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Part 2: The core concepts
In my experience, it is easier to divide ECG's into observing two core themes of the heart:
The electrics (Bundle branches, etc)
The plumbing (Coronary arteries)
An ECG records the summation of activity from cardiac muscle activation.
That is to say, we observe the result of the impulse - not the impulse itself.
It is thus from this baseline, that any deviations/abnormalities are detected - and interpreted.
Leads are divided into limb leads + praecordial leads.
Limb leads are named as such because they are attached to the limbs.
The mnemonic"Ride Your Green Bike" is often used to remember this.
"R" being the right hand, and going in a clockwise direction.
The Praecordial leads are named as such because they are, shock horror, placed upon the praecordium.
There are 6 leads to place.
By definition, they should be placed on the following landmarks:
Emphasis on "should."
The above is the exam answer.
In the real world, you will find that individual practices may vary.
And with that out of the way, here is the main event:
Behold, an ECG in its often-seen paramedic form!!
The observant amongst you will notice that there are 12 leads on this standard ECG.
But wait! Mathematics demands answers!
4 limb leads + 6 praecordial leads does not = 12!
Before we degenerate into a witchhunt and recreate the Salem trials, let us try explain this wizardry.
The praecordial leads V1-V6 are accounted for.
It is at this stage I remind the viewer that I am an EM Doctor, not a physicist.
Why a ground lead is required is beyond my scope.
Only that i know it exists. Which is why out of the 4 limb leads, only 3 are "active" so to speak.
Behold, the authors (horribly amateurish) drawing of the Einthoven triangle.
Notice how it has three leads, and it (tries) to make an equilateral triangle around the heart.
The astute reader may recall that the limb leads are placed in a roughly rectangular formation, given the way our bodies are designed.
And yet, a equilateral triangle emerges.
The author apologises that this mathematical geometrical obfuscation will simply be explained as "just run with it."
For the truly nerdy, the limb leads are "bipolar leads" because they have two electrode points.
This is opposed to the "augmented" limb leads - which are unipolar.
So named because they are from the electrical "zero" in the middle of the heart to the limb leads.
They are termed:
"Augmented Voltage Right" (aVR)
"Augmented Voltage Left" (aVL)
"Augmented Voltage Foot" (aVF)
And so the answer to why there are twelve leads is answered.
6 praecordial leads,
3 limb leads
3 augmented leads.
6+3+3=12.
And so balance has been restored.
Next : Understanding views and territories.
Click for part 3:
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