Curriculum · Cardiovascular and Hypertension
First-degree atrioventricular block
What it is
AV conduction blocks come in one of three degrees, and first degree heart block is the first of them. There is a prolonged AV delay, so conduction from the atria to the ventricles is slow, but it never fails: every P wave still has its QRS, the P:QRS ratio stays 1:1, and no beat is lost.
The list of AV blocks in order is first degree AV block; second degree AV block, Mobitz I or Wenckebach; second degree AV block, Mobitz II; second degree AV block, 2:1 block; and third degree AV block with junctional escape.
How it is diagnosed
The whole diagnosis sits in the PR interval. It is prolonged, greater than 0.2 sec, which is a PR interval longer than 200ms, or five small squares on the ECG. The P:QRS ratio stays 1:1.
What sets first degree apart from the second degree blocks is that no QRS is lost. In Mobitz type I there is gradual lengthening of the PR interval until a QRS is lost. In Mobitz type II there is intermittent loss of AV conduction with a fixed PR interval, and the block may last for 2 or more beats. In third degree, the complete heart block, there is complete AV conduction failure with no P:QRS relationship.