Atrial flutter and atrioventricular conduction disorder

ECG 6A: This tracing shows a bradycardia with a ventricular rate of 40 bpm; common atrial flutter pattern (sawtooth in inferior leads, positive atrial activity in V1 and negative in V6); incomplete right bundle branch block; the QRS complexes are slow and regular corresponding to an altered atrioventricular conduction (absence of fixed ratio between atrial and ventricular activities);

This patient underwent an ablation of the cavo-tricuspid isthmus; the next tracing was recorded after the ablation procedure.

ECG 6B: Atrial sinus activity and second-degree AV block with Wenckebach periodicity;

Comments: This patient presented an atrial flutter and a high-grade atrioventricular conduction disorder, which is characterized by a slow ventricular rate and a lack of direct relationship between atrial activities and ventricular activities. The highlighting of a second-degree AV block after return to sinus rhythm confirms the presence of this likely junctional conduction disorder (Wenckebach periods with a significant increment in PR interval suggesting a nodal block). This patient was not under any treatment aimed at slowing atrioventricular conduction. The electrical pattern evolved toward an escalation of the conduction disorder with 2:1 atrioventricular rhythm and symptomatic bradycardia of 30 beats/minute. The patient therefore underwent implantation of a dual-chamber pacemaker.

Take-home message: A second-degree AV block type 1 corresponds in the majority of instances to a conduction disorder located in the atrioventricular node and to an exaggeration of decremental conduction. This location explains the predominantly normal morphology of the QRS complexes. Electrophysiological study hence reveals a gradual prolongation of the AH interval, followed by a blocked atrial activity but not followed by a His potential (supra-Hisian block).