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The 12 leads ECG.
1.
2.
It is possible to record the ECG everywhere
In fact, you can put the recording electrodes
because the current from the heart flows
anywhere you like as long as it is on the skin, throughout the whole body. This current can
such as on your chest, on your belly, on your flow because the body consists for a large
foot and even on top of your head!
part of fluid (water) and ions.
3.
A mistake that is often made is the belief that
the ECG current flows with the blood. That is
nonsense. The current flows through all
fluids; blood, interstitial fluid and
intracellular fluids.
4.
You can record the ECG everywhere but
they will all look different.
That is because if the electrodes are for
example in front of the heart they will mainly
record the current originating from the front
of the heart. If the electrodes are located at
the back of the body, or at the side, then the
electrodes will especially collect the current
from the back or from the side.
5.
To be able to compare ECG’s between
different people or at different times it is
therefore necessary to standardize the
locations from where you record the ECG
6.
The three Einthoven leads:
The first standard locations were determined
by Einthoven (in 1903) and are:
a. lead I: between right arm (RA) and
left arm (LA)
b. lead II: between right arm (RA) and
left foot (LF)
c. lead III: between left arm (LA) and
left foot (LF).
The 12 leads ECG
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7.
Bipolar leads:
8.
Unipolar leads:
The Einthoven leads are essentially bipolar
leads; that is, they record from two
electrodes simultaneously, one positive and
one negative and the signal is a composite of
the current picked up at both sites. These
leads are therefore called bipolar leads.
9.
Indifferent electrode:
Someone else, later, modified the connections
by recording from a single location and
connect that to the voltmeter.
But, a voltmeter needs two input’s: one
positive and one negative.
The solution is to connect the negative pole to
an indifferent electrode. This is like an
imaginary or a virtual or a reference
electrode. It is actually connected to all limbs
together.
10.
Augmented leads:
Again, later, someone else developed an even
more clever connection scheme which
augmented the amplitude of the signals. That
is why these leads are now called unipolar
and augmented. There are three of these
leads: aVR, aVL and aVF.
11.
Six limb leads:
There are now six leads: I, II, III, and aVR,
aVL and aVF. These are all “limb” leads
because they are connected to the arms and to
the (left) leg. Three of these are bipolar leads
(I, II, III) and three are unipolar leads (aVR,
aVL, aVF).
12.
Pre-cordial leads:
Another way to “look” at the heart is to place
an electrode on the chest, as close as possible
to the heart. These are therefore called
precordial lead (pre = in front; cordis = heart).
These are also unipolar leads (= connected to
an indifferent electrode).
The 12 leads ECG
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13.
The electrode locations on the chest are very
precise:
V1 -> 4th intercostal space, right of the
sternum
V2 -> 4th intercostal space, left of the
sternum
V3 -> halfway between V2 and V4
V4 -> 5th intercostal space in the midclavicular line
V5 -> 5th intercostal space in the anterior
axillary line
V6 -> 5th intercostal space in the mid-axillary
line
14.
The triangle of Einthoven.
The diagram to the right shows an easy way
to remember the limb leads and their polarity.
The are displayed in the so-called triangle of
Einthoven.
So, what is a lead?
This is sometime confusing for students. A "lead" is really the wire and the electrode to
connect the ECG recorder to the patient. But the word "lead" also means which connection is
made; for example Lead I (=Einthoven I; between right arm and left arm), or lead aVR, or
chest lead V1. Note that some leads are bipolar and others unipolar. Likewise, some leads
are limb leads while others are chest leads.
The 12 leads ECG
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