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.