What does Einthoven's triangle describe in ECG terms?

• A. The triangle around the heart formed by the bipolar limb leads
• B. The triangle around the heart formed by the chest leads
• C. The triangle formed by RL leads
• D. The triangle around the heart formed by the unipolar limb leads

Answer: (A)

Einthoven's triangle is the geometric representation of the three standard bipolar limb leads around the heart. It’s formed by the potentials measured between pairs of limb electrodes: right arm to left arm, right arm to left leg, and left arm to left leg. These leads project the heart’s electrical dipole onto three axes spaced roughly 60 degrees apart, creating a triangle that helps describe the heart’s electrical activity in the frontal plane. Chest leads (precordial) sit on the chest and are unipolar; they map the heart in a different plane and are not part of this triangle. The right leg electrode is used as a reference/ground and isn’t one of the triangle’s vertices. So the triangle around the heart formed by the bipolar limb leads is the concept described.