What does Lead II primarily record in an ECG?

Lead II primarily records the electrical difference between the right arm and the left leg. This orientation allows Lead II to capture the heart's electrical activity as it travels from the upper body (right arm) to the lower body (left leg). The right arm serves as the negative pole, while the left leg acts as the positive pole. This placement positions Lead II well to monitor the overall depolarization wavefront of the heart as it primarily moves in a downward and leftward direction during depolarization.

This positioning is particularly significant because it aligns closely with the natural axis of the heart, providing a clear view of the heart's rhythm and overall electrical conduction. Consequently, Lead II is one of the most commonly used leads for assessing heart rhythms, as it typically produces a prominent P-wave and QRS complex, making it easier to identify arrhythmias and other cardiac abnormalities.