How could someone with relatively healthy arteries be at risk for a heart attack?

Blood clots are pretty common. They're caused by a whole laundry list of conditions, including cancer, atherosclerosis, blood and circulation disorders and obesity. Smoking can help someone develop a blood clot, as can sitting motionless for very long periods of time, like during an international plane trip.

Deep vein thrombosis, or DVT, is a type of blood clot that forms in the deep veins of the leg or groin, and it occurs commonly on flights lasting longer than eight hours. Hundreds of thousands of people develop this clot, often termed economy class syndrome, as a result of sitting still too long, and many of them have perfectly healthy arteries [source: Ohio]. Most people develop tiny clots that don't produce any symptoms and dissolve slowly, causing no problems at all. Some people, though, end up with a clot that can cause death.

Clots develop when clotting agents in the blood, called platelets, stick together. Platelets are the particles in your blood that prevent you from bleeding to death from a paper cut; they stick together to stop the bleeding. When a clot develops in a place like a vessel of the heart or one leading to the brain, though, it can stop blood flow and cut off oxygen, possibly leading to a stroke or heart attack. When it ends up in the heart, it's called a pulmonary embolism. You don't need to have heart-disease risk factors for it to happen, although that is the most common cause. On a plane, for instance, a blood clot can develop when you don't move around enough and blood pools in your legs. Blood clots can also result from the use of hormonal birth control, especially if the woman also smokes cigarettes.

In the scenario of a DVT, a heart attack isn't a sure thing. People who receive treatment -- usually blood thinners -- within an hour of developing a clot will almost always be just fine. The problem is that many people don't know the signs - they think the pain in their leg or groin is a cramp from sitting on the plane so long. Untreated, a DVT can be disastrous because it may disconnect from the leg vein, leave the point of origin and end up in the bloodstream. If this happens, it can travel all the way to the heart and cause a heart blockage, and possibly a heart attack. This is what happened to reporter David Bloom in 2003 when he spent days sitting in a cramped tank covering the Iraq War. He was 39 years old and didn't have heart disease. A clot developed in his leg and traveled to his heart, and he died of a heart attack.

The best way to avoid a heart attack from a pulmonary embolism is to be aware of the risks. When you take a long flight, make sure to walk the aisles of the plane every now and then to keep the blood moving efficiently through your legs. If you take birth control pills, call your doctor immediately if you feel pain or pressure in your leg or groin. And by all means, don't smoke.

While the causes of blood clots are well-known, another heart problem that can strike someone with a previously healthy heart is harder to explain.

Coronary artery spasms are a bit of a medical mystery.

Occurring in approximately 0.0004 percent of the population, coronary artery spasms are less common than blood clots [source: MedLine Plus]. They're usually not fatal. As with blood clots, coronary artery spasms, also called variant angina, are usually related to coronary artery disease. But sometimes they happen to people with no apparent heart problems at all.

A coronary artery spasm is pretty straightforward: It occurs when a part of the arterial heart muscle contracts suddenly, or squeezes shut. You can think of it in terms of any other type of muscle spasm, like when your bicep muscle tenses up out of nowhere. It's usually quite painful, and it doesn't last very long.

Unlike a sudden contraction in an arm muscle, though, a contraction in your heart muscle can have very serious results very quickly. A coronary artery spasm might last anywhere from five to 30 minutes, and it can cause chest pain [source: MedLine Plus]. Chest pain is usually called angina. Variant angina is different in that it usually occurs at rest (often when a person is asleep) instead of during a period of high exertion, and it may not be connected with heart disease. In some instances, maybe one-third of cases, people with no major heart-attack risk factors can have a coronary artery spasm [source: ClevelandClinic]. If it lasts, say, five minutes, the damage is minimal, limited to chest pain (or possibly no symptoms at all). It's usually treated with pharmaceuticals like beta-blockers, calcium-channel blockers or nitroglycerin.

If it lasts for, say, 30 minutes, the results can be dire. If the contracted artery muscle cuts off blood flow to the heart for an extended period of time, a heart attack can result.

Medical science is still not sure why variant angina occurs in otherwise healthy people. It may have something to do with a dysfunction in the endothelium cells that line blood vessels. Doctors do know what can increase the risk, though. Aside from heart disease, using drugs like cocaine, tobacco and amphetamine can lead to coronary spasm, as can severe stress and alcohol withdrawal.

Variant angina is a chronic condition and needs to be treated long-term. With use of the proper drugs when chest pain first occurs, a heart attack becomes far less likely.

For more information on variant angina, blood clots and other heart-related conditions, look over the links on the next page.