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the 12 leads ECG

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A. Why and how 12 leads of the ECG?

1.

In fact, you can put 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 the top of your head!

2.

It is possible to record everywhere the ECG because the current from the heart flows throughout the whole body. This current can flow because the body consists for a large part of fluid (water) and ions. 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.

 

3.

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.

 

Einthoven ECG leads

4.

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

 

5. the three Einthoven leads:

The first standard locations were determined by Einthoven (in 1903) and are:

    • lead I: between right arm (RA) and left arm (LA)
    • lead II: between right arm (RA) and left foot (LF)
    • lead III: between left arm (LA) and left foot (LF).

6. Bipolar leads:

These 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 simultaneously. These leads are therefore called bipolar leads or bipolar recordings.

 

Note that now we have three voltmeters working at the same time; one voltmeter for lead I, one voltmeter for lead II and one voltmeter for lead III.

7. Unipolar leads:

Someone else, later, modified the connections by recording from a single location and connect that to the voltmeter.

augmented limb leads

8. Indifferent electrode:

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 simultaneously.

9. 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.

10. Six limb leads:

There are now six leads: I, II, III,  aVR, aVL and aVF. These are all “limb” leads because they are connected to the arms and left leg. Three of these are bipolar (I, II, III) and three are unipolar leads (aVR, aVL, aVF).

location of precordial electrodes

11. 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 leads (pre = in front; cordis = heart).

 

12. Location on the chest:

  • 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
connection to precordial electrodes

13. 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.

12-lead ECG Slide Show

 

So, what is a lead?

This is sometime confusing for students. A "lead" is really a wire or an electrode to connect to the patient. But the word "lead" also means which connection is made; for example Lead I (=Einthoven I), or lead aVR, or chest lead V1. Note that some leads are bipolar and others unipolar. Likewsie, some leads are limb leads while others are chest leads.

 

 

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