What are EKG leads I, II, and III referred to as?

Leads I, II, and III are referred to as standard limb leads because they are derived from the electrical activity of the heart as captured by electrodes placed on the limbs. These leads form what is often referred to as the Einthoven triangle, an important concept in EKG analysis where the electrodes placed on the right arm, left arm, and left leg create a triangular circuit to measure the heart's electrical activity from different angles or perspectives.

The standard limb leads provide a view of the heart’s electrical activity in the frontal plane, which is crucial for diagnosing a variety of cardiac conditions. Each lead offers a unique perspective: Lead I measures the potential difference between the left arm and the right arm, Lead II measures it between the left leg and the right arm, and Lead III measures it between the left leg and the left arm. By interpreting these leads, clinicians can assess the heart’s rhythm and identify issues such as arrhythmias, myocardial infarction, and other cardiac pathologies.

The other options refer to different types of leads. Augmented limb leads consist of a different set formed by combining signals from the standard leads, while precordial leads are placed on the chest wall to provide a view of the heart's electrical activity in the horizontal