Effects of high‐intensity interval training on trajectories of gas‐exchange measures and blood lactate concentrations during cardiopulmonary exercise tests in cardiac rehabilitation—A randomized controlled trial

Background The optimal allocation of training time to different intensities in cardiac rehabilitation is still under debate. The objective of this study was to explore whether in a 12‐week cardiac rehabilitation program, replacement of two of four usual continuous endurance training (CET) sessions per week with energy expenditure‐matched high‐intensity interval training (HIIT) affects the trajectories of cardiopulmonary exercise test (CPET) variables such as ventilatory equivalents for O 2 (EqO 2 ) and CO 2 (EqCO 2 ), and blood lactate (BLa) during CPET. Methods Eighty‐two male patients undergoing outpatient cardiac rehabilitation after an acute coronary syndrome were randomized to CET (age [mean ± SD] 61.7 ± 9.8 years, body mass index [BMI] 28.1 ± 3.4) or HIIT+CET (60.0 ± 9.4 years, BMI 28.5 ± 3.5). CPET was performed at baseline, after 6 and after 12 weeks. HIIT consisted of ten 60‐s bouts of cycling at an intensity of 100% of the maximal power output ( P max ) achieved in an incremental test to exhaustion, interspersed with 60 s at 20% P max . CET was performed at 60% P max with equal duration. Training intensities were adjusted after 6 weeks to account for the training‐induced improvement in cardiorespiratory fitness. The entire functions defining the relationship between EqO 2 , EqCO 2 , and BLa, with power output were modeled using linear mixed models to assess how these trajectories are affected by HIIT. Results After 6 and 12 weeks, P max increased to 112.9% and 117.5% of baseline after CET, and to 113.9% and 124.7% after HIIT+CET (means). Twelve weeks of HIIT+CET elicited greater reductions of EqO 2 and EqCO 2 than CET alone ( p < 0.0001 each) in a range above 100% baseline P max . Specifically, at 100% of baseline P max , least squares arithmetic mean EqO 2 values of CET and HIIT+CET patients were 36.2 versus 33.5. At 115% and 130% of baseline P max , EqO 2 values were 41.2 versus 37.1 and 47.2 versus 41.7. Similarly, corresponding EqCO 2 values of CET and HIIT+CET patients were 32.4 versus 31.0, 34.3 versus 32.2, and 37.0 versus 34.0. Conversely, mean BLa levels (mM) were not differently affected ( p = 0.64). At 100%, 115%, and 130% of baseline P max after 12 weeks, BLa levels did not differ to a relevant extent (least squares geometric means, 3.56 vs. 3.63, 5.59 vs. 5.61, 9.27 vs. 9.10). Conclusions While HIIT+CET reduced ventilatory equivalents more effectively than CET alone, specifically when patients were approaching their maximal performance during CPET, both training strategies were equally effective in reducing BLa levels.