Which lead measures the potential difference between the right arm and left arm electrodes?

Which lead measures the potential difference between the right arm and left arm electrodes?

• A. Lead I
• B. Lead II
• C. Lead III
• D. aVR

Answer: (A)

In limb-lead ECGs, the different leads measure voltage differences between pairs of arm and leg electrodes. Lead I specifically compares the right arm to the left arm: it uses the right arm as the negative terminal and the left arm as the positive terminal, giving the potential difference along the horizontal axis from right to left. That direct RA-to-LA comparison is why this lead reflects the voltage between the two arms. The other limb leads involve different pairings (one includes the left leg as well), and the augmented lead is formed by combining limb potentials to create a single reference toward the right, not a simple RA versus LA difference.

In limb-lead ECGs, the different leads measure voltage differences between pairs of arm and leg electrodes. Lead I specifically compares the right arm to the left arm: it uses the right arm as the negative terminal and the left arm as the positive terminal, giving the potential difference along the horizontal axis from right to left. That direct RA-to-LA comparison is why this lead reflects the voltage between the two arms.

The other limb leads involve different pairings (one includes the left leg as well), and the augmented lead is formed by combining limb potentials to create a single reference toward the right, not a simple RA versus LA difference.