This topic will introduce you to the leads and electrodes of a standard 12-lead ECG, as well as some of the common variations such as right and posterior leads.
Wondering whether to skip ahead? Here are some of the questions you should be able to answer by the end of this topic:
First, there's the 12 leads of the ECG, which are like different channels or views of the heart. Each channel records electrical activity in a single direction. Having 12 different views lets us know more about how electricity moves through the heart in 3 dimensions.
Common lead names include I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5 and V6.
Then there's the 10 electrodes that are placed on the skin to record the ECG. These are also called leads by some people. Only 10 electrodes are used to record the 12 lead ECG because each lead compares two or more electrodes to make the trace. For example, the V2 channel uses electrodes LA, RA, LL, RL and V2 to make the trace.
Common electrode names include LA, RA, LL, RL, V1, V2, V3, V4, V5 and V6.
A 12-lead ECG is recorded by placing 10 electrodes on the skin in very specific locations.
The first 4 electrodes are placed on the limbs. They are named after each limb, including Left Arm (LA), Right Arm (RA), Left Leg (LL) and Right Leg (RL).
There are 6 other electrodes that are placed on the anterior chest wall. They are named V1-V6.
Limb leads include I, II, III, aVL, aVR and aVF. Generally, limb leads look at the heart in a frontal plane (i.e. they measure electrical activity moving up, down, left and right through the heart).
The chest leads include V1, V2, V3, V4, V5 and V6. They are also called precordial leads. It is often said that the chest leads look at the heart in a transverse plane (even though technically the chest leads are not quite all on exactly the same horizontal level).
The limb leads I, II and III are bipolar leads, whereas the rest are unipolar leads.
Each lead represents electrical activity in one direction.
Lead I is the easiest lead to remember. It records the horizontal electrical activity through the heart from right to left by comparing the two arm electrodes (RA --> LA). Lead aVF records the vertical electrical activity of the heart by comparing the arm electrodes RA + LA against LL.
Leads II and III look down towards the feet. Lead II compares RA (-) to LL (+), making it look in the direction of 5 o'clock. Lead III compares LA (-) to LL (+) (7 o'clock). As a result, lead II looks diagonally across the heart down to the left while lead III looks more to the right.
Lead aVR compares LA + LL (-) to RA (+), so it looks up towards the right shoulder (10 o'clock). Lead aVL compares RA + LL (-) to LA (+), so it looks up towards the left shoulder (2 o'clock).
(Note: the 'a' in aVF, aVR and aVL means they are 'augmented' leads. Augmented leads have to be mathematically amplified to make the waves large enough to see clearly. These special leads are also unipolar leads, because they record electrical activity between one electrode and the average of two other electrodes instead of simply using one electrode each end. By contrast, leads I, II and III are bipolar. )
If you combine all of these directions on the one diagram, you can see that the 6 limb leads cover a wide spectrum of electrical activity in the frontal plane.
Chest leads use the average of all the limb electrodes as their negative (-) electrode. They are also unipolar. Each chest lead records electrical activity from the centre of the heart out towards each chest electrode.
It is useful to divide the 12 leads into anatomical lead groups, because some pathology (i.e. infarction) affects specific areas of the heart.
Inferior leads include II, III and aVF.
Septal leads include V1 and V2. They may also be called right precordial leads.
Anterior leads include V3 and V4. They may also be called mid precordial leads.
Lateral leads include V5, V6, I and aVL. V5 and V6 may also be called left precordial leads, while I and aVL may be called high lateral leads.
When you are trying to localise an infarct, it is sometimes useful to record extra leads by moving the electrodes to the right or posterior surface of the heart.
Right sided leads can be used to identify a right ventricular infarction. They are recorded by swapping one or more of the chest electrodes to the same position on the right side of the chest. The most commonly used right lead is V4R, but there are several options depending on how many right sided leads you wish to record.
Posterior leads can be used to identify a posterior ventricular infarction, by moving chest electrodes into new positions in the same horizontal plane as V6.