Heart Rate Recovery Index and Improved Diastolic Dyssynchrony in Fusion Pacing Cardiac Resynchronization Therapy

<b>Background:</b> Restoring electrical synchrony with cardiac resynchronization therapy (CRT) reverses the heart failure phenotype developed by left-ventricular (LV) dyssynchrony. This study aimed to identify new predictors of response to LV-only fusion pacing CRT. <b>Methods:</b> A select group of patients with CRT-P indications received a right atrium (RA)/LV DDD pacing system. LV dyssynchrony was assessed via offline TDI timing focusing on the temporal difference between peak septal (E″T) and lateral wall (A"T) motion. CRT effectiveness was evaluated at each follow-up, involving the heart rate recovery index (HRRI) parameter (acceleration/deceleration time) derived from exercise testing along with the echocardiographic parameters. Patients were classified into super-responders (SR), responders (R), and non-responders (NR). <b>Results:</b> Baseline initial characteristics: 62 patients (35 male) aged 62 ± 11 y.o. with non-ischemic dilated cardiomyopathy (DCM). Ejection fraction (EF) 27 ± 5.2%; QRS 164 ± 18 ms; 29% had type III diastolic dysfunction (DD), 63% type II DD, and 8% type I DD. Average follow-up was 45 ± 19 months: 34% of patients were SR, 61% R, and 5% NR. The E″T decreased from 90 ± 20 ms to 25 ± 10 ms in SR, with a shorter deceleration time (DT) during exercise test compared to NR (109 ± 68 ms vs. 330 ± 30 ms; <i>p</i> < 0.0001). The responders present a higher HRRI (2.87 ± 1.47 vs. 0.98 ± 0.08; <i>p</i> = 0.03) compared to NR and a significantly decrease in E"T and A"T from 76 ± 13 ms to 51 ± 11 ms (<i>p</i> < 0.0001). Prolonged DT was associated with an accentuated LV dyssynchrony and nonoptimal response to CRT. <b>Conclusions:</b> The study identified new parameters for assessing responsiveness to LV-only fusion pacing CRT, which could improve candidate selection and CRT implementation.