Module title = Tutorial: Origins of Activity

Lesson title = Upright P waves

Rule: P waves from the SA node are upright

If a P wave comes from the SA node (image below), it will travel "downward" and to the patient's left. When measured in most ECG leads, this will create an upright P wave.
 
 


Rule: P waves from the atria are usually inverted

When a P wave comes from the atrium (image below), it will travel upward and often to the patient's right. This will usually (but not always) create an upside down P wave, also called an inverted P wave.
 



Rule: P waves from the AV node are inverted

Similarly, when a P wave comes from the AV node (image below), it will also travel upward. This will also create an inverted P wave:



Rule: P waves from the ventricle are inverted

This is a rare event. Sometimes with a ventricular origin, the electrical signal can travel in reverse direction through the AV node and cause a P wave. If this happens (again, rare), the P wave will be inverted.


Rule: No P waves rules out SA node and atria

The SA node and atria will both generate P waves when they are the origins of activity. Therefore, no P waves rules out the SA node and atria.
Caution: sometimes P waves are very very hard to see.
Exception to this rule: if you have a wavy baseline (discussed in Lesson #4 ... 2 after this one).


Summary:

• upright P waves are usually (> 95%) from the SA node
• inverted P waves are usually (> 95%) from the atria, AV node, or ventricles
• no P waves rules out SA node and atria
• P waves that come from the ventricles is a rare event