Table 4.

HIIT studies in cardiac disease

Article	HIIT/SIT protocol	Outcomes	Feasibility/tolerability
Angadi et al. (2015) [59]	Frequency: 3×/week for 4weeks Intervals: 4 intervals at 85–90% HRpeak for 4 min Rest: Active recovery for 3 min Modality: Treadmill	Diastolic BP was reduced after HIIT only. VO2peak increased by 9% post-HIIT but not post-MCT. Ventilation threshold, HRpeak, respiratory exchange ratio was unchanged in both groups. Brachial artery FMD was unchanged post-intervention in both groups. Diastolic dysfunction was reduced after HIIT by approximately 1 grade.	Dropouts: 4 participants excluded (2—noncompliance with baseline teste procedures, 1—change in employment status, 1—noncardiovascualr illness. Compliance: 13 subjects completed 100% of sessions, 2 subjects completed 11 of 12 sessions. AEs: No reported musculoskeletal injuries and no significant cardiac events reported.
Ellingsen et al. (2017) [60]	Frequency: 3×/week for 12 weeks Intervals: 4 intervals at 90–95% HRmax for 4 min Rest: 3-min active recovery at moderate intensity Modality: Treadmill or cycle ergometer	At 12 weeks post-baseline, both changes in left ventricular end-diastolic diameter and in VO2peak were similar between HIIT and MCT groups but larger than the control group. No difference was seen in LVEF or in respiratory quotient between groups. No differences in endpoints were seen between groups at 52 weeks.	After initiating the training program Dropouts: 9 participants dropped out due to serious adverse events, 7 withdrew or were lost to follow-up. Compliance: Adherence to supervised training ranged from 34-36 of 36 sessions. AEs: No statistically significant difference of serious AEs between groups but the HIIT group had more cardiovascular AEs during the intervention period and for the remainder of the year.
Fu et al. (2013) [61]	Frequency: 3×/week for 12 weeks Intervals: 5 intervals at 80% VO2peak for 3 min Rest: 3 min at 40% VO2peak Modality: Cycle ergometer	Aerobic fitness was significantly increased in HIIT group only (increase in ventilatory efficiency and cardiac-cerebral-muscular hemodynamic response to exercise).	Dropouts: HIIT—1, MCT—2, control—2; Results of participants who dropped out were included in pre-intervention data.
Iellamo et al. (2014) [62]	Frequency: 3×/week for 12weeks Intervals: 4 intervals at 75–80% HRR for 4 min Rest: Recovery at 45–50% HRR for 3 min Modality: “Uphill” treadmill walking	Ambulatory blood pressure did not significantly change but trended towards decreasing in both groups. Daytime diastolic BP was reduced significantly in HIIT compared to MCT. No significant change in LVEF or LVDD see in either group compared to baseline. VO2peak increased significantly and similarly in both groups. Both groups showed significant and similar decrease in fasting glycaemia, insulin and homeostatic model assessment-IR except the HOMA-IR was further reduced in HIIT than MIT.	Dropouts: 1 participant in HIIT group and 2 in MCT group discontinued study to due unwillingness to continue in study. Compliance: HIIT group average = 31.6/36 sessions; MCT group average = 30.1/36 sessions AEs: None reported
Isaksen et al. (2015) [63]	Frequency: 3×/week for 12 weeks Intervals: 4 intervals at 85% HRmax for 4 min Rest: Recovery at 60–70% HRmax for 3 min Modality: Cycle ergometer /treadmill	Significant increase in VO2 uptake (5.7% increase in HIIT vs 4.1 decrease in control), cycle ergometer workload, and endothelial function was seen in HIIT compared to control. See feasibility and safety as further outcomes.	Dropouts: 35 of 38 recruited completed the study: one from control group, two from HIIT due to medical complications: repeated haematuria following exercises and diagnosed with urothelial carcinoma and device-related infection. Compliance: Average attendance rate was 98% with none completing less than 75% of the planned sessions and 20 completing 100% AEs: None reported, including symptomatic arrhythmias, sustained arrhythmias, antitachycardia pacing, or implantable cardioverter defibrillator discharge.
Isaksen et al. (2016) [64]	Frequency: 3×/week for 12 weeks Intervals: 4 intervals at 85% HRmax for 4 min Rest: Active recovery for 3 min Modality: Cycle ergometer /treadmill	In HIIT only, significant increase in VO2peak was seen. Some improvements in anxiety and depression scores (SF-36 and HADS-D) were seen in HIIT group at 12 weeks. At 2-year follow-up, the HIIT group had maintained scores, or scores trended towards baseline values. This was significantly improved over controls who had no change at 12 weeks and had deteriorated scores at 2-year follow-up. At 2-year follow-up, the control group reported significantly more time spent sitting during the day compared to the HIIT group. Non-significantly, the HIIT group also had more physical activity per week. No significant differences between groups regarding hospitalization and implantable cardioverter defibrillator shocks at 2-year follow-up.	Dropouts: 1, not in results Compliance: 26 completed 2-year follow-up assessment. Mean attendance rate = 97.5%. Mean reported Borg score was 15.2 during intervals.
Munch et al. (2018) [65]	Frequency: 3×/week for 6 weeks Intervals: 8 intervals at 90% 1-leg Wmax for 4 min (alternating legs) Rest: 1.5–2-min recovery Modality: Cycle ergometer (1-legged)	In both HF and healthy populations, HIIT increased aerobic capacity and improved ability to override sympathetic vasoconstriction (arterial infusion of tyramine) during exercise. The peak vasodilatory responsiveness to ATP infusion was less in the HF population. Acetylcholine-induced vasodilation in the HF population was increased after HIIT.	Not given
Spee et al. (2020) [66]	Frequency: 3×/week for 3 months Intervals: 4 intervals at 85-95% VO2peak for 4 min Rest: 3 min active rest Modality: Cycle ergometer	After cardiac resynchronization therapy (both groups), VO2peak increased (17 ± 5.3 to 18.7 ± 6.2 ml/kg/min, p < 0.05). After HIIT there was a non-significant increase of 1.4 ml/kg/min (p = 0.12). Peak cardiac output did not change significantly after cardiac resynchronization therapy or HIIT. LVEF increased 25% after resynchronization therapy but not after HIIT.	Dropouts: After randomization, two participants could not complete the protocol due to orthopedic complaints.
Thijssen et al. (2019) [67]	Frequency: 2×/week for 12 weeks Intervals: 10 intervals at 90% Wmax for 1 min Rest: 30% Wmax for 2.5 min Modality: Cycle ergometer	VO2peak (as percentage of predicted VO2peak) and maximum workload increased after training with no difference seen between training groups. No significant change in FMD, cardiac function, or health-related quality of life (SF-36 total score) was seen.	Dropouts: 4 dropouts after allocation (2 due to musculoskeletal complaints and 2 due to progression of HF—one of each in each group) Compliance: 100% as missed sessions were rescheduled

HIIT high-intensity interval training; MCT moderate-intensity continuous training; HR heart rate; VO₂ volume of oxygen consumption; BP blood pressure; FMD flow-mediated dilation; LVEF left-ventricular ejection fraction; HF heart failure; AE adverse events