In making an ECG, lead II configuration compares the potential difference between lead connections:

In an ECG (electrocardiogram), the lead II configuration specifically measures the electrical activity of the heart by comparing the potential difference between two specific electrodes: the right arm (RA) and the left leg (LL). This configuration is pivotal because it aligns with the heart’s electrical axis in a way that maximizes the detection of the heart's electrical signals, particularly those generated during the depolarization and repolarization phases of the cardiac cycle.

When the electrodes are placed on the right arm and left leg, the ECG lead II provides a waveform that represents the heart's electrical activity in a standard view, typically referred to as the "normal" lead position. This leads to better visualization of the PQRST cycles of the heartbeat, allowing healthcare providers to diagnose cardiac conditions effectively.

The other electrode configurations do not align with this specific view of the heart's electrical activity. For instance, connections between right arm and left arm, or right arm and right leg, do not represent the same angle of electrical potential and would not offer the same insights into the heart's function as lead II does. Therefore, the choice of RA and LL is essential for capturing a clear and accurate ECG trace that reflects the heart's rhythm and electrical conduction.