An electrocardiogram (ECG or EKG) is a standard non-invasive diagnostic test used to measure and record the heart’s electrical activity. This activity originates from the heart’s natural pacemaker, causing the heart muscle to contract and pump blood. The ECG machine captures these electrical impulses from the body’s surface using conductive patches called electrodes. The term “lead” refers not to a physical wire or electrode, but to a specific, mathematically derived perspective or view of the heart’s electrical current.
The Bipolar Limb Leads
Leads I, II, and III are specific measurements called the bipolar limb leads, not physical electrodes. They are “bipolar” because they measure the voltage difference between two distinct electrode points, one positive and one negative. Conceptually, these three leads form Einthoven’s Triangle, an imaginary inverted equilateral triangle with the heart at its center.
This triangle provides the basis for understanding the leads’ relationship. Lead I measures the difference between the left arm (positive) and the right arm (negative). Lead II measures the difference between the left leg (positive) and the right arm (negative), and Lead III measures the difference between the left leg (positive) and the left arm (negative). These derivations adhere to Einthoven’s Law, where the activity recorded in Lead II equals the sum of the activity in Leads I and III.
Anatomical Placement of Limb Electrodes
The three bipolar limb leads are derived from signals recorded by four physical electrodes placed on the limbs or torso. Four limb electrodes are required: Right Arm (RA), Left Arm (LA), Left Leg (LL), and Right Leg (RL). The RL electrode serves as an electrical ground or neutral reference point to minimize interference and is not used in the voltage calculation of Leads I, II, or III.
Electrodes are typically placed on the fleshy, non-muscular areas of the limbs, such as the wrists and ankles. Since the limbs act as long conductors, placing the electrodes more proximally on the shoulders and hips achieves the same result and can reduce movement artifacts. The physical locations of the RA, LA, and LL electrodes form the corners of the theoretical Einthoven’s Triangle, from which the three leads are mathematically calculated.
Standardized color coding helps ensure correct placement, as incorrect placement can lead to misdiagnosis. Although color codes vary internationally, the common scheme uses white for the Right Arm, black for the Left Arm, red for the Left Leg, and green for the Right Leg ground electrode.
Clinical Perspective of Leads I, II, and III
Leads I, II, and III provide a specific directional view, or vector, of the heart’s electrical activity in the frontal plane. Lead I is oriented horizontally at 0 degrees and offers a view of the lateral wall of the left ventricle. This lateral perspective assesses electrical events moving across the heart from right to left.
Lead II is oriented at approximately +60 degrees and provides a view toward the inferior surface of the heart, including the bottom wall of the left ventricle. Lead III is oriented at approximately +120 degrees, also offering an inferior view from a slightly different angle. Leads II and III, along with the augmented lead aVF, are grouped as the “inferior leads” and are used to detect conditions like an inferior wall myocardial infarction.
By analyzing the combined patterns from these three leads, along with the other nine leads of a standard 12-lead ECG, physicians determine the heart’s electrical axis. An abnormal electrical axis, indicated by deviations in the waveforms of Leads I, II, and III, can suggest hypertrophy of a heart chamber or a disruption in the normal electrical conduction pathway. This multi-angled electrical picture is fundamental for diagnosing a wide range of cardiac conditions.