Low-Volume, High-Intensity Interval Training in Patients With CAD

Katharine D. Currie; Jonathan B. Dubberley; Robert S. Mckelvie; Maureen J. Macdonald

Disclosures

Med Sci Sports Exerc. 2013;45(8):1436-1442. 

In This Article

Discussion

This is the first study to examine the effectiveness of low-volume HIT in patients with CAD. We demonstrated comparable increases in brachial artery FMD and cardiorespiratory fitness in both END and HIT training programs, although the END group performed approximately twice as much total work as the HIT group. Schnohr et al.[29] recently demonstrated that cycling intensity rather than duration is more closely associated with determining the risk of all-cause and cardiovascular mortality in men and women. Our findings lend further support to the notion that exercise intensity may be more important than exercise duration in terms of cardiovascular health. Recent work has also demonstrated the safety of high-intensity interval exercise in patients with cardiovascular disease.[28] Similar to previous investigations using isocaloric high-intensity interval exercise prescriptions,[19,20,27,35,36] we reported no adverse events in either our low-volume HIT or END groups. Taken together, the findings from this study provide preliminary evidence that low-volume HIT provides the same benefit as END and therefore may be a suitable exercise prescription for patients with CAD.

Cardiorespiratory Fitness

Previous investigations in patients with CAD demonstrate comparable[35] and superior[27,36] improvements in cardiorespiratory fitness with interval exercise training when compared with traditional endurance exercise. Although we did not observe a difference in the magnitude of change in fitness between END and HIT, the 24% increase in relative V·O2peak observed after HIT is larger than the changes observed in these previous interval exercise investigations.[19,21,27,35] Low-volume HIT also increased relative V·O2peak at anaerobic threshold, similar to the findings reported by Warburton et al.[35] Previous work has demonstrated a 12% improvement in survival for each 3.5-mL·kg−1·min−1 increase in V·O2peak.[22] On average, V·O2peak increased 3.6 and 4.7 mL·kg−1·min−1 in the END and HIT groups, respectively, which may indicate the potential for improvements in survival rates after both training programs.

Brachial Artery Endothelial Function

Brachial artery endothelial function is an accepted noninvasive surrogate for coronary artery endothelial function.[31] Endothelial dysfunction is associated with the presence and severity of CAD,[14,24] making it an important therapeutic target. The observation of improved brachial artery FMD after both END and low-volume HIT was not surprising, given previous evidence of training-induced improvements in brachial artery FMD after lower-limb interval[19,21,36] and endurance[9,36] exercise training in CAD patients. Currently, there is no consensus regarding a clinical cutoff value for brachial artery FMD;[32] however, increased cardiovascular morbidity and mortality has been shown to be associated with a persistently impaired relative FMD value <5.5% after optimized anti-atherosclerotic therapy in patients with existing CAD.[17] The pretraining FMD averages for both END and HIT were less than 5.5%, suggesting our sample may have been at an increased risk of future cardiovascular events. We observed a 34% and 33% increase in endothelial-dependent function in the END and HIT groups, respectively, resulting in posttraining group averages higher than 5.5%.

Posttraining improvements in brachial artery FMD were likely elicited by repetitive exercise-induced increases in localized shear stress in the brachial arteries, which has been shown to occur during lower-limb cycling.[33] In addition, previous work has demonstrated the necessity of localized shear stress for FMD adaptations. In a previous study, the attenuation of brachial artery shear rate using forearm cuff inflation prevented the brachial artery FMD improvements, which were observed in the uncuffed arm, after 8 wk of lower limb cycle training.[4] In the present study, patients were not using a recumbent cycle ergometer. Therefore, we cannot rule out the potential localized effects of gripping the handlebars during training and the contribution it may have had on the observed training responses in the brachial artery.[34] Mechanisms mediating the observed improvement in brachial artery FMD may include, but are not limited to, increased nitric oxide bioavailability,[8,11] decreased basal oxidative stress,[1,36] or decreased sympathetic activity.[12] We observed no change in NTG-mediated vasodilation after both training programs, which is consistent with other training studies.[16,21,36] The absence of endothelial-independent improvements with exercise training lends further support to the ability of both HIT and END to selectively improve endothelial-dependent pathways. In addition, the absence of a change in preocclusion diameters and peak reactive hyperemic blood flow and shear rate AUC between pre- and posttraining assessments suggests a comparable FMD stimulus, lending further support to an increased capacity of the endothelium to respond to a given stimulus after 12 wk of exercise training.

Resting Hemodynamic Indices

Resting heart rate and diastolic blood pressure were decreased after training in both END and HIT, which is consistent with findings from previous interval exercise training studies.[19,21] An elevated resting heart rate is associated with increased risk of mortality from CAD.[23] For each increment of 10 bpm, there is approximately an 18% and 10% increased risk of death for women and men, respectively. The average reduction in heart rate observed in both END and HIT was <10 bpm, therefore not clinically significant. Diastolic blood pressure reductions equated to 10% and 3% for END and HIT, respectively. Although the reductions observed with HIT were smaller, they are comparable with the reductions reported by a meta-analysis of exercise training studies.[15] Systolic blood pressure was not significantly lower after training, which contradicts the findings of the meta-analysis. However, previous interval exercise studies in CAD reported no training-induced reductions in systolic blood pressure.[21,27,35,36] The absence of a change in systolic blood pressure and the minimal reductions in heart rate and diastolic blood pressure after training could be attributed to the optimal medical management and normotensive state of our patients or the sample size.

Limitations

We aimed to recruit an equal number of men and women; however, we had limited access to female patients with CAD. Therefore, only two women were included in the study. Although they were distributed between the two exercise groups, a larger female sample would be more representative of the cardiac rehabilitation population. Program wait times at our cardiac rehabilitation center delayed patient start times. On average, patients initiated exercise training 4–5 months after their CAD event, which is longer than desired based on cardiac rehabilitation guidelines.[30] Although our patients were not undergoing previous exercise training during the waiting period, the use of a CAD sample closer to their date of event may have altered the training responses observed in our sample. The study did not include a control group with no exercise intervention. Rather, we used the current standard of care (END) as the control group. Previous interval training studies using a nonexercise control group found no change in physiological indices.[21,36] Participants attended two supervised training sessions per week and exercised one additional day per week. Although our prescription satisfies the cardiac rehabilitation guidelines of three exercise sessions per week,[30] public health agencies advocate for individuals to participant in activity during most days of the week. We observed significant training adaptations with only 3 d of exercise per week; however, larger adaptations may have been observed by increasing training frequency. Lastly, there are a variety of interval exercise training protocols currently being used. We chose the ratio of 1:1 for the high- and lower-intensity exercise intervals based on its success in middle-age populations.[13,18] However, alternative time-efficient interval protocols should be tested. What is important is that our protocol was time efficient and involved approximately half of the work involved in a traditional endurance exercise session. We believe these features may be appealing to populations who are not motivated, or do not have the time to exercise.

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