How do doctors know what percentage of you is burned?

Answers to your questions about the news.
July 11 2006 6:20 PM

How Much of Me Is Burned?

Thirty percent? Forty?

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A four-story building on the Upper East Side of Manhattan blew up on Monday morning in what authorities think may have been a suicide attempt. The building's owner, Nicholas Bartha, was pulled from the rubble with second- and third-degree burns covering 30 percent of his body. How did doctors figure out that percentage?

They might have used "the Rule of Nines." In the 1950s, doctors developed an easy way to estimate the ratio of the area of a patient's burns to the total area of his skin. The system works by assigning standard percentages to major body parts. (Most of these happen to be multiples of nine.) The skin on each arm, for example, covers 9 percent of a patient's total surface area. Each leg comprises 18 percent, as do the front and back of the torso. The head and neck together make up another 9 percent, and the last bit (or 1 percent) covers the genitalia and perineum. This breakdown makes it easy for doctors to estimate the size of a burn in relation to a body—a burn that covered half the arm would add 4 or 5 percent to the total figure.

Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

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The Rule of Nines works pretty well for adults, but it can lead to significant distortions with children. That's because a child's body proportions vary dramatically as she gets older—especially in the legs and head. To account for this development, many hospitals use a Lund-Browder chart (first published in 1944) to determine burn percentages. The Lund-Browder system uses fixed percentages for the feet, arms, torso, neck, and genitals, but the values assigned to the legs and head vary with a child's age.

Another method uses the size of a patient's palm as a reference. As a general rule, the skin on the palm of your hand comprises 0.5 percent of your total surface area. (For children, it's 1 percent.) A doctor can check the size of a patient's hand and compare it with the size of a burn to make a quick guess about the percentage.

All these methods have their flaws. For starters, body shapes have changed significantly during the last 60 years. An obese patient will have different proportions than a skinny one. (For example, the abdomen grows bigger as compared with the palm of the hand.) The Rule of Nines and the Lund-Browder chart can also produce significant errors in burn victims with particularly large breasts.

Computer-assisted methods provide better estimates for burn percentages. Some of these use three-dimensional models of the human body. Others let doctors draw burn shapes with a mouse or the stylus of a Palm Pilot.

Bonus Explainer: Why does the percentage matter? The total surface area of the burn correlates with the outcome for the patient. Doctors can use the percentage to decide the course of treatment. A burn victim might be hospitalized if she has moderate burns over more than 10 percent of her body, or third-degree burns over more than 1 percent. Doctors can also use the burn percentage to figure out how much fluid should be given intravenously. The Merck Manual suggests that a 160-pound patient with a 40 percent burn should receive about 8 liters of fluid over the first day of treatment.

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Explainer thanks Raphael Lee of the University of Chicago.

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