Wagner_Marriot's Practical Electrocardiography, 12e

SEVERITY OF ATRIOVENTRICULAR BLOCK First-Degree Atrioventricular Block

Rhythm V1

Rhythm V1

Rhythm II

Rhythm II

Rhythm V5

Rhythm V5

A

B

F I G U R E 2 2 . 2 . Simultaneous three-lead (V1, II, and V5) rhythm strips showing examples of first-degree AV block from healthy patient (A) and a (B) woman receiving no medications. Arrows indicate PR intervals of 0.25 (A) and 0.35 second (B) .

The “normal” PR interval has a duration of 0.12 to 0.20 second. First-degree AV block is generally defined as a prolongation of AV conduction time (PR interval) to 0.20 second. In analyses of records from normal young persons, the incidence of first-degree block by this definition ranged from 0.5% 1 to 2%. 2 In healthy middle-aged men, a prolonged PR in- terval in the presence of a normal QRS complex was found not to affect prognosis and to be unrelated to ischemic heart disease. 3 Figure 22.2 illustrates two examples of first-degree AV block. The first of these (see Fig. 22.2A) is minor, with a PR interval of 0.24 second, and the second (see Fig. 22.2B) shows extreme PR lengthening. Note that in Figure 22.2B, the P wave is superimposed on the T wave of the preceding cycle.

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CHAPTER 22: Atrioventricular Block