Lead I records electrical activity between which two electrodes?

• A. RA to LA
• B. RA to LL
• C. LA to LL
• D. RA to RL

Answer: (A)

Lead I measures the heart’s electrical activity as the potential difference between the right arm and left arm electrodes. In practice, one limb electrode (the right arm) serves as the negative reference and the other (the left arm) as the positive input, so the signal represents the voltage of the left arm relative to the right arm. The right leg electrode is used as a ground reference and doesn’t form part of this lead. By comparison, the other limb leads pair different limbs (for example, right arm to left leg, or left arm to left leg), which is why Lead I specifically corresponds to the right arm to left arm connection.