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The optimal programming of a pacemaker (PM) voltage output considers both efficiency (prolonging battery cell longevity) and patient safety (adequate safety margin). Currently, automatic capture (AC) algorithms are designed to ensure safe automatic stimulation threshold determination and pacing with a safety margin.The aims of this prospective observational study were (1) to evaluate, over a short-term follow-up, the extent of backup pacing in patients implanted with an AC-featured PM produced by Boston Scientific (Insignia) and a wide range of ventricular leads; (2) to identify patient- or lead-specific predictors of ventricular threshold increase or missed detection of the ventricular pacing threshold; and (3) to analyze day-to-day fluctuations in the ventricular pacing threshold and the relationship between their magnitude, the characteristics of patients, and the system implanted.Five hundred and seventy-nine patients implanted with 89 different leads were followed up for a median of 2.1 months. Five hundred and thirty-six patients (92.5%) never experienced failure of automatic threshold testing; 571 (98.6%) did not experience permanent failure requiring continuous backup pacing at high energy. On multivariate analysis, none of the patient or lead characteristics predicted the occurrence of high-energy backup pacing during the study period. Day-to-day threshold fluctuations were associated only with higher thresholds (>1 V).AC algorithm reliably measures ventricular pacing thresholds in most patients: in only 1.4% of patients the system is permanently unable to detect the ventricular threshold. Backup pacing is not dependent on lead or patient characteristics, including lead polarization, polarity, and maturation.