BACKGROUND: Although differential pacing (DP) has been conventionally used to confirm the achievement of block across linear lesion sets, high-resolution mapping (HRM)can unmask pseudoblock and persistent residual conduction across the lines. OBJECTIVES: This study aimed to compare conventional DP criteria with HRM for line assessment and to evaluate the impact of persistent residual conduction across a line on the risk of developing atrial tachycardia (AT) during follow-up. METHODS: Our study included consecutive patients who underwent AT or persistent atrial fibrillation ablation with ≥1 atrial linear lesion. We systematically evaluated the existence of a residual conduction through all linear lesions using HRM and compared this strategy with the results of DP. RESULTS: In a cohort of 102 patients, 101 cavotricuspid isthmus lines, 85 roof lines, and 84 mitral isthmus lines were systematically evaluated using DP and HRM. Among the cavotricuspid isthmus lines, 38 lines (37.6%) exhibited residual conduction as determined by HRM. In these 38 cases, DP yielded a false-positive result in 14 cases (37%). For the roof lines, 40 lines (47.1%) showed residual conduction, with DP yielding false-positive results in 16 (40%) of these cases. Regarding the mitral isthmus lines, 24 lines (28.6%) displayed residual conduction, and DP provided false-positive results in 13 (54%) of these cases. The 1-year AT-free survival rate was 95.4% in patients where all attempted lines were successfully blocked, compared to 31.9% in those with ≥1 line that remained unblocked despite multiple ablation attempts. CONCLUSIONS: The systematic use of HRM for line assessment allows to unmask a substantial proportion of pseudoblocks not identified by DP. The existence of a persistent residual conduction through a line is associated with a high risk of AT during follow-up.< Réduire