In both the 5- and 12-lead configuration, leads I, II and III are called limb leads. The electrodes that form these signals are located on the limbs—one on each arm and one on the left leg. The limb leads form the points of what is known as Einthoven's triangle.
- Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode.
- Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode.
- Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode.
BIPOLAR & UNIPOLAR LEADS
There are two types of leads: unipolar and bipolar. Bipolar leads have one positive and one negative pole. In a 12-lead ECG, the limb leads (I, II and III) are bipolar leads. Unipolar leads also have two poles, as a voltage is measured; however, the negative pole is a composite pole (Wilson's central terminal) made up of signals from lots of other electrodes. In a 12-lead ECG, all leads besides the limb leads are unipolar (aVR, aVL, aVF, V1, V2, V3, V4, V5, and V6).
Wilson's central terminal is produced by connecting the electrodes, RA; LA; and LL, together, via a simple resistive network, to give an average potential across the body, which approximates the potential at infinity (i.e., zero).LEADS avr, avl and avf
Leads aVR, aVL, and aVF are augmented limb leads. They are derived from the same three electrodes as leads I, II, and III. However, they view the heart from different angles (or vectors) because the negative electrode for these leads is a modification of Wilson's central terminal. This zeroes out the negative electrode and allows the positive electrode to become the "exploring electrode" or a unipolar lead. This is possible because Einthoven's Law states that I + (−II) + III = 0. The equation can also be written I + III = II. It is written this way (instead of I − II + III = 0) because Einthoven reversed the polarity of lead II in Einthoven's triangle, possibly because he liked to view upright QRS complexes. Wilson's central terminal paved the way for the development of the augmented limb leads aVR, aVL, aVF and the precordial leads V1, V2, V3, V4, V5, and V6.
- Lead augmented vector right (aVR) has the positive electrode (white) on the right arm. The negative electrode is a combination of the left arm (black) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the right arm.
- Lead augmented vector left (aVL) has the positive (black) electrode on the left arm. The negative electrode is a combination of the right arm (white) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the left arm.
- Lead augmented vector foot (aVF) has the positive (red) electrode on the left leg. The negative electrode is a combination of the right arm (white) electrode and the left arm (black) electrode, which "augments" the signal of the positive electrode on the left leg.
The augmented limb leads aVR, aVL, and aVF are amplified in this way because the signal is too small to be useful when the negative electrode is Wilson's central terminal. Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system, which is used to calculate the heart's electrical axis in the frontal plane.Einthoven's triangle
LEFT AXIS DEVIATION (LAD)
Left axis deviation occurs when additional electrical forces move to the left (hypertrophy), or when the time required for the electrical activity to move over the ventricle is prolonged (LBBB, left ventricular dilation).
Causes of left axis deviation include hypertension, aortic stenosis or regurgitation, subaortic stenosis, mitral regurgitation, and left ventricular conduction defects.
The QRS axis may shift during the respiratory cycle if elevation of the diaphragm changes the physical position of the heart. Beat-to-beat variation in QRS axis (an every-other-beat change in QRS shape) is called “electrical alternans.” This is thought to be caused by the heart physically swinging back and forth in a pericardial effusion.
RIGHT AXIS DEVIATION (RAD)
Right axis deviation is seen on the ECG when more electrical forces are moving to the right than normal. This is usually due to hypertrophy of the right ventricle (RVH). Causes of right axis deviation include COPD, pulmonary emboli, valvular disease, septal defects, and pulmonary hypertension.
An axis of +90 is common in persons with emphysema. This so-called “vertical heart” reflects both the rotation of the heart downward as the diaphragm position drops due to air trapping, and some degree of hypertrophy of the right ventricle.
EXTREME AXIS DEVIATION
RIGHT SUPERIOR AXIS DEVIATION (RARE CASES)
ROTATION OF THE CARDIAC AXIS
The QRS axis is the “average” direction of electrical activity during ventricular depolarization. The QRS axis may shift due to physical change in the position of the heart, chamber hypertrophy, or conduction block.