Mixing 1:1 HIIT and MICT in Early to Very Early Phase Heart Transplant (HTx) Recipients : a Retrospective Study From a 510 Patients/12 Years Prospectively-collected Database

INTRODUCTION The incidence and prevalence of heart failure (HF) are increasing globally. While its prevalence is estimated at 2% in developed countries, a similar upward trend has been observed in developing countries since the 2000s. This growing burden has led to a parallel rise in cases of advanced HF, a trend primarily driven by an aging population and improved patient survival rates. For patients with advanced HF and no contraindications, heart transplantation (HTx) remains the gold-standard treatment.

Concurrently, exercise capacity is a critical health indicator. In the general population, it is recognized as the strongest predictor of good health and survival, regardless of underlying pathologies. This importance is mirrored in heart transplant recipients (HTR), where enhanced exercise capacity is associated with improved functional status, better quality of life and lower re-hospitalization rates. Furthermore, emerging evidence suggests an inverse relationship with post-transplant mortality. For instance, a +14% increase in maximal oxygen consumption (VO2peak) has been estimated to yield a 32% reduction in mortality.

Exercise-based cardiac rehabilitation programs are the primary intervention for improving exercise capacity after HTx. Their effectiveness in increasing VO2peak was confirmed by a meta-analysis of nine randomized controlled trials (284 patients), the majority of which (n=8) utilized Moderate-Intensity Continuous Training (MICT), yielding a significant mean difference of +2.49 ml/kg/min (95% CI 1.69 to 3.49). However, these studies have limitations. With one exception from the smallest, the trials were conducted in the stable, chronic phase post-HTx (median 12 months) and exhibited significant heterogeneity in the type, volume, and intensity of aerobic exercise prescribed. Notably, despite being a major determinant of program effectiveness, patient adherence was not reported in any of these studies.

A few retrospective studies with small to medium sample sizes, more recently, the largest randomized controlled trial (n=81) to date have explored the effects of early cardiac rehabilitation. While these programs started between 2 and 3 months after HTx, few data exist on very early (≈1 month) standardized rehabilitation programs. In addition, if the ability of MICT to improve cardiorespiratory fitness (CRF) in HTx has been demonstrated, Nytroen et al. demonstrated the efficacy and safety of high-intensity interval training (HIIT) over MICT (SMD +1.8 mL/kg/min), in a long-term (9 months), high-volume (58 sessions x 25 minutes), high-intensity, good adherence, out-patient program.

Yet, recommendations still suggest starting at low intensity (≤50% VO2peak or -10% below the VO2ventilatory threshold), from the 2nd or 3rd post-operative week for the first few weeks, then increasing to MICT and then HIIT depending on tolerance, whereas HIIT should be introduced " only in the further course of clinically stable patient ". However, it may be that initial adherence to and enjoyment of exercise is a cornerstone of long-term physical activity maintenance Rodrigues, especially in the context of HF Klompstra. As described in other population, early in-center peer emulation and addressing the monotony of exercise programs seem important for improving initial management and long-term adherence.