Which ECG lead measures the heart’s activity from the left arm to the left leg?

Lead II is significant in electrocardiography as it measures the heart's electrical activity from the left arm to the left leg. This lead provides a view of the heart's rhythm and is commonly used for both diagnostic and monitoring purposes because it typically presents the largest and most easily interpretable P waves and QRS complexes. The vector of electrical activity during depolarization travels toward the left leg from the heart, making Lead II particularly useful for visualizing the overall cardiac electrical activity.

The placement of electrodes for Lead II creates a positive electrode on the left leg and a negative electrode on the right arm, allowing for a diagonal view of the heart's electrical conduction. This orientation effectively captures the activity of the SA node in the right atrium, as well as the depolarization across the atria and ventricles. As a result, Lead II is favored in both standard ECG recordings and continuous monitoring in clinical settings.