Case Report. A 25-year-old man presented to the emergency room with sudden onset of palpitations, diaphoresis and lightheadedness while working. He denied chest pain or shortness of breath and had not lost consciousness. On further questioning, he recalled prior episodes of intermittent, self-limited episodes of rapid heart beating for the past 7 years, but denied a family history of cardiac disease or sudden cardiac death. On arrival to the emergency room, the patient was awake and alert with a blood pressure of 126/80, and his initial ECG demonstrated an irregular, wide complex tachycardia at a rate of up to 260 beats per minute (Figure 1A). The patient received intravenous adenosine and lidocaine without any change in rhythm, and he was subsequently cardioverted to sinus rhythm utilizing 70 Joules. Post-cardioversion ECG demonstrated sinus rhythm with a PR of 0.10 sec, QRS of 0.13 sec, and QTc of 0.42 sec. There was evidence of pre-excitation with a delta-wave polarity suggestive of a posteroseptal AP (Figure 1B). An electrophysiology study was subsequently performed and confirmed the posteroseptal AP with an inducible orthodromic atrioventricular reciprocating tachycardia using the AP, as well as episodes of atrial fibrillation with rapid antegrade conduction via the AP. Radiofrequency catheter ablation was performed using the cooled-tip Chilli catheter (at low power, 10-20 W, and a maximum temperature of 50 °C) through the coronary sinus at the ostium of the middle cardiac vein. The procedure was technically difficult, but there were no signs of ischemia based on symptoms or ST-segment shifts during or immediately after the delivery of RF current. Post-ablation ECG showed no evidence of pre-excitation, however there were new inferolateral ST-T wave changes (Figure 1C). Troponin-I was subsequently elevated at 63 ng/ml and continued to rise to 190 ng/ml in the absence of symptoms of ischemia. Although a transthoracic echocardiogram revealed no segemental wall motion abnormalities, there was a continued rise in troponin-I and persistent ECG changes, thus the patient was advised to undergo cardiac catheterization. Coronary angiography demonstrated 100% occlusion of the posterolateral (PL) branch of the RCA without collateral flow (Figure 2A). Biplane left ventriculography showed only mild posterolateral hypokinesis. Balloon angioplasty of the occluded PL branch was successfully performed using a Choice PT wire and a 2.0 mm balloon with no residual stenosis (Figure 2B). Coronary stenting was not performed due to the small caliber vessel. Following the coronary intervention, ECG demonstrated improvement in the ST-segment changes (Figure 1D). The patient was subsequently discharged on aspirin and metroprolol. Discussion. Although complications occured after 3.8-4.4% of radiofrequency catheter ablations, acute coronary occlusion was not among the reported sequelae in these large series.1,2 Early studies demonstrated no significant change in coronary anatomy or development of coronary lesions immediately following RFCA or even days to months later.3,4 Subsequently, mechanisms emerged to explain rare coronary injury following RFCA.5 It is more likely to occur during delivery of RF energy within the coronary sinus (because of the close proximity to the epicardial surface) or with RF energy on the atrial side of valve annuli.5,6 Coronary injury can also result from transmural lesions during ablations on the ventricular side.5,6 Coronary injury due to RFCA is usually attributed to spasm, especially in the setting of a normal coronary angiogram.7,8 Thermal injury as a consequence of RFCA may also cause edema followed by thrombosis and complete obstruction.3,4 Other possible mechanisms of acute vessel closure include coronary embolization of a thrombus at the site of RFCA, or direct intimal trauma of the coronary artery as a consequence of crossing the aortic valve. Chronic occlusion due to progressive scarring can occur as a late complication.5 Two case reports in pediatric patients have described successful bail-out stenting for acute coronary occlusions of the RCA complicating ablation.6,9 A recent report described an acute distal RCA occlusion in a pediatric patient during RFCA of a posteroseptal AP that was successfully treated with an angioplasty guidewire.10 Acute coronary occlusions following RFCA for accessory pathways in adult patients have been described in three patients.11-13 Calkins et al. described a patient with left circumflex artery occlusion after inadvertent intracoronary delivery of RF energy.12 Hope et al. described a case of left main occlusion occurring 12 hours after the ablation of a right posterolateral bypass tract treated with coronary angioplasty.11 A third case involved acute occlusion of the first obtuse marginal branch of the left circumflex artery following RFCA of a left lateral accessory pathway.13 In our case, we postulate that the RF current delivered at the ostium of the middle cardiac vein caused injury to the PL branch of the RCA because of the close anatomic proximity of these structures. The mechanism most likely involved thermal injury to the vessel wall from the delivery of RF current, leading to wall edema with subsequent thrombosis and complete occlusion of the PL branch. Myocardial infarction was confirmed based on the combination of significant ST-segment changes on ECG, troponin elevation, and segmental wall motion abnormality on left ventriculography. Coronary angioplasty was successfully performed with improvement in the ST-segment changes and a decline in the troponin level. Our case represents the first adult patient with successful PTCA of an acute RCA occlusion subsequent to RF catheter ablation of a posteroseptal AP through the coronary sinus (CS) at the ostium of the middle cardiac vein. When crossing the aortic valve, careful catheter manipulation must be performed to prevent intimal injury to the coronary ostium. If myocardial ischemia is suspected, coronary angiography should be performed immediately to establish a definitive diagnosis. Cases of acute coronary artery occlusion in the setting of RF catheter ablation of an AP have been reported up to 12 hours later, and chronic occlusions can also occur from progressive scarring years later. This case reemphasizes the importance of recognizing the potential complication of acute coronary artery occlusion as a consequence of RF catheter ablation of accessory pathways. Reprinted with permission from the Journal of Invasive Cardiology 2004;16:657-659.