This risk is higher in left ventricular outflow tract obstructions, heterotaxy, and atrioventricular septal defects. Arrhythmias such as tachycardia, bradycardia, atrioventricular block, and premature ventricular beats have also been reported. The valves between the atria and ventricles are called the atrioventricular valves. The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum. Many involve stenosis of the semilunar valves or regurgitation of the atrioventricular valves. Many involve stenosis of the atrioventricular valves or regurgitation of the semilunar valves. A dromotrope affects atrioventricular node (AV node) conduction. It is located in the endocardium of the atrial surface of the right atrioventricular valve. This can lead to abnormal heart rhythms including atrioventricular re-entrant tachycardia. Four different septal defects exist, with perimembranous most common, outlet, atrioventricular, and muscular less commonly. They start from either the atria or atrioventricular node. The blood supply of the AV node is from the atrioventricular nodal branch. In the atrioventricular node, the resting potential is abbreviated, which facilitates conduction. AVNRT occurs when a reentrant circuit forms within or just next to the atrioventricular node. It is contraindicated in patients with AV (atrioventricular) heart block or digoxin toxicity.