To the Editor:
As Moraes et al.  point out, the exercise capacity is subnormal in most heart transplant (HTx) recipients, although a few well-trained subjects achieve normal VO2peak levels.
It is important to underscore that this study being debated [1, 2] found that high-intensity interval training (HIIT) was well tolerated by stable, long-term HTx recipients and had no adverse effects. Furthermore, the VO2peak increased from 27.7 ± 5.5 mL/kg/min to 30.9 ± 5.3 mL/kg/min in the HIIT group, corresponding to 80% and 89% of predicted values. Previous studies that have documented greater improvements in VO2peak, including Braith et al., referred to by Moraes et al. , who showed an increase from 15.4 ± 4.3 mL/kg/min to 19.4 ± 5.5 mL/kg/min in the exercise group, have solely had much lower mean baseline VO2peak levels, and thus, can document a larger VO2peak increase. It is well known that untrained subjects with a low VO2peak achieve a greater change than those with fairly high baseline values.
We agree with Moraes et al.  that future studies need to investigate the effect of HIIT with respect to post-HTx time and degree of reinnervation. Chronotropic incompetence due to denervation is regarded an important factor influencing exercise capacity and is probably also limiting HIIT shortly after HTx. We recently demonstrated that the heart rate (HR) response improves significantly during the first 6–12 months after HTx , with a further increase thereafter, confirmed by the study population in this study  with a baseline, mean time after HTx of 4.1 years and a mean maximum HR of 94% of predicted . A close to normal chronotropic response was, of course, an important basis when planning the fairly ambitious intervention of the current study, as commented on by Moraes et al. , and we were anxious to see whether the HIIT protocol would and could be sustained by the participants for a full year. Since the results showed that 96% of the planned HIIT sessions were completed at target intensity throughout the year, without any adverse events, we feel it is safe to conclude that HIIT is an applicable, safe, and effective form of exercise in stable, long-term HTx recipients. Thus, as questioned by Moraes et al.; if HIIT could be started earlier; that is, shortly after HTx, needs to be investigated in forthcoming studies.
We also agree with Moraes et al.  that the greatest limitation of this study is that the control group did not undergo a specific exercise strategy, and therefore we cannot conclude with certainty that the HIIT program is superior to moderate intensity with respect to VO2peak increase. However, since the control group also performed a certain amount of exercise during the study period, according to general recommendations (67% exercised more than two times per week, and only 33% little or nothing); we believe that this HIIT program is likely to have induced a greater effect than that of moderate training, which is in accordance with previous studies among patients with coronary artery disease  and left ventricular dysfunction  which have used the same HIIT protocol as in this study .