Einthoven's triangle
Imaginary triangle of limb leads used in electrocardiography.
Einthoven's triangle is an imaginary, inverted equilateral triangle used in electrocardiography, with its points at the two shoulders and the pubis and the heart at its center. Willem Einthoven, who first proposed the concept, used salt-water buckets to immerse the hands and feet as contact points for his string galvanometer, the first practical ECG machine.
The triangle defines three standard limb leads. Lead I runs horizontally from the right shoulder (negative) to the left shoulder (positive), giving a 0-degree angle. Lead II goes from the right arm (negative) to the left leg (positive), with a +60-degree orientation. Lead III runs from the left shoulder (negative) to the left leg (positive), oriented at +120 degrees. Electrodes can be placed anywhere on the limb without affecting the recording, and the right leg electrode, which reduces interference, can be placed anywhere without altering results. Each lead measures the voltage difference between two electrodes, capturing the heart's continuously changing electric field as a vector during depolarization and repolarization.
The triangle also helps identify lead misplacements. Reversing the arm electrodes flips lead I's polarity and swaps leads II and III. Swapping the right arm and leg electrodes flips lead II's polarity and makes lead I become lead III, and vice versa. Reversing the left arm and leg flips lead III's polarity and swaps leads I and II.
- field
- Electrocardiography
- known_for
- Theorizing Einthoven's triangle and developing the first practical ECG machine
Lore & Background
Einthoven's triangle is an imaginary formation of three limb leads in a triangle used in the electrocardiography, formed by the two shoulders and the pubis. The shape forms an inverted equilateral triangle with the heart at the center. It is named after Willem Einthoven, who theorized its existence. Einthoven used these measuring points, by immersing the hands and feet in pails of salt water, as the contacts for his string galvanometer, the first practical ECG machine.
Reader's Guide
Einthoven's triangle provides the foundational framework for standard limb leads in electrocardiography. The three leads—Lead I (shoulder to shoulder, 0 degrees), Lead II (right arm to left leg, +60 degrees), and Lead III (left shoulder to leg, +120 degrees)—each measure the electric field created by the heart during depolarization and repolarization of myocytes. The electric field can be represented as a vector that changes continuously and can be measured by recording the voltage difference between electrodes. Electrodes may be placed distally or proximally on the limb without affecting the recording, and the right leg electrode acts to reduce interference. Einthoven's triangle is also helpful in identifying incorrect placement of leads, which can lead to error in the recording and ultimately to misdiagnosis. For example, if the arm electrodes are reversed, lead I changes polarity, causing lead II and lead III to switch. Reversal of the right arm with the leg electrode changes lead II polarity, causing lead I to become lead III, and vice versa. Reversal of the left arm and leg causes a change in polarity of lead III and switching of leads I and II.
Did You Know?
- Einthoven's triangle is an inverted equilateral triangle with the heart at its center.
- Einthoven used pails of salt water to immerse the hands and feet as contacts for his string galvanometer.
- Lead I has a 0-degree angle of orientation, Lead II has +60 degrees, and Lead III has +120 degrees.
- The right leg electrode acts to reduce interference and can be placed anywhere without affecting ECG results.
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