A type of supraventricular tachycardias (SVT). Accounts for about 60% of all fast heart rates that start in the upper part of the heart. AVNRT occurs when the A-V node has an extra electrical pathway in the heart linking the right atrium to the A-V node, a kind of "short circuit." The paths are different and unique; one is fast and the other slow. Symptoms include attacks of palpitations, dizziness, and syncope; heart rates can achieve 150-230 beats per minute. These attacks start and stop suddenly. .