Impact of oxygen uptake efficiency slope as a marker of cardiorespiratory reserve on response to cardiac resynchronization therapy.

BACKGROUND: Cardiac resynchronization therapy (CRT) is limited by the high numbers of nonresponders. This study analyzed the impact of the cardiorespiratory functional reserve to predict the response to CRT. METHODS AND
RESULTS: Twenty-eight patients (age 67 ± 9 years, LVEF < 35%, NYHA class III, QRS 158 ± 25 ms) underwent submaximal cardiopulmonary treadmill exercise testing prior and 6 months after implantation of a CRT device. Breath-to-breath gas analysis was used for calculation of the oxygen uptake efficiency slope (OUES = non-effort-dependent index of cardiorespiratory functional reserve) in the responder and nonresponder group. Responders to CRT [defined by a decrease in left ventricular end-systolic volume (LVESV) > 15%] showed a significant lower cardiorespiratory reserve at baseline (prior CRT) as compared to the nonresponders (OUES 1,235 ± 651 vs. 2,480 ± 590; p < 0.01). Responders showed an increase in OUES during CRT at the 6 months follow-up (1,879 ± 663; p < 0.05) whereas nonresponders showed no significant changes from baseline (2,194 ± 422; ns). Both responders and nonresponders showed an improvement in LVEF at the 6 months follow-up (23 ± 5 vs. 31 ± 9% and 26 ± 7 vs. 32 ± 3%; p < 0.05). Responders to CRT showed a decrease in NYHA classification (3.0 vs. 2.6 ± 0.5; p < 0.05) and a decrease in LVESV (175 ± 58 vs. 128 ± 40 ml; p < 0.05).
CONCLUSIONS: Nonresponders to CRT showed a more preserved cardiorespiratory functional reserve as compared to responders despite similar NYHA classification. Evaluation of the OUES by submaximal exercise testing improves identification of responders to CRT.