The Class 2 Review: In the beginning... there was the P wave, and it was pretty good. Part I

The P wave, the beginning of any good EKG. But, what does it really mean? What exactly is it showing? How do I know that the P wave is abnormal?

In this entry, let's review the P wave. It's largely taken for granted, but there can be times where paying special attention to the little hump before the big complex can be warranted.

The Holy Grail of Basic* EKG interpretation, in most circles, is Dr. Dubin's Rapid Interpretation of EKG's. In his book, he suggests that the best lead to view the P wave is V1. Why is that?

You can see that V1 rests directly over the right atrium, which is were the SA node is located. This is what makes it the best candidate to evaluate the P wave. It gives direct access to the electrical forces leaving the atrium of the heart. Another great lead for looking at P waves is lead II, which follows the line of the electrical vector in a normal axis. (Axis Is Important!)

In sinus rhythm, the P wave should be upright in all leads, except aVR. If you think about it, it makes perfect sense, the electrical impulse is traveling from the SA node (located high in the right atrium) in a right to left, top to bottom fashion towards the AV node. This would cause a (mostly) positive deflection in any view where the positive lead is located below or to the left of the SA Node. The only lead that does not fit this description is aVR.

Tech Specs:

Less than 120 ms wide
Less than 2.5 mm tall

Fun Fact: Atrial reploarization occurs, but the amplitude is too low to show up on the EKG

And some food for thought, here is Dr. S. Venkatesan:

What are the components of Pwave ?
The SA node depolarises the RA (right atrium) first , so the initial part of P wave represents RA after about 40msec the wave front reaches LA (left atrium) and it begins it’s depolarisation By the time LA is maximally depolarised the RA already starts its repolarisation.So there is overlap and also a short time lag between these two wave forms.

Makes sense if you think about it.

In the next installment, we'll brush up on some of the abnormalities of the P waves.

*I say basic, because this book does a great job at getting you to a basic understanding of reading 12 Lead EKG's, however, there is so, so much more to learn that is not included in the book. Here are some great websites to help your skills up a notch:

EMS 12 Lead

Dr. Steve Smith's EKG Blog

Harvard's EKG Case Library

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