To characterize myocardial swelling in response to application of endocardial radio-frequency ablation lesions.In individual patients, we have observed that ablation in the posterior right atrium was associated with echocardiographic evidence of atrial and contiguous right pulmonary vein wall swelling.1. Human Subjects: "linear" ablation was performed in the posterior right atrium in 10 subjects; a portion of the ablation lesion was contiguous to the right pulmonary vein; this area was defined as the "contiguity zone". In the contiguity zone, right atrial wall thickness and pulmonary vein lumen diameter were measured utilizing intracardiac echocardiography. Measurements were made just prior to (baseline) and immediately after ablation.2. Porcine Subjects: linear ablation was performed in the posterior right atrium of 14 pigs. In the contiguity zone, atrial wall thickness, interstitial space thickness, right pulmonary vein wall thickness and lumen diameter were measured using intracardiac echocardiography. Measurements were made at baseline, immediately after ablation, and at 1, 4, 8 or 12 weeks after ablation (followup). Post-mortem pathologic evaluation of the contiguity zone was performed.1. Human Subjects: Immediately after ablation, relative to baseline right atrial wall thickness was significantly increased (9.4+/-3.1mm versus 5.4+/-1.5 mm) and right pulmonary vein lumen diameter was significantly decreased (6.2+/-2.9 mm versus 8.1+/-2.9 mm).2. Porcine Subjects: Immediately after ablation, right atrial wall thickness (4.1+/-1.2 mm), interstitial space thickness (1.9+/-1.1mm), and right pulmonary vein wall thickness (1.2+/-0.4 mm) were each significantly increased relative to baseline (1.0+/-0.3 mm, 0+/-0 mm, and 0.7+/-0.2 mm, respectively) and pulmonary vein lumen diameter was significantly decreased (5.0+/-1.4 mm versus 6.9+/-2.2 mm). Similar findings were made at the 1 week followup interval. At 4, 8 and 12 week followup intervals, thicknesses and lumen diameter were not significantly different from baseline. At post-mortem examination, direct measurements of wall thickness were significantly correlated with echocardiographic measurements. Histologic analysis demonstrated edema to be the cause of the early wall thickness and lumen diameter changes. Ablation lesions were transmural in the right atria of all animals; in some animals, lesion formation was also observed in the pulmonary vein wall.Cardiac edema resulting from right atrial linear ablation results in swelling of atrial and contiguous right pulmonary vein walls, as well as the interposed extracardiac interstitial space. These changes are associated with a decrease in pulmonary vein lumen diameter. Swelling evolves rapidly and resolves within 4 weeks.