An ECG is made by measuring the electrical activity that goes through the heart. This activity measured using external electrodes:
- six chest electrodes: V1 – V6
- four extremity electrodes: left arm, right arm, left foot, and a neutral electrode
With these electrodes it is possible to derive twelve different leads: six precordial leads and six extremity leads. The extremity leads are:
- I: electrical activity measured from the right to the left arm
- II: electrical activity measured from the right arm to the left leg
- III: electrical activity measured from the left arm to the left leg
- aVL: electrical activity moving towards the left arm
- aVR: electrical activity moving towards the right arm
- aVF: electrical activity moving towards the feet
When the electrical activity moves towards the leads, it creates a positive deflection of the QRS-complex of that lead. Electrical activity that moves away from the lead will create a negative deflection of the QRS-complex of that lead.