C13. Atropine Challenge: Bedside Diagnosis of Block Level

The concept of 2:1 AV block remains poorly understood by many physicians even after so many years of advancement in the field of electrophysiology. It cannot be classified into type I or type II second-degree AV block because there is only one PR interval to examine before the blocked P wave.

Case 1  . A 78-year-old male with asymptomatic bradycardia was referred to our hospital. Resting ECG revealed 2:1 conduction AV block. In order to define the site of block we performed the atropine challenge test, revealed improvement of AV block to Mobitz I and then 1^st degree AV block, suggesting the suprahissian block. Case 2. A 74-year-old male with inferoposterior and right ventricular STEMI was referred to our hospital. His resting ECG revealed 2:1 conduction of P wave. The atropine challenge test revealed improvement of block to 1^st degree AV block, suggesting the suprahissian block.

It is often difficult to distinguish intranodal and infranodal blocks when 2:1 conduction is present. Therefore, a careful search of a long ECG tracing for two consecutive P waves should be made. In addition, atropine can improve AV node conduction due to sympathetic and/or parasympatholytic stimulation. Therefore, it may increase AV node conduction but worsen infranodal block without affecting the refractory period of the infranodal. This case illustrates the importance of a noninvasive test for confirming the location of a 2:1 conduction AV block, as guide to diagnostic and therapeutic determinations.