What is the shape of Einthoven's triangle?

• A. Right triangle
• B. Isosceles triangle
• C. Acute triangle
• D. Inverted equilateral triangle

Answer: (D)

Einthoven's triangle is an imaginary triangle formed by the three limb leads—the right arm, left arm, and left leg. The triangle is oriented with its apex pointing downward toward the left leg, so it’s inverted relative to a typical upright triangle. Because the limb electrodes are placed roughly at equal distances around the body, the sides are treated as equal, giving the geometry of an equilateral triangle in theory. So the shape is an inverted equilateral triangle. This concept helps explain the relationships between the limb leads and underpins Einthoven's law.