What does prolongation of the PR interval on an electrocardiogram (ECG) to more than 200 msec indicate?

Prolongation of the PR interval to more than 200 milliseconds indicates a first-degree AV block. In a first-degree AV block, there is a delay in conduction through the atrioventricular (AV) node, which is reflected in the ECG as a prolonged PR interval. While all PR intervals are measured from the beginning of the P wave to the beginning of the QRS complex, a normal PR interval ranges from 120 to 200 milliseconds. When it exceeds 200 milliseconds, it characterizes the first-degree AV block.

In this context, the signal from the atria is delayed before being transmitted to the ventricles, but all impulses ultimately get through. Importantly, this condition is often asymptomatic and can be observed without significant clinical ramifications.

Other possibilities like second-degree AV block or complete heart block involve more complex conduction issues, where not all atrial impulses are transmitted to the ventricles, which is not reflected solely by PR interval prolongation. A normal variant does not typically involve a PR interval that exceeds the defined limit, and thus would not fit the criteria of the question.