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Sudden death is always shocking, but there was something particularly unsettling about the way actor John Ritter died last week. He suffered what is known as an aortic dissection, an uncommon condition in which the major artery that carries blood from the heart to the rest of the body basically tears itself apart. More precisely, blood, which is propelled by the beating heart, gets between layers of the arterial wall and pushes them apart, moving down the blood vessel like a run racing down a nylon stocking.

As you might expect, if the tear occurs close to the heart, death can occur in minutes. Indeed, half of all patients who suffer this kind of aortic catastrophe die within 24 hours. "The key is to make the diagnosis," says Dr. Michael Deeb, a cardiac surgeon at the University of Michigan in Ann Arbor. The pain of aortic dissection is often mistaken for that of a heart attack. But if the dissection is detected in time and there's no damage to other organs in the body, surgeons can successfully replace the torn section of aorta with a synthetic graft.

Unfortunately, aortic dissections are impossible to predict. "You can't get screened for dissection," says Dr. Ann Bolger, a cardiologist at the University of California, San Francisco, and a spokeswoman for the American Heart Association. "It's not there until it starts, and when it starts, it's almost over."

There are tests that can detect small aneurysms — tiny bulges in the arterial wall that sometimes precede aortic dissections. But finding these aneurysms in the chest is a major production, and just because you have one doesn't mean you'll get a dissection. (Abdominal aneurysms are easier to spot.)

You can, however, decrease your risk of developing an aortic tear the same way you decrease your risk of heart disease — stop smoking, start exercising, and if you have either high blood pressure or diabetes, get it under control.

Search medlineplus.gov for more on aortic dissection

• Christine Gorman