The normal QRS interval is 0.12 seconds (3 mm or 3 small squares) on the ECG. To correctly determine the QRS interval, use the lead with the widest QRS complex. If the QRS complex is less than or equal to 0.12 seconds, then no further analysis is necessary. If it is greater than 0.12 seconds, then you should try to determine the reason for the abnormally long QRS interval.

A simple approach is to consider the following three possible causes for QRS widening:

The type of bundle branch block can usually be determined from the examination of three key leads: I, V1 and V6.