First degree AV block is simply an increase in the time it takes for the impulse from the atrium to reach the ventricles. In a normal heart rhythm, the PR interval is in the range of .12 to .20 seconds. In first degree AV block, that interval will exceed .20 seconds and can be as long as .50 seconds in extreme examples. The cause for this delay lies in the AV node. The AV node is suppose to cause a certain amount of delay in the impulse reaching the ventricles to allow for the ventricles to fill with blood, but in first degree AV block, this delay is increased.
In the above example, the heart rate is 55 beats per minute and the PR interval is .27 seconds. (In addition, the QRS is .13 seconds which means this patient also has a bundle branch block, which is described below.) It is not uncommon to have an AV block and a bundle branch block in the same patient. First degree AV block is the most common conduction disturbance. It may occur in healthy as well as diseased hearts. It also can vary with heart rate. As the rate decreases, the PR interval can get longer and as the rate increases, it can get shorter. First degree AV blcck is not uncommon in well conditioned athletes with slow resting heart rates. It is also quite common in elderly patients without heart disease. Occasionally, it can be a result of medications. Patients will not generally have any symptoms.