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Figure 3-1: Explanation

Figure 3–1A

A 30-year-old man with a history of PSVT undergoes electrophysiologic study. During ventricular pacing several electrophysiologic phenomena occur. How many can you name?

First of all, note that during ventricular pacing there is ventriculoatrial (VA) block. The fourth pacing stimulus does not capture the ventricle because it occurs during ventricular refractoriness that is present shortly after activation from the sinus complex. The third and fifth QRS complexes conduct from the sinus origin to the ventricle. The first sinus complex occurs very soon after the ventricular paced beat and does not conduct through the atrioventricular (AV) node as noted by absence of a His bundle electrogram. Thus, there is concealed conduction into the AV node.

The fifth QRS complex is a result of conduction from the atrium over an AV accessory pathway. It is able to conduct because the time from the previous ventricular paced beat to the sinus complex, 510 milliseconds, is sufficient to allow conduction over the accessory pathway. Since this pathway is not noted on the first sinus complex, either there is also concealed conduction into the accessory pathway preventing AV conduction over the accessory pathway or the ventricle is simply refractory since it comes too early after the initial paced beat.

The third QRS complex is not normal but it does not show conduction over the AV accessory pathway. The paced V to the subsequent atrial interval is 300 milliseconds, and there is substantial prolongation of the AH interval suggesting concealed conduction into the AV node but not to the degree that it causes anterograde block. The HV interval is very short. No matter what the AH interval was in this patient, whenever the AV accessory pathway was blocked there would be a similar short HV interval. This represents conduction over a fasciculoventricular (Mahaim) pathway. Clearly there is concealed conduction into the AV accessory pathway here because the sinus complex can conduct to the ventricle over the normal system as well as the fasciculoventricular pathway. With the longer paced V to A interval of 510 milliseconds, there is now manifest conduction over the AV accessory pathway with a shorter AH interval, but the His is within the QRS complex, characteristic of this type of accessory pathway.

In summary, the electrophysiologic observations in this tracing include concealed conduction into both the accessory AV pathway and the AV node, manifest preexcitation over an AV accessory pathway, and manifest preexcitation over a fasciculoventricular pathway.

Figure 3-2: Explanation

Figure 3–2A

At electrophysiologic study, atrial pacing at 400 milliseconds produced the following observation in a 36-year-old woman with history of wide QRS complex tachycardia. Explain the observations.

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