Where is the heart located in relation to Einthoven's triangle?

• A. At the left vertex
• B. At the center
• C. Outside the triangle
• D. At one side

Answer: (B)

Einthoven's triangle is a frontal-plane representation formed by the three limb leads around the body. The heart’s electrical activity can be visualized as a single vector (the cardiac dipole) whose projection falls inside that triangle. Because the limb leads surround the heart, the overall depolarization direction is balanced in all directions, placing the heart roughly at the center of the triangle. If the heart were at a vertex or outside, the leads would produce very unbalanced deflections, which isn’t how the ECG behaves. So the heart sits at the center of Einthoven's triangle, roughly speaking.