Rhythms and Arrhythmia
First Degree Atrioventricular Block
If each wave of depolarisation that originates in the sinus node is conducted to the ventricles, but there is a delay in conduction through the atrioventricular node, then the PR interval will be prolonged.
If the PR interval is more than 0.20 seconds (one box) then this prolongation is referred to as first degree atrioventricular block.
In the ECG trace below, the PR interval (shaded) is prolonged to wider than one box.