This will probably already be famililar, but just to recap:
The P-wave corresponds to atrial depolarisation.
The QRS complex corresponds to ventricular depolarisation (these are discussed in further detail below).
The T-wave corresponds to ventricular repolarisation.

THE LEADS

A 12-lead ECG shows the heart from various different angles – 6 vertical leads that are arranged around the heart in a coronal plane of section and a further 6 horizontal leads arranged around the heart in a transverse plane of section. As the electrical events in the heart follow specific vectors (directions), a lead placed at a different point will record different electrical activity.
In fact it has 10 leads that you attach to the patient – the ECG machine will then calculate the what it should be seeing from the remaining angles (the term ‘leads' can be misleading here, so remember that a ‘lead' on an ECG trace doesn't always correspond to a lead on a patient!).

Vertical Leads
There are 3 leads that are attached to the patient, from which 6 ECG traces are calculated.
One lead on the right arm (usually red)
One lead on the left arm (usually yellow)
One lead on the left leg (usually green)
An earth lead should also be placed on the right leg (usually black)
This creates Eindhoven's Triangle around the heart:
The three unipolar leads (aVR, aVL and aVF, also known as the limb leads ) show the views of the heart from the leads on the limbs (the three corners of the triangle). The three bipolar leads (I, II and III) are calculated from the differences between points on Eindhoven 's triangle.
This creates the 6 vertical leads:

Horizontal Leads
V1-V6 are the horizontal leads, arranged around the heart in a transverse plane of section (fig 1.9 from ECG made easy). V1 and V2 mainly view the right ventricle, while V3-V6 mainly view the left.

They are placed on the body as follows:
V1 on the 4 th intercostal space, to the right of the sternum.
V2 on the 4 th intercostal space, to the left of the sternum.
V3 inbetween V2 and V4.
V4 on the 5 th intercostal space in the midclavicular line (the apex beat).
V5 inbetween V4 and V6.
V6 in the midaxillary line, on a horizontal level with V4.

 

 

eindhoven's rules for labelling qrs complexes

The first wave upwards is labelled R (irrespective or whether or not it is preceeded by a downwards wave).
The first downwards wave before R is labelled Q.
The first downwards wave after R is labelled S.
Any upwards wave after S is labelled R` (pronounced R-prime).

 

Vectors of ventricular depolarisation

1.   Depolarisation spreads across the ventricular septum from left to right.
2. The depolarisation then spreads through the mass of the ventricles from inside to outside – as the left ventricle has thicker walls it dominates the vector, which is downwards and to the left.
3.  Finally, the remaining areas of the heart (the posterior aspect) depolarise with a depolarisation running upwards and to the right.
An electrical event coming towards a lead is recorded as a positive (upwards) wave, while one moving away from it is recorded as negative (downwards). If an electrical event is perpendicular to the lead, then no wave is created.

You should hopefully now be able to see how the same electrical event, recorded in different leads may have different letters as these leads are recording the event from different directions. For the electrical events above, viewed from 2 different leads:

Electrical event

Wave created in lead II

Wave created in aVR

1

Q

R

2

R

S

3

S

R`

Look at the diagram showing the arrangement of the leads above and this should make sense to you.