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Background: Adequate synchronization between the passive ("E") and active ("a") left ventricular (LV) diastolic filling contributes to the efficiency of the heartbeat. E/a superposition in dual-chamber pacemaker (PM) recipients is an under-recognized phenomenon that may be corrected by shortening the atrio-ventricular interval (AVI). We aimed at establishing the prevalence of E/a superposition in PM patients and to analyze the clinical, echocardiographic, and biological impact of AVI shortening. Methods: Seventy patients with dual-chamber PMs (74 ± 8 years old, 12 women) were consecutively enrolled in this study. Patients with baseline E/a superposition were crossed over from default to manually shortened AVI or vice versa in a case–control fashion (intervention group). Patients without baseline E/a superposition (controls) served as a reference for a descriptive comparison with the intervention group. Results: Thirty-three patients had E/a superposition after PM implantation (47%). Controls (n = 37) had higher LV ejection fraction (59 ± 8% vs. 53 ± 10%, p = 0.048) and lower levels of high sensitive troponin T and ST2 (p < 0.05) than intervention group patients. The AVI was shortened at 48 ± 9 ms in order to ensure adequate E/a separation. The walked distance increased from 75 ± 17 to 78 ± 10% (p = 0.049) and the Euro-QoL score from 0.50 ± 0.27 to 0.63 ± 0.19 (p = 0.011) with short AVI. Conclusions: E/a superposition occurs in approximately half of dual-chamber PM recipients and is associated with reduced LV function and increased myocardial injury biomarkers. AVI shortening produces a modest but significant effect in functional capacity and quality of life. [ABSTRACT FROM AUTHOR]