What lead is positioned on the left leg in the ECG system?

In the ECG system, Lead III is positioned on the left leg as part of the standard limb leads. The limb leads are set up to measure the electrical activity of the heart from various angles, and Lead III specifically records the difference in electrical potential between the left leg and the left arm.

Positioning Lead III on the left leg allows the ECG to capture the vector of electrical activity that travels from the left arm to the left leg, providing important information about the heart’s electrical conduction and helping in the assessment of various cardiac conditions. This positioning is integral to forming a complete picture of the heart's electrical behavior and contributes to the overall analysis when reading an ECG trace.