Exploring Limb Leads: Capturing the Heart’s Vertical Plane

The six limb leads help us understand the electrical flow of impulses on the frontal plane. They detect vertical but not anteroposterior ones. These leads are generated by the six chest electrodes, with the EKG machine automatically assigning positive or negative charges to each electrode to form the leads.

Standard Bipolar Limb Leads

Among the six limb leads, three are standard- These are bipolar leads- they measure the voltage difference between two electrodes, one of which is positive and the other negative. A wave of depolarization toward a positive electrode would record a positive deflection. Repolarization would have the opposite effect.

• Lead I is formed by making the left arm positive and the right arm negative (0° orientation).
• Lead II is formed by making the legs positive and the right arm negative (60° orientation).
• Lead III is formed by making the legs positive and the left arm negative (120° orientation).

Augmented Unipolar Limb Leads

The remaining three limb leads are augmented. Unlike the standard limb leads, these leads are unipolar- they measure the electrical potential at one electrode relative to a calculated central reference point, the central terminal.

Central Terminal

The central terminal is an average of the electrical potentials from the three limb electrodes: V central terminal=(RA+LA+LL)/3. This central terminal acts as a neutral reference point, approximating the heart’s electrical center.

• Lead aVL is created by making the left arm positive and other limbs negative (-30° orientation).
• Lead aVR is created by making the right arm positive and other limbs negative (-150° orientation).
• Lead aVF is created by making the legs positive and other limbs negative (90° orientation).

When using the central terminal as the reference, the voltages recorded by the unipolar limb leads are naturally smaller than those in the bipolar leads.To make these signals clinically useful, they are mathematically augmented (amplified) by approximately 50% in the EKG machine. This enhancement is why they are called augmented leads. If the amplitude of the recorded wave in aVF is 3mm, the actual voltage is only 2mm but to make it easier to see, the output is augmented.

Limb Leads and the view they provide

Based on their orientation, each lead provides a different view of the heart on the frontal plane:

• Leads II, III, and aVF are known as inferior leads: As the direction of the vector of each lead suggests, they “look” at the inferior surface of the heart, composed primarily of the inferior aspect of the left ventricle. The Right Coronary Artery usually supplies this part of the heart.
• Leads I and aVL are high lateral leads: They look at the high, left lateral side of the heart, primarily composed of the left ventricle.
• Lead aVR is often overlooked but holds significant clinical importance. It views the heart from the right shoulder, essentially looking at the upper right side of the heart. Typically shows negative deflections because the heart’s overall electrical activity moves away from the right shoulder.

A special focus on aVR, the most overlooked lead?

Abnormalities in aVR are frequently overlooked. However, a few critical conditions may be diagnosed by looking at the lead that is frequently not looked at.

• An ST-segment elevation in aVR with ST-depressions and T-wave inversions in other leads may signify global subendocardial ischemia or significant left main lesion or significant left anterior descending lesion.
• Upright P-waves suggest severe right atrial abnormality.
• In pericarditis, we note widespread ST elevation with PR depression, but ST elevation in aVR is typically absent
• Differentiating Ventricular arrhythmia from Supra ventricular tachycardia with aberrant conduction: The Vereckei algorithm is a stepwise approach using lead aVR to differentiate VT from SVT with aberrancy. The following suggest VT as against SVT with aberrant conduction:
  1. Initial dominant R-Wave in aVR
  2. Initial q- or r-wave in aVR ≥40 ms
  3. Notching on the initial Downstroke
  4. Vt≥Vi in aVR is suggestive of VT: In SVT with aberrant conduction, the initial conduction happens via the Bundle of His: So, the vertical distance travelled by the QRS complex during the initial 40 msec (Vi) is greater than the terminal vertical distance travelled by the QRS complex during the last 40 msec (Vt) (i.e. Vi > Vt). However in VT, typically, the Bundle of His is the last to be depolarized- So the terminal vertical distance travelled by the QRS in the last 40 msec will be ≥ the total vertical distance travelled by QRS complex in the initial 40 msec.