Where is V3 placement located?

• A. between V3 and V5
• B. between V2 and V4
• C. between V1 and V3
• D. between V4 and V6

Answer: (B)

Understanding how chest leads map the heart is the key here. The precordial leads run from right to left across the chest to image the heart’s anterior surface. V2 is placed at the left sternal border in the 4th intercostal space, and V4 is placed at the left midclavicular line in the 5th intercostal space. V3 is positioned halfway between those two placements, i.e., between V2 and V4. This midway position captures the electrical activity over the part of the heart between what V2 and V4 each monitor, giving a more complete view of the anterior wall.

If you tried placing V3 elsewhere, such as between V1 and V3 or between V4 and V6, you would not sample the intended anterior region in continuity with V2 and V4, which would reduce the usefulness of the comparison across adjacent leads.