What is the primary characteristic of a first degree heart block on an EKG?

The primary characteristic of a first-degree heart block is a PR interval that is prolonged beyond the normal range, specifically greater than 200 milliseconds. In first-degree heart block, there is a delay in the conduction of electrical impulses through the atrioventricular (AV) node, resulting in this extended PR interval. The interval indicates the time it takes for the impulse to travel from the atria to the ventricles, and in first-degree heart block, this conduction is impaired but all impulses still reach the ventricles.

A normal PR interval is typically between 120 to 200 milliseconds; thus, when the interval exceeds this duration, it is an indicator of first-degree heart block. This characteristic makes it different from other types of heart block, where there are significant changes in the rhythm or the occurrence of dropped beats. In contrast, the QRS complexes in first-degree heart block are usually normal in width, as the ventricular conduction is unaffected in this type of block.