First degree atrioventricular (AV) block

First degree AV block is caused by a prolongation or delay in impulse conduction through the AV node. It is defined as a PR interval >0.20 seconds. In this case the PR interval (blue lines) is approximately 0.22 seconds.

The normal P wave in sinus rhythm is slightly notched since activation of the right atrium precedes that of the left atrium. The P wave is upright in a positive direction in leads I and II. A P wave with a uniform morphology precedes each QRS complex. The rate is between 60 and 100 beats per minute and the cycle length is uniform between sequential P waves and QRS complexes. In addition, the P wave morphology and PR intervals are identical from beat to beat.