Last month, we included two mystery electrocardiograms for the first installment of this special monthly feature. Below we reveal the answers! Description: These recordings were obtained at the first clinic visit for this post-pacemaker- implant patient. The patient had been feeling increased fatigue in the last week. Questions: 1. What is the rhythm? 2. What is occuring with the atrial electrogram? Answers: 1. The rhythm is atrial flutter with variable A-V block. Cycle length of the atrial flutter is 320 ms. The cycle length is long because the patient is fully loaded on amiodarone. In Figure 1, the marker channel does not show all the atrial impulses because every other one is in the post-ventricular atrial blanking (PVAB) interval, which is programmed to180 ms. In Figure 2, there is a higher degree of block shown in the last two paced ventricular beats. All the atrial impulses are sensed. 2. The atrial lead is positioned with an electrode on either side of the crista terminalis. As the atrial flutter activation wavefront circulates through the atrium, the atrial lead records a potential on either side of the crista. Since the sides activate with a 30 ms delay from each other, a double potential is recorded. This is of no clinical significance, as the second potential is in the atrial blanking interval. (Analysis contribution by Pradeep Maheshwari, MD and Esther Weiss, RN, MSN) Think you can stump us? Do you have images from a puzzling case study that you would like to share? Please email us your images as well as a brief description and any questions you would like to ask. In addition, please let us know if you would like your name published. Send it to Jodie Miller at: firstname.lastname@example.org It may appear in an upcoming issue of EP Lab Digest!