When John Ritter arrived at the hospital Sept. 11, 2003, he was nauseated and vomiting, and he felt a dull tightening in his chest. Doctors ordered an EKG, which proved inconclusive. But an hour or so later, Ritter's chest tightness was worse, and a second EKG was more dire. Besides, his blood pressure had gone down, his heart rate had gone up, and he seemed to be developing congestion in his lungs.

These were all signs of a man having a heart attack.

The standard of care for heart attack victims is to try to open their arteries as soon as possible, and that is what doctors did for the popular actor, treating him in the cardiac catheterization lab where they inserted an intra-aortic balloon pump.

Unfortunately, this was exactly what Ritter didn't need.

Ritter's heart attack was not typical. It was not caused by plaque blocking his arteries but by a sudden tear in the inner lining of his aorta, a very dangerous, and rare, condition known as an aortic dissection.

The aorta, leading from the heart, is the body's largest blood vessel. The lining of Ritter's aorta tore in a critical spot: The flap of loose tissue intermittently covered his left main coronary artery, interrupting the supply of blood -- and oxygen -- to the left ventricle of his heart.

What Ritter needed was emergency surgery to repair the tear.

After Ritter's death, his widow, Amy Yasbeck, and his children sued the cardiologist who treated Ritter in the emergency room that night, as well as a radiologist who had examined Ritter two years before. They contended that the cardiologist should have diagnosed Ritter's real problem and given him more appropriate treatment and that the radiologist should have found that Ritter had an enlarged aorta, which increases the risk for a dissection.

Earlier this month, the doctors were found blameless, with a jury saying they had done all they could.

That may have done little to reassure Americans afraid of suffering their own aortic dissection -- and of not being diagnosed correctly.

So, the first thing to remember is that you probably won't have an aortic dissection. Only about 10,000 Americans a year do, says Dr. Kim Eagle, a cardiology professor at the University of Michigan at Ann Arbor and a director of the university's Cardiovascular Center, who testified for the defense in the Ritter case. (Heart attacks are far more common; there are 450,000 of them annually in the U.S.)

A second thing to know is that your risk for having an aortic dissection is largely determined by your genes, although some lifestyle choices can affect your chance of having one and recovering from it.

Finally, if you ever think you might be having a dissection -- or a heart attack -- be sure to go to a hospital immediately. Early diagnosis and treatment can make all the difference.

Aortic dissections come in two kinds -- varying by where they occur. Type A's, the kind Ritter had, occur in the upper part of the aorta where it first leaves the heart. Type Bs occur farther along, at or below the point where arteries branch off to the arms.

Type A dissections -- the most common and most serious kind -- can cause hemorrhaging in the sac around the heart or interfere with the blood supply to heart or brain. Ritter's intermittently blocked the supply to his heart, causing his heart attack. About 3% to 5% of dissections cause heart attacks, Eagle says.

Type B dissections can interfere with the blood supply just about anywhere at or below the chest, including the pancreas, liver, kidneys, intestines and legs.

Symptoms vary a lot

Symptoms of a dissection of either type can vary from something as small as a painful jaw or cold leg to paralysis or stroke. In fact, because dissections are so rare, and so many of their symptoms can have other, more likely causes, "between 35% and 45% of patients with dissections are initially suspected of having something else," Eagle says.

"It's the great masquerader," says Dr. John Elefteriades, professor and chief of cardiac surgery at Yale University in New Haven, Conn., who testified for the plaintiffs in the Ritter trial.

The most common symptom among dissection patients is sudden, catastrophic pain that some describe as worse than childbirth. "They have almost a freeze-frame memory of what they were doing when the dissection occurs," Eagle says. "A heart attack takes places over a period of minutes. With a dissection, there's a moment."