Membranous septal aneurysm (MSA) is a rare anomaly and known to cause ventricular tachycardia and atrioventricular block. However, underlying mechanisms have not been addressed in its long history. We report first 3-D electro-anatomical mapping of MSA during and three years following the surgery. An elderly patient underwent a surgery for MSA. In the mapping, we located the His bundle near MSA and observed delayed potentials around MSA. Our report showed that electrophysiological character of myocardium was changed around MSA and this change might be a reason for ventricular tachycardia and atrioventricular block. An ordinary surgery for MSA might not resolve this problem since we still observed delayed potentials three years after the surgery.
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