A combination of two simultaneous tachycardias in the right atrium close to the atrio-ventricular node and within the coronary sinus in a post-operative cor triatriatum patient

A 71-year-old male was referred to another hospital for dizziness. A bradycardia -tachycardia syndrome and Cor triatriatum were detected, and an operation to resect the membrane in the left atrium and implant a pacemaker epicardially was performed. However, no suitable site could be found on either atria and therefore, a single chamber ventricular pacemaker was implanted. In the electrophysiological study performed in our hospital, we could not detect any atrial potentials in either atria, excluding the region close to the His bundle (HB) and within coronary sinus (CS), in spite of extensive catheter mapping. A regular atrial rhythm with a cycle length of 820 ms, which was synchronous with the rate of the QRS complex on the surface ECG, was recorded only at the HB. Meanwhile, the CS catheter recording exhibited regular focal activity with a cycle length of 150 ms, and this focal activity did not conduct to the atrium close to the HB. Furthermore, this activity was dissociated from the ventricular activity recorded from the CS catheter. During an isoproterenol infusion, an atrial tachycardia with a cycle length of 380 ms was recorded only at the HB, and the twelve-lead ECG exhibited a regular tachycardia with the same cycle length as this tachycardia. Meanwhile, the focal activity within the CS persisted without any change in the cycle length. These findings suggested that there was dissociation between the right atrium (RA) and CS. Furthermore, partial atrial standstill was observed in both atria, excluding the RA close to the atrio-ventricular (AV) node and area within the CS. These rare electrophysiological features were considered to play an important role in the genesis of a simultaneous combination of the two tachycardias at their respective sites.