First degree heart blocks are common and often the result of high vagal tone. In this situation, increasing the heart rate to a higher level, i.e. 100 bpm or more, by doing some exercise, and repeating the ECG, should see a normalisation of the PR interval. A conduction defect must be suspected if the PR interval does not normalise, or if the PR interval increases over time.
The PR interval can occasionally be found to be very prolonged. If normalising with exercise, this is likely to be due to a very high vagal tone in a healthy individual.
The combination of AV block and a bundle branch block however suggests the existence of a conduction tissue disorder.
Mobitz I (Wenkebach) AV block may be seen in normal individuals during sleep. However, the presence of second degree block requires investigation as those can occasionally progress to complete block.
Mobitz II AV block and complete heart blocks are generally not acceptable.
On the first occasion that the PR interval is abnormal (i.e. ≥210 ms); or if the PR interval has increased since last investigated.