Atypical atrial flutter with RBBB aberration and conduction improvement every third beat resulting from concealed type I (Wenckebach) second-degree block in the right bundle branch with 3:2 ratio
C. Start treatment with low-molecular-weight heparin and amiodarone
The atrial rate during adenosine administration was identical to the ventricular rate during tachycardia, implying that the arrhythmia in Figure 1A reflects an atrial tachycardia with 1:1 AV conduction. The medical history strongly suggests a macroreentrant circuit (macroreentrant atrial tachycardia, also called atypical atrial flutter) developing around the atriotomy scar related to mitral valve surgery, possibly a left-sided atrial flutter such as mitral annular flutter. The R-R interval prolongation due to the shift from 1:1 to 2:1 AV ratio restored the normal right bundle branch conduction resulting in narrow QRS complexes. This supports an intermittent phase 3 RBBB. The most challenging issue in the tracing on presentation is the periodic disappearance of the RBBB aberration. The key to the correct diagnosis is the unexpected normalization of the QRS morphology occurring periodically, every third beat, despite constant cycle lengths.1- 4