Atrioventricular and Intraventricular Conduction Abnormalities

This chapter focuses on conduction abnormalities—delays, blocks, and pre-excitation. The first element is consistent with a first-degree atrioventricular (AV) delay, also known as a first-degree AV block. This finding is not a block in the strictest sense, as there is no “dropped” QRS complex. This fact contrasts with both types of second-degree AV block, wherein the QRS complex is “dropped.” The type I variety of second-degree AV block involves a progressive prolongation of the PR interval with a concomitant shortening of the RR interval. In second-degree AV block Mobitz Type II, a beat is still dropped, despite no changes in these intervals. Finally, third-degree AV block involves a complete dissociation of conduction between the atria and ventricles, with the atrial rate being faster than the ventricular rate. Intraventricular conduction abnormalities, such as hemiblocks and bundle branch blocks, add further nuance to ECG interpretation. In contrast to delays and blocks, pre-excitation facilitates rapid conduction through an abnormal pathway. The above conduction abnormalities may seem overwhelming, but diligent assessment of the ECG will surely reveal these findings—whether they are obvious or subtle.