Inadvertent anastomosis of the left internal mammary artery to the great cardiac vein

We present an uncommon and underreported complication of coronary artery bypass graft surgery: erroneous anastomosis of the left internal mammary artery to the great cardiac vein. The iatrogenic aorto-coronary arteriovenous fistula with left-to-right shunting resulted in dilation of the coronary sinus, a supporting secondary finding. Factors predisposing to this inadvertent anastomosis include an intramyocardial segment of the left anterior descending coronary artery, demonstrated in this case, as well as epicardial fat and potentially fibrosis of the underlying myocardium.