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In fact, to record an ECG, you can place the recording electrodes anywhere you like as long as it is on the skin, such as on your chest, on your belly, on your foot and even on top of your head!
It is possible to record the ECG anywhere on the skin because the current generated in the heart flows throughout the whole body. This current can flow because the body consists for a large part of fluid (water) and ions (electrolytes).
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.
You can record the ECG everywhere but they will all look different.
That is because if the electrodes are for example on the chest, 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 collect the current especially from the back or from the side of the heart.
The three Einthoven leads:
The first standard locations were determined by Einthoven (in 1903) and are:
Note: these leads are always named ‘I’, ‘II’ and ‘III’ (Roman letters) and NOT ‘1’, ‘2’ or ‘3’!!
Bipolar 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.
Unipolar leads:
Someone else, later, modified the connections by recording from a single location and connect that to the voltmeter.
But a voltmeter needs two inputs’: 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.
Again, later, someone else developed an even more clever connection scheme, which augments (=increases) 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.
(The ‘a’ in front of VR, VL and VF means ’augmented’!)
Six limb leads:
We now have six leads: I, II, III (from Einthoven), and aVR, aVL and aVF (the augmented leads). These are all “limb” leads because they are connected to the arms and to the left leg.
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 mid-clavicular line
V5 -> 5th intercostal space in the anterior axillary line
V6 -> 5th intercostal space in the mid-axillary line
1.
In summary, we now have 12 standard leads!
The diagram to the right shows an easy way to remember the limb leads and their polarities. They are displayed in the so-called triangle of Einthoven.
Tips: to help remember all this:
– The polarities at the right arm are always negative.
– The polarities at the (left) leg are always positive.
– The polarities at the left arm follow those at the right arm and the left leg.
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 are unipolar. Likewise, some leads are limb leads while others are chest leads (V1 to V6).
