Wednesday, 5 January 2011

ECG101:P-Wave

It’s all in a P-Wave

You know, the more you learn about medicine, the more you will realize how much there is out there that you will never know. ECGs are no exception. My obsession with cardiology began in paramedic school and it was only natural for me to grow a similar love for ECG interpretation.

Unfortunately, the usual prehospital training for 12-lead ECGs is limited, and generally doesn’t cover much more than STEMI and bundle branch blocks. The traditional resistance to further education usually involves a phrase like “well how is that going to change your treatment?” Nothing irks me more than hearing that stupid question. As if gaining new treatment strategies was the only reason to learn something. In this collection of posts, we will discuss some of the other things that an ECG can be used for, and you may find that it just might change your treatment.

The p-wave
The p-wave is the graphical representation of atrial depolarization. With normal physiology, this is initiated by the sinoatrial, or SA node. The p-wave can tell us many different things. A normal p-wave, married to a normal QRS lets us know that the patient has normal conduction.

There are many abnormals that may clue us in to a few pathologies though. If a p-wave is too tall, or peaked, right atrial enlargement is likely. A p-wave with two humps might indicate left atrial enlargement. The absence of a p-wave lets us know that the pacemaker is somewhere else in the heart. A p-wave that follows the QRS is likely not being used, and indicates that the AV junction is probably controlling the heart rate.

Maybe you have a p-wave that is slowly gaining distance between itself and it’s QRS complex, until finally there is no QRS; which indicates a Wenkebach pattern. The p-wave could be completely divorced from the QRS complex, indicating atrioventricular disassociation, or a complete heart block. It’s funny how much that little deflection can tell us. We often take it for granted, but when it is absent, we want it back desperately.

Above you can see a relatively normal location for a P-wave. 99% of your patients will probably have a similar variation to this.
Below is an example of p-pulmonale, which is an indication of right atrial enlargement.


Why is it called p-pulmonale?

Well, this is enlargement if the right atrium.  The right atrium pumps its contents into the right ventricle which eventually empties its contents into the pulmonary trunk, pulmonary arteries, and eventually the lungs.  When the pressure is backed up or increased in the lungs from chronic respiratory conditions like COPD, or asthma, or as a result of left ventricular failure, the right ventricle has to work harder.  When the right ventricle works harder, so does the right atrium.  Consisting of mostly muscle, when the heart works harder it gets bigger, just like your biceps.  Consequently, right-sided heart failure is known as cor pulmonale.  There are other causes, such as a right ventricular myocardial infarction, but the concept remains the same.


Above, you will see a p-wave with two humps.  These p-waves are usually wider than normal, and are an example of p-mitrale, which is indicative of left atrial enlargement.

How does the left atrium enlarge?

The left atrium generally hypertrophies as a result of heart failure.  To be more specific, left-sided heart failure.  Left ventricular hypertrophy can be expected as well.  This doesn't have to be the case though, anything problem that causes the left atrium to increase its workload may result in an increased size.

While biphasic p-waves may be a normal finding in V1 on a 12-lead ECG, if it is deeper than it is tall, it is likely representing left atrial enlargement as well.


You may find that many of your patients have both atria enlarged, and maybe even all four chambers.  This cardiomegaly is a very good indicator that your patient may be suffering from heart failure.  When you see left atrial enlargement on the 12-lead, don't forget to look for LVH.  Remember that LVH can present with a left ventricular strain pattern, and this can mimic STEMI quite well.  Check out ems12lead.com for more about that STE-Mimic.

The p-wave and its relationship with the QRS complex can tell you about abnormal conduction from the atria to the ventricles.  More specifically, you can diagnose an atrioventricular block by closely examining the p-wave and pr-interval.

How to identify which AV block you are looking at:


Is the PR-Interval a constant length?
Yes > Are there any dropped beats?                                                     No > P wave for every QRS?

- Yes = Mobitz 2                                                                                   - Yes = Mobitz 1/Wencheback

- No = 1st Degree AV Block                                                                  - No = Complete Heart Block