Always begin from the left of the complexes and move to the right.  Start with the WIDTHS of the complexes.  If you move from left to right, the first width you come across is the P wave width.  Look specifically in lead II.  If the duration of the P wave is greater than 0.12 seconds (3 small boxes) left atrial enlargement (LAE) exists.

As you move across the complex from left to right, the next WIDTH you encounter is the PR segment.  This segment encompasses the entire P wave from its beginning up until the first deflection of the QRS complex.  It should be LESS THAN 0.2 SECONDS (1 large box) to be considered normal in adults.  If the PR segment is greater than 0.2 seconds, first degree AV block exists.

The next WIDTH you encounter is the QRS complex width.  It should be LESS THAN 0.12 SECONDS (3 small boxes).  If it is greater than 3 boxes, either a bundle branch block (BBB) exists (partial or full) or the complex did not take the conduction pathway at all and had a ventricular origin as is seen in a premature ventricular complex or PVC.

The next WIDTH that we come across as we move from left to right is the QT segment.  Prolongation or shortening of this segment is neither specific nor sensitive for any single diagnosis.  However you should know that the corrected QT or the QTc should be less than 450 milliseconds.  If prolongation occurs, it may be a tip-off that electrolyte abnormalities exist or some toxin is present.  While this concept is more complex than depicted here, it is not so important and can be discussed later when looking at specific EKG’s.

Source: https://kchemekg.wordpress.com/ekg-test-home/