Basic Arrhythmias with 12-Lead EKG Practice Test 2026 – Complete Exam Prep

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.