First degree heart block is a disease of the electrical conduction system of the heart.
In first degree heart block, the disease is almost always at the level of the atrioventricular node (AV node).
In normal individuals, the AV node slows the conduction of electrical impulse through the heart. This is manifest on a surface EKG as the PR interval. The normal PR interval is from 120 ms to 200 ms in length.
In first degree heart block, the diseased AV node conducts the electrical activity slower. This is seen as a PR interval greater than 200 ms in length on the surface EKG.
Isolated first degree heart block has no clinical consequences. There are no symptoms or signs associated with it, and there is no danger of progression to complete heart block.
In a subset of individuals with the triad of first degree heart block, right bundle branch block, and either left anterior or left posterior fascicular block (known as trifascicular heart block) may be at an increased risk of progression to complete heart block.
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