Atrial threshold variability: Implications for automatic atrial stimulation algorithms

Background: Automatic management of atrial stimulation by verification of atrial threshold (ACM) has recently been made feasible. We investigated circadian atrial threshold variability over the long term and the predictors of successful automatic atrial threshold measurement, in order to provide practical clues for programming ACM features, in such a way as to achieve daily threshold verification and > 99% effective atrial stimulation. Methods: Six daily attempts to measure atrial threshold were programmed in patients receiving an EnPulse (TM) pacemaker (Medtronic Inc., Minneapolis, MN, USA). Atrioventricular (AV) conduction was maximized by programming Search AV+ (SAV+) to a resting Paced AV delay = 400 ms in the first month, and 600 ms thereafter. Results: Seventy-six patients had a median follow-up of 12 months. Median ACM success was 77%. Concordance between automatically and manually measured thresholds was observed during the entire follow-up (Rho = 0.82, P < 0.001). Daily variability in atrial threshold was < 0.5 V in > 94% of measurements in the first trimester after implantation, and < 0.5 V in > 99% of measurements thereafter, as well as any time after pacemaker replacement. Atrial threshold was measured on 86% of days: the predictors of ACM failure were AV block (AVB), high%Atrial pacing, and atrial fibrillation. Programming SAV+ to achieve 600 ms resting Paced AV decreased%Vpacing in patients with normal AV conduction and first-degree AVB, improving the ability to detect atrial threshold. Conclusions: The reliability of ACM is high over a long follow-up. On the basis of atrial threshold variability, a practical approach to ACM programming should be two daily atrial threshold measurements in patients with normal AV conduction and%Ap <= 40%, or with normal sinus activity and AVB, whereas 3-4 measurements should be recommended in patients with first-degree AVB and%Ap > 40% or with recurrent atrial fibrillation and AVB. The lowest adapted stimulation output should achieve at least threshold +1 V in the first trimester after implantation, then threshold +0.5 V thereafter, in order to achieve > 99.5% effective atrial stimulation.