2:1 atrioventricular block during atrioventricular node reentrant tachycardia

Area of Science:

• Electrophysiology
• Cardiology
• Cardiac Arrhythmias

Background:

• 2:1 atrioventricular (AV) block during AV node reentrant tachycardia (AVNRT) has been attributed to intranodal or lower common pathway block.
• Absence of a His bundle potential in blocked beats was previously considered indicative of intranodal block.

Purpose of the Study:

• To determine the incidence of 2:1 AV block during AVNRT.
• To clarify the mechanism of 2:1 AV block during AVNRT.

Main Methods:

• Consecutive patients undergoing electrophysiology studies for AVNRT were observed.
• The incidence of 2:1 AV block was recorded.
• Responses to atropine and ventricular extrastimulus were assessed.

Main Results:

• Persistent 2:1 AV block occurred in 13 of 139 (9.4%) patients with AVNRT.
• A His bundle deflection was present in blocked beats in 8 patients and absent in 5.
• Patients with 2:1 block had shorter tachycardia cycle lengths (312 ms vs. 353 ms).
• Atropine did not affect the block; ventricular extrastimulus consistently converted 2:1 to 1:1 conduction.

Conclusions:

• The incidence of 2:1 AV block during AVNRT is approximately 10%.
• The consistent conversion to 1:1 conduction with a ventricular extrastimulus indicates functional infranodal block.
• The presence or absence of a His bundle potential in blocked beats does not alter the conclusion of infranodal block.