An electrocardiogram (EKG or ECG) is a foundational diagnostic tool that records the heart’s electrical activity over time. This process involves placing electrodes on the body’s surface to detect the electrical changes that accompany the heart muscle’s depolarization and repolarization. The overall direction of this electrical flow is captured from different perspectives, known as leads. The standard procedure uses 12 specific views, six derived from limb electrodes, including the unipolar augmented limb leads: aVR, aVL, and aVF.
AVR, AVL, and AVF: What the Acronyms Mean
The initial letter “a” in each of the three leads stands for “augmented,” while the “V” signifies “voltage.” These leads are specifically named for the limb on which the positive electrode is electrically focused. The acronym aVR stands for “augmented voltage right” (right arm). Similarly, aVL means “augmented voltage left” (left arm). The third lead, aVF, represents “augmented voltage foot” (left leg). These three augmented limb leads are classified as unipolar, measuring the electrical potential at a single positive electrode against an averaged reference point. They work alongside the three standard bipolar limb leads (I, II, and III) to form the complete set of six limb leads used in the 12-lead EKG.
The Function of Augmented Leads
The “augmented” characteristic refers to an artificial amplification of the electrical signal they record. Unlike bipolar leads, which measure the voltage difference between two distinct limbs, unipolar leads measure the voltage at one limb against a neutral reference point. This neutral reference is a calculated average of the electrical potential from the other two limbs, known as the Goldberger’s central terminal. When the signal is compared to this central terminal, the resulting voltage amplitude is often quite small. The process of “augmentation” mathematically increases this signal’s strength by about 50%, making the tracing easier to interpret. This amplification is necessary because the positive electrode limb is also part of the central terminal’s average, which naturally reduces the recorded voltage difference.
Mapping the Heart’s Frontal Plane
The purpose of aVR, aVL, and aVF is to establish the heart’s electrical orientation within the body’s frontal plane. These three augmented leads, when combined with the three standard limb leads, create a six-axis reference system that surrounds the heart. This system allows doctors to determine the heart’s electrical axis, which is the general direction of the electrical spread during ventricular depolarization. The angles of these leads are fixed, with aVL positioned at -30 degrees, aVF at +90 degrees, and aVR at -150 degrees, relative to the horizontal axis of Lead I.
Each lead provides a distinct view of a specific anatomical area of the heart. Lead aVL views the high lateral wall of the left ventricle, while aVF offers a view of the inferior surface of the heart. Lead aVR points toward the upper right side of the chest, often displaying a negative deflection because the heart’s normal electrical flow moves away from this direction. Analyzing the electrical waves across this frontal plane is crucial for identifying conditions like axis deviation, ventricular hypertrophy, or certain conduction blocks.