The ECG showed normal sinus rhythm at 65 per min., with a very short P-R interval (0.10 sec.). On the fourth beat of the first lead set (simultaneous leads I, II, III), which is marked with an arrow, there is an atrial premature, contraction. This latter beat has a low, rather flat P wave preceding the QRS with a P-R interval of 0.20 sec. The contrast between the sinus beats (with short PR) and...
