Bipolar recordings utilize standard limb lead configurations depicted at the right. By convention, lead I has the positive electrode on the left arm, and the negative electrode on the right arm, and therefore measures the potential difference between the two arms. In this and the other two limb leads, an electrode on the right leg serves as a reference electrode for recording purposes. In the lead II configuration, the positive electrode is on the left leg and the negative electrode is on the right arm. Lead III has the positive electrode on the left leg and the negative electrode on the left arm. These three bipolar limb leads roughly form an equilateral triangle (with the heart at the center) that is called Einthoven's triangle in honor of Willem Einthoven who developed the electrocardiogram in 1901. Whether the limb leads are attached to the end of the limb (wrists and ankles) or at the origin of the limb (shoulder or upper thigh) makes no difference in the recording because the limb can simply be viewed as a long wire conductor originating from a point on the trunk of the body.
Based upon universally accepted ECG rules, a wave a depolarization heading toward the left arm gives a positive deflection in lead I because the positive electrode is on the left arm. Maximal positive ECG deflection occurs in lead I when a wave of depolarization travels parallel to the axis between the right and left arms. If a wave of depolarization heads away from the left arm, the deflection is negative. Also by these rules, a wave of repolarization moving away from the left arm is recorded as a positive deflection. Similar statements can be made for leads II and III in which the positive electrode is located on the left leg. For example, a wave of depolarization traveling toward the left leg produces a positive deflection in both leads II and III because the positive electrode for both leads is on the left leg. A maximal positive deflection is recorded in lead II when the depolarization wave travels parallel to the axis between the right arm and left leg. Similarly, a maximal positive deflection is obtained in lead III when the depolarization wave travels parallel to the axis between the left arm and left leg.
If the three limbs of Einthoven's triangle (assumed to be equilateral) are broken apart, collapsed, and superimposed over the heart, then the positive electrode for lead I is said to be at zero degrees relative to the heart (along the horizontal axis) (see figure at right). Similarly, the positive electrode for lead II will be +60º relative to the heart, and the positive electrode for lead III will be +120º relative to the heart as shown to the right. This new construction of the electrical axis is called the axial reference system. With this system, a wave of depolarization traveling at +60º produces the greatest positive deflection in lead II. A wave of depolarization oriented +90º relative to the heart produces equally positive deflections in both lead II and III. In this latter example, lead I shows no net deflection because the wave of depolarization is heading perpendicular to the 0º, or lead I, axis (see ECG rules).
