Chronotropic index, heart rate and exercise capacity in diabetes mellitus: comparing two different high-intensity interval training protocols

Background The chronotropic response is associated with a higher risk of cardiovascular disease and exercise intolerance. It has been reported that poor glycemic control and low exercise capacity are associated with chronotropic insufficiency in type 2 diabetes mellitus (T2DM) patients. The effects of different high-intensity interval training (HIIT) protocols on the chronotropic index, heart rate, and exercise capacity are not apparent in the literature. Purpose The aim of study was to compare the effects of different HIIT protocols on the chronotropic index, heart rate, and exercise capacity. Methods Thirty individuals with T2DM patients (27F, 3M) participated in this study. Demographic and laboratory values were recorded. Exercise capacity was evaluated using an incremental symptom-limited maximal exercise test on a bicycle ergometer. Heart rate, blood pressure, and perceived exertion were assessed. In addition, the peak heart rate and workload reached by the patients were recorded during the test. Abnormal heart rate and chronotropic index were calculated. The patients were divided into two groups according to randomization. The first group was trained with the short-interval HIIT protocol (SI-HIIT) (30 s x 4, Workload peak 100%), and the second group was trained with the long-interval HIIT protocol (LI-HIIT) (4 min x 4, Workload peak 85%) three times a week for eight weeks. Results The ratio of patients having chronotropic insufficiency (<0.80) decreased in SI-HIIT (before 60% vs. after 33.3%; p = 0.42) and LI-HIIT (before 47.7% vs. after 40.8%; p = 0.035) with HIIT training. After training, the peak workload achieved during the cycling test increased within each group (p<0.05). The resting heart rate decreased in both groups compared to baseline (p<0.05). The ratio of patients having chronotropic insufficiency, the ratio of patients having abnormal heart rate peak workload achieved during a cycling test, and resting heart rate were similar between the two groups after training (p > 0.05) Conclusion Chronotropic index and resting heart rate improve with HIIT in T2DM. A time-efficient protocol should be chosen to restore cardiac autonomic responses in T2DM, as SI-HIIT and LI-HIIT have similar effects on cardiac function.