OBJECTIVES: The study explored the potential for tissue swelling and venous occlusion during radiofrequency (RF) catheter ablation procedures using intracardiac echocardiography (ICE). BACKGROUND: Transient superior vena cam occlusion has been reported following catheter ablation procedures for inappropriate sinus tachycardia (IST). Presumably, venous occlusion could occur owing to thrombus formation or tissue swelling with resultant narrowing of the superior vena cava-right atrial (SVC-RA) junction. METHODS: Intracardiac echocardiography (9 MHz) was used to guide ablation catheter position and for continuous monitoring during RF application in 13 ablation procedures in 10 patients with IST. The SVC-RA junction was measured prior to and following ablation. Successful ablation was marked by abrupt reduction in the sinus rate and a change to a superiorly directed p-wave axis. RESULTS: Eleven of 13 procedures were successful, requiring 29 ± 20 RF lesions. Prior to the delivery of RF lesions, the SVC-RA junction measured 16.4 ± 2.9 mm. With RF delivery, local and circumferential swelling was observed, causing progressive reduction in the diameter of the SVC-RA junction to 12.6 ± 3.3 mm (24% reduction, p = 0.0001). A reduction in SVC-RA orifice diameter of ≥30% compared to baseline was observed in five patients. CONCLUSIONS: The delivery of multiple RF ablation lesions, often necessary for cure of IST. Can cause considerable atrial swelling and resultant narrowing of the SVC-RA junction. Smaller venous structures, such as the coronary sinus and the pulmonary veins, would also be expected to be vulnerable to this complication. Thus, ICE imaging may be helpful in preventing excessive tissue swelling leading to venous occlusion during catheter ablation procedures.