In the Modified Chest Left lead, the negative electrode goes on the left upper chest, with the positive electrode at the right side of the sternum in the 4th intercostal space. Which description is correct?

In the Modified Chest Left lead, the negative electrode goes on the left upper chest, with the positive electrode at the right side of the sternum in the 4th intercostal space. Which description is correct?

• A. Negative on the left upper chest; positive on the right sternum in the 4th intercostal space
• B. Negative on the right upper chest; positive on the left sternum in the 2nd intercostal space
• C. Negative on the left lower chest; positive on the right sternum in the 5th intercostal space
• D. Negative on both sides of the chest; positive on the back

Answer: (A)

The key idea here is how electrode polarity and position define the lead’s vector. For the Modified Chest Lead, the negative electrode sits on the left upper chest and the positive electrode is placed to the right of the sternum at the 4th intercostal space. This sets the lead axis to run from the left upper chest toward the right sternal border, so the recorded signal reflects the electrical activity along that left-to-right anterior direction.

Because the positive electrode is on the right sternum at the 4th intercostal space, this orientation captures a specific frontal-plane vector across the chest. Placing the negative electrode on the left upper chest ensures the measured potential difference points in that direction, producing the characteristic waveform expected for this lead configuration.

Other placements would shift the axis or place one electrode in a different region (for example, on the opposite chest, at a different intercostal level, or on the back), which changes what the lead “sees.” Hence, the described setup—negative on the left upper chest and positive on the right sternum in the 4th intercostal space—best matches the Modified Chest Lead described.