Stump the Experts

Questions:

1. What is the exact diagnosis of this tachycardia: description, mechanism?

2. What diagnostic maneuver or treatment could be initiated to determine the diagnosis?

Answers:

Question #1

The diagnosis is AV reentrant tachycardia using a left lateral accessory pathway and ipsilateral bundle branch block (BBB). The first part of the ECG shows a wide-QRS tachycardia at a rate of 192 beats per minute (BPM) and a left bundle branch block. The last part of the ECG is a narrow-QRS tachycardia with a rate of 212 BPM. Loss of BBB with acceleration of SVT is highly suggestive of AVRT with an accessory pathway ipsilateral to the site of the BBB. If the BBB results in prolongation of the V-A interval commensurate with the increase in cycle length of the SVT, this is diagnostic of orthodromic AVRT (Coumel s Rule). An AV reentrant tachycardia may start with a narrow-QRS and the bundle branch block develop later. shows an accessory pathway in the left lateral position connecting the atrium and the ventricle. The tachycardia circuit is orthodromic, going down the His-Purkinje system in the antegrade direction. Upon coming back up to the base of the heart, the impulse is able to conduct retrograde over the accessory pathway to the atrium, where it traverses back to the AV node and the circuit continues persistently. On the intracardiac electrograms one can see that the earliest retrograde atrial activation is seen on the CS catheter at the 2-1 poles. This identifies the site of the accessory pathway. The V-A interval is 75 ms. The V-A interval represents the time taken for the electrical impulse to pass from the bundle branches simultaneously, back to the base of the heart and up to the atrium via the accessory pathway. shows a bundle branch block on the side of the heart where the accessory pathway is located. The V-A interval lengthens to 105 ms because the impulse is blocked in the left bundle (as demonstrated on the 12-lead ECG), travels down the septum via the right bundle, crosses over the septum to the left bundle, through the ventricles to the base of the heart followed by the atrium via the accessory pathway. This takes longer than if the impulse were to be able to travel down both bundle branches at once, therefore the tachycardia slows with ipsilateral bundle branch block. In this case, the accessory pathway is shown to be located on the left side since the tachycardia slows down with left bundle branch block. The same phenomenon would occur with right bundle branch block when a right sided accessory pathway is responsible for tachycardia.

Question #2

Administration of adenosine which terminates the tachycardia demonstrates that the AV node is one limb of the tachycardia circuit. An EP study would provide a definitive diagnosis of an accessory pathway. One method of proving the AP is involved in the tachycardia would be to demonstrate the termination of the tachycardia by the introduction of a paced PVC when the His is refractory.