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)

In unipolar chest leads, the heart is viewed along a defined axis by placing a reference (negative) electrode at one chest site and an active (positive) electrode at another. The Modified Chest Left lead specifically uses a negative electrode on the left upper chest and a positive electrode on the right side of the sternum at the fourth intercostal space. This setup makes the lead vector point from the left upper chest toward the right sternum in that fourth intercostal space. That orientation is what the correct description describes: negative on the left upper chest and positive on the right sternum in the 4th intercostal space. Other configurations would place electrodes in different locations or sides and would alter the viewing direction and resulting waveform.

In unipolar chest leads, the heart is viewed along a defined axis by placing a reference (negative) electrode at one chest site and an active (positive) electrode at another. The Modified Chest Left lead specifically uses a negative electrode on the left upper chest and a positive electrode on the right side of the sternum at the fourth intercostal space. This setup makes the lead vector point from the left upper chest toward the right sternum in that fourth intercostal space. That orientation is what the correct description describes: negative on the left upper chest and positive on the right sternum in the 4th intercostal space. Other configurations would place electrodes in different locations or sides and would alter the viewing direction and resulting waveform.