How to read an Electrocardiogram (ECG) - Part 3, The P Wave

The P wave represents the spread of electrical activity over the atrium. The normal depolarization begins at the sinoatrial (SA) node near the top of the atrium. Because of the top-to-bottom, right-to-left path of the P wave, it’s normally largest in lead II. The normal P wave is upright in all leads except R.

• The P wave normally lasts less than 0.11 seconds (just less than three small boxes). 
• An abnormally long P wave occurs whenever it takes extra time for the electrical wave to reach the entire atrium. This occurs in left atrial enlargement.
• The height of the P wave is normally less than 2.5 small boxes (less than 0.25 milli volts). 
• An abnormally tall P wave is seen when larger amounts of electricity are moving over the atrium. This usually indicates hypertrophy of the right atrium. 
• The P wave may be decreased in height by hyperkalemia.

Part 1 PQRST Waves
Part 2 ECG Graph Interpretation
Part 3 The P Wave
Part 4 The PR Interval
Part 5, The QRS Complex
Part 6, The ST Segment