V1:
4th intercostal space to the right of the sternumThere are 12 leads in the standard surface electrocardiogram, 3 bipolar and 9 unipolar. The electrodes for bipolar and 3 of the unipolar are placed on the limbs, there are known as limb leads, the remaining 6 unipolar leads are on the chest.
Why are there 12 ECG leads ??
V1:
4th intercostal space to the right of the sternum
V2 :4th intercostal space to the left the of sternum
V4: 5th intercostal space in the midclavicular lineV3: Midway between V2 and V4
V5: 5th intercostal space in the anterior axillary line
V6: 5th intercostal space in the midaxillary line
Each lead look at the heart from different angle
click for ECG tracing of each lead
Bipolar limb leads record the differeces in potential between 2 limbs
Lead I : Between left arm and right arm with left arm is positive.
Lead II: Between right arm and left leg with left leg is positive.
Lead III: Between left arm and left leg with left leg is positive.
click for ECG tracing of each lead
The axis of three bipolar limb leads form a triangle known as the Einthoven's Triangle. The electrodes for all the 3 leads are about equidistant from the heart, thus the triangle they form is equilateral.
Unipolar limb leads record the potential difference between one limb and a neutral electrode. The recording obtained from unipolar limb leads are small, therefore the augmented limb leads are generally used. These leads record electrical activity between one lead and the other two limbs. The size of the potential is increased by 50 % without changing the configuration of the record.
aVR: Right arm positive. Click for electrode connection
aVL: Left arm positive. Click for electrode connection
aVF: Left foot positive. Click for electrode connection