly higher in the HIIT group but not different at 12 months. This is not the only study that failed to show a difference between HIIT and less intense training at 12 months probably because of reduced exercise adherence. There was only 1 adverse event attributed to the exercise training, an episode of atrial fibrillation in a HIIT subject.

So, is HIIT or MISS UK a hit or a miss? Completing such a large, well-designed, randomized, controlled, clinical trial of exercise training in cardiac patients is clearly a hit. The results showing larger early, but transisent, improvements in exercise performance with HIIT is probably a miss. This result was expected, but worthy of verification. Also, the conclusion that HIIT is safe is a miss because cardiac events in well-managed cardiac patients are too rare to evaluate without a much larger sample size. This is a good trial, that justifies its name because it made some hits and some misses. Nevertheless, it should remind clinicians of the physiological gains that can be attained by exercise training and prompt us to refer our cardiac patients to this therapy.

1. McGregor G, et al. High-intensity interval training versus moderate-intensity steady-state training in UK cardiac rehabilitation programmes (HIIT or MISS UK): study protocol for a multicentre randomised controlled trial and economic evaluation. BMJ Open. 2016 Nov 16;6(11):e012843. PMID: 27852718