The Electrocardiogram - Part VI


The Systematic Approach






I strongly believe that it is important to look at every ECG the same way. If you have a methodical way of interpreting ECGs you will miss very little. You can do this in any manor you would like as long as it is the same each time. I am going to go over it in the way in which I look at ECGs. I think this is probably the most common system.





Systematic approach:
  • Rate
  • Rhythm
  • P wave
  • PR interval
  • QRS Complex
  • Axis

The Rate & Rhythm are the first thing you should interpret. We went over this in Part V. You will determine the heart rate in beats per minute and whether the rhythm is regular or irregular.

Below is a regularly regular rhythm

Below is an example of an irregularly irregular rhythm

Below is a regularly irregular rhythm.
It may take a longer strip to determine the pattern.


The P wave:
  • Are they present?
  • Are they all the same?
  • Are they of normal morphology?
  • Are there more P waves than QRS complexes?
Are they present?
The P wave is indicator of a functioning sinoatrial node. Rhythms that have an adequate P wave are termed Sinus. P waves should be less than 120 ms and the amplitude should be less than 0.25 mv. Certain pathologies may alter this.

Below is an example of a sinus rhythm

Below is an example of a rhythm without P waves

Are they all the same?
Does each P wave on the ECG strip resemble each other? This is important.

Below are two examples of rhythms that have multifocal P waves

Are there more P waves that QRS complexes?

This may indicate an AV block or even worse.


Below is an example of an instance where you have more P waves than QRS complexes

PR Interval:
  • Is the PR interval long or short?
  • Does the PR interval change?
Is the PR interval long or short?
The normal PR interval range is 120ms to 200ms

Below is an example of a long PR interval

Below is an example of a short PR interval

Does the PR interval change?
The PR interval shows the correlation between the P wave and the QRS complex (the atria and the ventricles). This correlation should not change.

Below is an example of a fluctuating PR interval

QRS Complex:
  • How wide are the QRS complexes?
  • Are they all the same?
  • Is it low or high voltage?
How wide are the QRS complexes?
They should be between 40ms to 120ms

Below is an example of QRS complexes of normal width

Below is an example of an ECG rhythm with wide QRS complexes
All ECG images courtesy of Pace Symposia's ECG simulator

Are they all the same?
Each QRS complex should resemble the last. Certain pathologies may alter this.

Is it high or low voltage?
Make note of extremes in QRS voltage. This may indicate certain pathologies.


Axis:

Our concern is the QRS axis. The axis is the frontal view of the mean electrical vector.

To learn how to determine the axis within 15 degrees I will refer you once again to the prehospital 12 lead blog. Click here for the axis tutorial.

Here is a simple method to determine axis deviation which I will elaborate on in the future.

Look at leads I, II, & III. Another method uses just aVF instead of II & III, but is less specific.


[+] = positive
[-] = negative

Normal Axis: [+] I, [+] II, [+] III

Physiological left axis deviation: [+] I, [-/+] II, [-] III

Pathological left axis deviation: [+] I, [-] II, [-] III

Physiological right axis deviation: [-] I, [+] II, [+] III

Extreme right axis deviation: [-] I, [-] II, [-] III



Below is the lead I and aVF method:





6 comments:

Tom B said...

I teach the "Six Step Method" (which actually has seven steps).

1.) Rate and rhythm. If the exact rhythm is unknown, are we certain we're dealing with a supraventricular rhythm? This is critical because if the rhythm has wide QRS complexes, it's ventricular until proven otherwise.

2.) Axis determination. Is the axis in the normal range? Is it a left axis deviation, which might suggest left anterior fascicular block, inferior MI, or paced rhythm. Is it a right axis deviation, which might suggest left posterior fascicular block, lateral MI, or right ventricular hypertrophy or strain. Is it an extreme right axis deviation, which might suggest VT, electrolyte derangement, or misplaced limb lead electrodes?

3.) QRS duration (and other intervals like the PR interval and QT interval). There's a good chance you've already recognized these abnormalities, but this is your opportunity to re-check.

4.) Morphology. If the QRS duration is = or > 120 ms, what is the QRS morphology in lead V1? Is it RBBB morphology or LBBB morphology? Now check lead I to confirm. If it's RBBB morphology in lead V1, combine with axis to determine whether or not bifascicular block is present. If lead V1 and lead I don't match, then consider ventricular rhythms, including paced rhythms, non-specific intraventricular conduction defects, and electrolyte derangements or drug overdose.

5.) STE-mimics, QRS confounders, Imposters of AMI. By now we've already determined whether or not a bundle branch block or paced rhythm is present. How about left ventricular hypertrophy? Benign early repolarization or pericaditis?

6.) Ischemia, Injury, Infarct. Check out the ST-segments. In the case of a STE-mimic, consider the "rule of appropriate T-wave discordance" and Sgarbossa's criteria. Look for reciprocal changes.

7.) "Step 7". Clinical correlation! Interpret the ECG in light of the hisotry and clinical presentation.

It's been working pretty well for me for the last 10 years!

Tom

Adam Thompson, EMT-P said...

Tom B,

Yet another good example of a systematic approach. If I ever get to 12-leads, I will make sure to mention your method.

Tom B said...

Sorry, Adam! I should have realized my post was out of context. You're just the only person I know who includes axis with basic rhythm interpretation! :)

Tom

Adam Thompson, EMT-P said...

Yea, I only started doing that recently. It comes in handy when differentiating a few rhythms. I usually just scroll through the leads to get an idea prior to doing a 12-lead. I guess it may be a bit much for this tutorial.

Have you done a post based on your method? I think I am going to practice this. I just read it again, and I think it is better than what I have been doing.

Tom B said...

I have not written about the Six Step Method on the Prehospital 12 Lead ECG blog. Just haven't gotten around to it! :)

Tom

C. Watford said...

Great post, quick note, you have "intermediate axis" rather than "indeterminate axis" in the last paragraph. Your steps are what we learned in school and Tom's are what we learned for 12-leads.