ECG EDUCATION--D

THE 12 LEAD ECG MACHINE

In the market ECG machines are available in various types such as 12-lead,6-lead,3-lead, and single lead types.

When we think to buy one for our home use we must first familiar with these types.

For example when we buy a 12-lead machine we should have a thorough knowledge regarding the number of electrodes (Leads) and what are the functions of each electrode and how they should be placed on our body.

See the following animated diagram for a 12-lead ECG machine:-            

                             

Fig-1

In Fig-1 we can see the various electrodes and how they are placed on a human body and brief details are given below:-

1.aVR-Lead:-This lead has been attached at the wrist below the palm at the right hand.

2.aVL-Lead:-This lead has been attached on the left-hand wrist similarly.

3.aVF(R)-Lead:-This lead has been attached at the right foot which you can see as a red dot at the right foot without naming as it serves merely as an earth connection without any further involvement in the ECG procedure.

4.aVF-Lead:-This is attached on the left foot similar to the right.

The above mentioned three unipolar leads namely aVR,aVL, and aVF are forming a triangle known as Einthoven's Triangle.

The above-mentioned triangle is shown with green lines in the above figure.

The triangle is very important as it adds three more virtual bipolar electrodes namely lead-I,

lead-II, and lead-III (Limb bipolar electrodes)

5. In the above diagram (Fig-1) the lead-I receives electrical signals from aVR, and aVL and sends it to the heart.

6. Similarly, the lead-II receives signals from the two unipolar leads namely aVR and aVF and send to the heart

7. The last bipolar lead receives signals from aVL and aVF and sends it to the heart.

Now we attached 7 electrodes out of which one is earth (aVR-F) and the three bipolar electrodes are virtual and embedded in each side of the triangle.

Now we have to place six more electrodes known as chest electrodes as per the methods given below:-
V1-electrode:4th intercostal space at the right sternal border. This means from the first rib next to the shoulder bone (clavicle) below the neck to the space between 4th and 5th ribs and right to the chest big bone(sternum)
V2-electrode:-Similar to the above lead but to the right of the sternum.
V3-electrode:-Midway between V2 and V4.
V4-electrode:-5th intercostal space at the midclavicular line
V5-electrode:-Left anterior axillary line horizontally next to V4.
V6-electrode:-Left midaxillary line on the same horizontal pane next to V4 and V5.
Now the views of the heart by the electrodes are diagrammatical as follows:-

                                 Fig-2 In the above diagram (Fig-2) there are three views such as from the anterior(upper), posterior(lower), and lateral(side) views.
aVR, I,aVL, V1, V2, V3 are viewing the heart from the upper side.
aVF is viewing the heart from the lower side
II, III, V4'V5'and V6 are viewing from the lateral sides.
Terminology:-
Electrodes-----Electrical conductor or receiver
Lead         ---- The electrical potential difference                              between two electrodes
aVR------Augmented Voltage Right

aVL ------Augmented Voltage Left
aVF ------Augmented Voltage Foot

Note:-Many times these two terms 'lead and electrode' are used interchangeably and it doesn't matter.
The bipolar electrodes are virtual and hence they are leads only.

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                                                           Continued in the next post (E)...