Background: First-degree atrioventricular block (AVB) might not be benign. Markedly long PR intervals may cause cardiac dyssynchrony, with many consequences. Restoring optimal AV synchrony represents a reasonable option for hemodynamic and clinical improvement.Objectives: To compare 2 cardiac pacing strategies for bradycardia associated with firstdegree AVB: (1) long PR interval (PRi)-narrow intrinsic QRS, avoiding ventricular pacing but potentially causing AV dyssynchrony (AVD); vs (2) optimized AV interval (oAVi)wide paced QRS, potentially inducing ventricular dyssynchrony.Methodology: Prospective cohort study with patients with permanent DDD pacemakers due to sinus disease associated with first-degree AVB (binodal disease). We analyzed diastolic filling time (DFT), defining 2 groups: patients with AV synchrony (AVS) and AVD. Clinical and echocardiographic follow-up was performed for a year. Results:We studied 43 patients (mean age 71 years; 51.2% female). Longer PRis were associated with worse baseline ventricular systolic function. The AVD group (24/43) showed longer PRi (mean=283.5ms; p≤0.001) and reduced ventricular DFT (p=0.032). Firstdegree AVB with PRi>263ms (relative risk [RR]=1.84; p=0.024; specificity=78.9%; 95% confidence interval [CI] 0.43-0.79) and DFT<40% of the cardiac cycle duration (RR=0.99; p<0.001) were independent predictors of AVD. When PRi>300ms, dyssynchrony was not correctable by AVi optimization. The AVS group (controls, n=19; mean PRi=252.4ms), despite maintaining synchrony, had worsened mitral regurgitation (p=0.008) at follow-up.Conclusions: First-degree AVB comprehends significantly different patients: those with AVD and AVS, determined by DFT and PRi length. In those with AVD, we hypothesized the existence of the "long PR syndrome", defined from a PRi>263ms associated with overt DFT impairment.

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