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Background: Cardiac resynchronization therapy (CRT) is a cornerstone treatment for heart failure, yet the optimal pacing mode remains uncertain. This study investigates the impact of DDDR (rate-adaptive atrial support) versus VDD pacing modes on functional capacity and echocardiographic outcomes in patients with CRT-defibrillators., Methods: In a multicenter, double-blind, crossover trial, 26 sinus rhythm patients undergoing CRT-defibrillator implantation were randomized to DDDR at 60 beats-per-minute or VDD at 30 beats-per-minute. Each pacing mode was maintained for 6 months (period 1), followed by crossover to the alternate programming for additional 6 months (period 2). Distance covered in the 6-minute walk test (6MWT) and echocardiographic parameters were evaluated at baseline and after each period., Results: The proportion of CRT responders was 73% at 6 months and 79% at 12 months. DDDR pacing yielded a median atrial pacing percentage of 55% (interquartile range, 48%-71%). No significant difference was found between DDDR and VDD modes in 6MWT distance (448 m [369-538] vs. 428 m [360-535], p=0.71). The sequence of pacing modes was also not significant (p=0.17), while the study period (p=0.03) and the distance covered at baseline (p<0.01) were positively correlated with the distance walked at follow-up. Echocardiographic parameters, including left ventricular ejection fraction and ventricular volumes, showed no significant differences between the pacing modes., Conclusions: In this cohort, rate-adaptive atrial support did not offer any advantage over VDD pacing in terms of functional capacity or echocardiographic response. The findings support the recommendation to maximize intrinsic sinus rhythm sensing in CRT devices., Trial Registration: ClinicalTrials.gov Identifier: NCT06592690., Competing Interests: Declaration of competing interest D.G. is an employee of Biotronik Italia S.p.a. The remaining authors declare no conflicts of interest., (Copyright © 2024. Published by Elsevier Inc.)