This ECG is that of an 85 year old woman who was receiving digoxin for the treatment of congestive heart failure. Note that the atrial rate is 155/min and that every other P wave is not conducted (i.e.is blocked). Note also that the P waves are abnormal. They are inverted in leads II,III and aVF and upright in aVR with a frontal plane axis of -90 degrees, indicating that they probably arise from an ectopic focus in the inferior aspects of the atrium, near the coronary sinus, and are superiorly directed. The electrocardiographic diagnosis is ectopic atrial tachycardia with 2:1 A-V block and is indicative of digitalis toxicity.