Despite described benefits of aerobic high-intensity interval exercise (HIIE), the acute responses during different HIIE modes and associated health risks have only been sparsely discovered in heart disease patients. Therefore, the aim of this study was to investigate the acute responses for physiological parameters, cardiovascular and inflammatory biomarkers, and catecholamines yielded by two different aerobic HIIE protocols compared to continuous exercise (CE) in phase III cardiac rehabilitation. Eight cardiac patients (7 with coronary heart disease, 1 with myocarditis; 7 males, 1 female; age: 63.0 ± 9.4 years; height: 1.74 ± 0.05 m; weight: 83.6 ± 8.7 kg), all but one treated with ß-blocking agents, performed a maximal symptom-limited incremental exercise test (IET) and three different exercise tests matched for mean load (P_mean) and total duration: 1) short HIIE with a peak workload duration (t_peak) of 20 s and a peak workload (P_peak) equal to the maximum power output (P_max) from IET; 2) long HIIE with a t_peak of 4 min, P_peak was corresponding to the power output at 85 % of maximal heart rate (HR_max) from IET; 3) CE with a target workload equal to P_mean of both HIIE modes. Acute metabolic and peak cardiorespiratory responses were significantly higher during long HIIE compared to short HIIE and CE (p < 0.05) except HR_peak which tended to be higher in long HIIE than in short HIIE (p = 0.08). Between short HIIE and CE, no significant difference was found for any parameter. Acute responses of cardiovascular and inflammatory biomarkers and catecholamines didn’t show any significant difference between tests (p > 0.05). All health-related variables remained in a normal range in any test except NT-proBNP, which was already elevated at baseline. Despite a high P_peak particularly in short HIIE, both HIIE modes were as safe and as well tolerated as moderate CE in cardiac patients by using our methodological approach.

• High-intensity interval exercise (HIIE) with short peak workload durations (t_peak) induce a lower acute metabolic and peak cardiorespiratory response compared to intervals with long t_peak despite higher peak workload intensities and identical mean load. No significant difference for any physiological parameter was found between short HIIE and CE.

• Between short HIIE, long HIIE, and CE, no significant difference was found in the increase (or decrease, respectively,) of health related markers such as cardiovascular biomarkers, catecholamines, or inflammatory parameters during exercise.

• During all exercise modes, all risk markers remained in a normal range except for NT-proBNP which was, however, already elevated at baseline.

• Short HIIE, long HIIE, and CE were safely performed by patients with CHD or myocarditis in cardiac rehabilitation by using our methodological approach to exercise prescription. This approach included the prescription of exercise intensities with respect to LTP₁, LTP₂, and P_max as well as a conscious setting of P_mean at a moderate level (80 % of PP_LPT2). Importantly, all exercise modes were matched for PP_mean and exercise duration in order to enable a comparison of the three protocols.