Voltage differences might be used to distinguish normal atrial tissue from abnormal atrial tissue. This study was aimed at identifying lowest voltage areas in patients with atrial tachycardia after surgical correction of congenital heart disease and to evaluate if identification of these areas in diseased hearts facilitates selection of critical conduction pathways in reentrant circuits as target sites for catheter ablation. Ten patients (four men, age 39 +/- 15 years) with normal sized atria and atrioventricular reciprocating tachycardia (control group) and ten patients (5 men, 32 +/- 7 years) with congenital heart disease and postoperative atrial tachycardia (CL 281 +/- 79 ms) referred for radiofrequency catheter ablation were studied. Mapping and ablation was guided by a three-dimensional electroanatomic mapping system (CARTO) in all patients. In the control group, voltage maps were constructed during sinus rhythm and during tachycardia to evaluate the voltage distribution. The amplitude of bipolar signals was 1.90 +/- 1.45 mV (0.11-8.12 mV, n = 660) during sinus rhythm and 1.45 +/- 1.66 mV (0.12-5.83 mV, n = 440, P < 0.05) during atrioventricular reciprocating tachycardia. In the study group, the amplitude of 1,962 bipolar signals during tachycardia was 1.01 +/- 1.19 mV (0.04-9.40 mV), which differed significantly from the control group during tachycardia (P < 0.0001). No significant difference in the tachycardia cycle length was found (P < 0.05) between the control and study groups. As the lowest voltage measured in normal hearts was 0.1 mV, this value was used as the upper limit of the lowest voltage areas in the patients with congenital heart disease. These areas were identified by detailed voltage mapping and represented by a gray color. Activation and propagation maps were then used to select critical conduction pathways as target sites for ablation. These sites were characterized by fragmented signals in all patients. Ablation resulted in termination of the tachycardia in eight (80%) of ten patients. Complications were not observed. Identification of the lowest voltage areas using a cut-off value of 0.1 mV in congenital heart disease patients with postoperative atrial reentrant tachycardia facilitated the selection of critical conduction pathways as target sites for ablation.