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.
Firstdegreeheartblock may be due to conduction delay in the AV node, in the His-Purkinje system (made up by the bundle of His and the Purkinje fibers), or a combination of the two.
In a subset of individuals with the triad of firstdegreeheartblock, right bundle branch block, and either left anterior or left posterior fascicular block (known as trifascicular heartblock) may be at an increased risk of progression to complete heartblock.
This is because if an individual had complete block at this level of the conduction system (which is uncommon), the secondary pacemaker of the heart would be at the AV node, which would fire at 40 to 60 beats a minute, which is enough to retain consciousness in the resting state.
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