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In tetralogy of Fallot (TOF), the dominant ventricular tachycardia substrates are slow-conducting anatomical isthmuses. Surgical correction has evolved, which might have influenced isthmus presence and dimensions.One hundred and forty-two postmortem TOF specimens (84/58 corrected/uncorrected) were studied for isthmus presence. Isthmus 1 is located between the tricuspid annulus and right ventricular (RV) outflow tract (RVOT) patch/RV incision, isthmus 2 between RVOT patch/RV incision and pulmonary valve, isthmus 3 between pulmonary valve and ventricular septal defect (patch), isthmus 4 between ventricular septal defect (patch) and tricuspid annulus. Isthmus width and thickness were measured.Of 84 corrected postmortem TOF specimens (death: 6.6 years (4.0–11.5)), 83 demonstrated isthmus 1 (99%, width=25±10 mm, thickness=5±2 mm), 35 isthmus 2 (42%, width=10±9 mm, thickness=3±2 mm), 83 isthmus 3 (99%, width=10±6 mm, thickness=5±2 mm), and 5 isthmus 4 (6%, width=4±2 mm, thickness=2±1 mm). Transatrial-transpulmonary correction (n=49) as compared with transventricular correction (n=35) prevented isthmus 2 (0% vs 100%, P<0.001). Transatrial-transpulmonary correction at age <1 year (n=7) as compared with ≥1 year (n=42) required a smaller transannular RVOT patch (28±15 vs 45±14 mm, P<0.001). Mode and timing of correction did not influence presence and dimensions of isthmus 3. In corrected and uncorrected TOF specimens (death 1.8 years (0.5–6.6)), the range of isthmus 3 dimensions was broad (width: min=2 mm, max=32 mm; thickness: min=1, max 13 mm) across all ages. Isthmus 3 width and thickness were strongly correlated (r=0.65, P<0.001).In TOF, the current routine use of transatrial-transpulmonary correction prevents isthmus 2. Correction <1 year reduces transannular patch size, which may influence isthmus 1 width later in life. Mode and timing of correction did not change prevalence and dimensions of isthmus 3, in which dimensions varied widely in uncorrected and corrected TOF.