This was a functional slowing bundle branch block typical of a left lateral WPW Indeed, the first part of the tachycardia had a cycle at 340ms with wide QRS and a LBBB morphology. The second part was marked by a narrow QRS with a faster tachycardia cycle at 270ms. Therefore, the conduction delay on the left branch has led to a slowing of the tachycardia, which proved that the left branch is part of the tachycardia pathway. This is found only when the junctional tachycardia descends through the atrioventricular node, passes through the left bundle branch, ascends through the left lateral accessory pathway and then through the left atrium.
