When one P wave begets two QRS complexes: 1:2 AV conduction

The cause is dual AV nodal pathways where the slow pathway is so much slower than the fast pathway that by the time the impulse traverses it the infranodal tissues are no longer refractory. Although it is believed to be rare its true frequency is unknown, and the surface electrocardiac patterns it produces are common, often called by the computer or the casual observer “supraventricular bigeminy.” It may also be considered to be “narrow complex tachycardia with alternating RR intervals.” If the second impulse experiences aberrant conduction it may present as “ventricular bigeminy.” It may also be mistaken for atrial fibrillation. Case report here, editorial here, blog post here.