We covered the first 6 leads of the 12 lead ECG in the 6-part tutorial on axis determination, so it’s time to discuss the precordial leads (V1-V6). These are the unipolar leads that are placed directly on the patient’s chest.
The heart is a 3 dimensional object, and calculating the heart’s electrical axis in the frontal plane tells us nothing about whether or not the heart’s electrical axis is anterior or posterior, for example. This 3rd dimension that can be calculated using the precordial leads is sometimes referred to as the Z-axis (think of the Cartesian coordinate system).
I’m never one to say “you don’t need to know that”. However, I will say that I have not found an exact calculation of the Z-axis to be clinically useful. Maybe it is useful to cardiologists. I really don’t know, and it’s possible that my opinion will change in this matter.
The Textbook of Medical Physiology 9e; © 1996 Guyton AC, Hall JE; WB Saunders
Here’s what I teach my students.
The QRS complex should start out negative in lead V1. The QRS complex should end up positive in lead V6. Somewhere in between, there should be an equiphasic QRS complex and this is referred to as the transition lead. This most often occurs in lead V3 but is highly dependent on lead placement. When the transition happens in lead V1 or V2 it is referred to as an early transition. When it happens in leads V4, V5, or V6 it is referred to as a late transition.
In addition, there should be a gradual increase in the amplitude of the R-wave between leads V1-V4. This is referred to as R-wave progression. Lead V1 may or may not have an R wave, but one should show up by lead V2 and get a little taller in lead V3 and reach its maximum height in lead V4 or V5. Again, this is highly dependent on lead placement.
With an early transition, the R-wave is often taller than the S-wave in lead V1 or V2. This is referred to as an R/S ratio > 1 it could indicate a problem like posterior STEMI. When accompanied by “down-up” ST-segment depression, it’s what Tomas Garcia, M.D. refers to as a “carousel pony” which is diagnostic of acute posterior STEMI.
We’ll cover this in more detail on another day, but for now, here is the differential diagnosis of tall R waves in lead V1. This is taken from Mattu, Brady, et al. Prominent R Wave in Lead V1: Electrocardiographic Differential Diagnosis. Am J Emerg Med 2001;19:504-513.
- Right bundle branch block
- Left ventricular ectopy
- Right ventricular hypertrophy
- Acute right ventricular dilation
- Wolff-Parkinson-White syndrome Type A
- Posterior myocardial infarction
- Hypertrophic cardiomyopathy
- Progressive muscular dystrophy
- Misplaced precordial leads
- Normal variant
Poor R-wave progression (or “poor anterior R-wave progression”) is a non-specific finding on the 12 lead ECG. However, it could be an indicator of a more serious problem. This is an issue because EMS does not transport every patient to the hospital and we almost never have an “old” ECG for comparison. Many, if not most elderly patients do not have a “normal” ECG.
I hear all the time how paramedics don’t need to know how to interpret a 12 lead ECG to a high level. Of course we do! How else can we correctly advise patients of the risk of refusing care? It’s impossible to correctly interpret ST segment depression, ST segment elevation, and T wave abnormalities if you don’t understand the various conditions that cause them.
Take for example the ECG used in Part IV of the axis determination tutorial.
This ECG was captured on an emergency call of a 16 year old girl with a congenital heart defect. The call was for “acute shortness of breath” and the family advised dispatch that she was in “heart failure”. Look at the T waves in the right precordial leads. They’re inverted! And the ST segments are depressed! That’s acute coronary ischemia, right? Wrong. It’s a strain pattern from right ventricular hypertrophy. Note the tall R waves in lead V1. It turns out the girl was having a panic attack. Do you see why paramedics need to understand 12 lead ECGs?
Here are some parting words from Chou’s Electrocardiography in Clinical Practice, Fifth Edition, 2001 Saunders, ISBN: 0-7216-8696-4:
“The widely used term “poor R wave progression” is not helpful. In many cases the abnormally low R amplitude extending from the right into the mid or left precordial leads indicates myocardial infarction of the anterior wall. Such a pattern occurs also in the presence of left ventricular hypertrophy and in normal subjects without cardiac or pulmonary disease. It may be caused by a shift of the transitional zone to the left or by an atypical (abnormally high) placement of the mid-precordial chest electrodes. For this reason it is advisable to report the most likely cause of “poor R wave progression” in each case.”
Importantly, R-wave progression can help differentiate between early repolarization and LAD occlusion. R-wave progression tends to remain intact for the former and is often obliterated by the latter.
Stephen Smith, M.D. of Dr. Smith’s ECG Blog has a decision rule that states:
If the mean R-wave amplitude from V2-V4 is less than 5 mm, then it is almost certainly MI. If greater than 5 mm, it is probably early repolarization. A cutoff of 5 mm gives a sensitivity for MI of about 70%, but a specificity of greater than 95%.
You can see an example of LAD occlusion with poor R-wave progression here.
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