Cardiology Corner - Dr. L. Littmann

I. Bifascicular Block and Second Degree AV Block

1. In asymptomatic individuals, chronic bifascicular block in itself does not usually require cardiac work-up; the prognosis is generally benign
2. The following high-risk features, however, warrant urgent evaluation:
   

• Bifascicular block and syncope
  
• Bifascicular block and intermittent second degree AV block
  
• 1:1 AV conduction at slower sinus rates but higher grade block (i.e., 2:1 AV block) at faster sinus rates (“acceleration-dependent AV block”)
  

     3. Your role in the evaluation and management of patients with bifascicular block:

• Actively search for nonconducted P waves in the 12-lead ECG
  
• Also search for nonconducted P waves (second-degree AV block) in telemetry strips
  
• In patients with bifascicular block who develop acceleration-dependent AV block with a very slow ventricular rate, carotid massage or IV beta blocker, by decreasing the sinus rate, can paradoxically restore 1:1 AV conduction; IV atropine, on the other hand, can increase the degree of block
  
• Patients with bifascicular block and syncope require admission and cardiology consultation for possible pacemaker implantation
  
• Patients with bifascicular block with intermittent second degree AV block require admission and cardiology consultation for possible pacemaker implantation
  

II. Second Degree AV Block with Narrow QRS complexes
           · The block is located within the AV node or the His bundle
           · The vast majority is in the AV node and is usually benign
           · Type I AV block (Wenckebach periodicity) confirms AV nodal block
           · Even if the AV block appears to be type II, it is most likely localized within the AV node and is usually benign (“pseudo-type II AV block”)

• Confirm pseudo-type II block by demonstrating that the block occurred simultaneously with an abrupt deceleration of the sinus rate
• Pseudo-type II AV block occurs in autonomic dysfunction, sleep apnea, obesity-hypoventilation, coughing spells, suctioning, vomiting etc.
  

          · 1:1 conduction at slower sinus rates but 2:1 block at faster atrial rates (i.e., acceleration-dependent block) strongly suggests the block to be at the level of the His bundle rather than the AV node

• Implantation of a permanent pacemaker is usually indicated