The following tracings
is obtained during EP evaluation of AV conduction in different patients. HRA (high right atrium) and HBE (His bundle electrogram) are the intracardiac electrograms, recording from the high right atrium and the His bundle regions, respectively. Which of the following is true for these tracings?
CHB at the infra-Hisian (below the His bundle) level. In this tracing, there is NSR with CHB and a relatively wide escape rhythm. In the HBE tracing, each atrial deflection is followed by an initial His deflection and a third, smaller deflection, H', indicating that there is conduction delay within the His bundle itself. This is suggestive of significant His-Purkinje conduction disease. Therefore, the atrial impulse enters the AV node, conducts down to the His bundle (normal AH interval), where it encounters conduction delay (a “split” His made up of both an H and an H'), and then fails to propagate to the ventricle, indicating that the level of block is at or below the level of the bundle of His. There is obvious AV dissociation with a ventricular escape rhythm.
Second-degree AV block at an infra-Hisian level. In this tracing, the surface ECG shows NSR with 2:1 AV block. The HBE tracing shows constant AH with 2:1 block below the level of the His bundle.
Second-degree AV block at an infra-Hisian level. In this tracing, the surface ECG shows NSR with second-degree type I AV block (Wenckebach). This pattern of block is usually localized to the AV node. However, in rare circumstances the block can occur within or below the His bundle. The wide QRS seen on the surface leads are a clue in this case that the patient has conduction disease below the level of the AV node; however, the site of block can only be determined by reviewing the His bundle recordings. In this situation, the HBE tracing shows progressive prolongation in the HV interval before it blocks in a 3:2 conduction pattern. Therefore, the conduction delay is not at the level of the AV node but at or below the His bundle. As opposed to Wenckebach in the AV node, which is usually benign in nature, this type of infra-Hisian block indicates HisPurkinje conduction system disease and is an indication for pacemaker placement, as it may progress to CHB.
A 38-year-old woman with congenital CHB undergoes a dual-chamber permanent pacemaker. A 12-lead ECG obtained after the procedure shows normal sinus rhythm (NSR) with atrial tracking (a sense–V pace behavior). The ventricular-paced complex has a right bundle branch block (RBBB) morphology.
Further evaluation should include:
Obtaining a two-view (anteroposterior and lateral) CXR to evaluate lead position. The presence of an RBBB-paced QRS complex pattern suggests that the ventricular lead may be in the LV. The lead may enter the LV through an atrial septal defect or ventricular septal defect or via perforation of the interventricular septum. It may also be inadvertently introduced into an artery and passed retrogradely through the aortic valve. Another possibility is placement into one of the LV branches of the coronary sinus. Although sometimes an apical position in the RV in a rotated heart can potentially give an RBBB-paced pattern, a two-view CXR should be obtained to rule out LV positioning. A single-view portable AP will not distinguish an LV from an apical RV placement. If LV placement is confirmed on the lateral radiograph, repositioning of the lead is indicated.
Which of the following is true regarding EP testing of the conduction system?
Patients with evidence of infra-Hisian block during EP testing should be considered for permanent pacing. Patients with symptomatic CHB do not need EP testing because the decision for a permanent pacemaker is already made. The sensitivity and specificity of sinus node recovery time are approximately 70%, making this test less than ideal; in most cases, the decision as to whether to implant a pacemaker in cases of suspected sinus node dysfunction depends on symptoms and correlation with ambulatory monitoring rather than results of EP testing. Patients with infra-Hisian block tend to have an unpredictable course and should be considered for permanent pacing.
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