You have just stored your carry-on bag overhead and are settling into the window seat of a crowded airplane. It's your first vacation in 11 months, and you're ready to relax, but the traffic jam getting to National Airport was a nightmare and your heart is still pounding after your race through the terminal.

As the plane begins to taxi and the seatbelt and no-smoking signs alight, you suddenly feel a cramp in your chest. If this is a heart attack, what are your chances of getting treatment in time -- right there at the airport?

There were four or five heart attacks at National last year, airport officials say, and at other airports there were many more. This year, millions of passengers will fly somewhere in the United States, and some will need medical care during their journeys. Some will suffer heart attacks; some will have injuries; some will experience motion sickness or anxiety attacks; some will be pregnant.

The Federal Aviation Administration requires that all airports be prepared to act in case of a major disaster, such as the recent Detroit crash. Usually this involves an arrangement to bring victims to local hospitals. At Detroit Metropolitan Wayne County Airport, there are 10 to 13 trained medics on hand at all times, and they were among the first at the crash scene.

Most plans of this nature at major airports are so comprehensive that airport officials like to tell stories of turning away ambulances from nearby communities because too much help sometimes descends on airports after a crash.

But for routine care of the millions of people passing through airports, the arrangements are much more haphazard. The FAA has no rule concerning medical care on the ground for travelers, and most airports simply rely on emergency medical technicians stationed at the fire rescue stations or call an outside ambulance.

Many airports have tried medical clinics staffed by doctors and nurses. While these have provided needed services, most of the clinics don't get enough patients to sustain a profit. As a result, clinics at airports in Denver and St. Louis have closed. At Dulles International, the clinic operated by the George Washington University Medical Center closed last February when its rent became too expensive.

Other airports are pushing ahead with more elaborate plans for on-site care.

In Chicago, O'Hare Airport is planning an expansion that could increase annual air traffic from a current 54 million passengers to 82 million passengers a year by 1995. Given that increase, airport officials are considering the possibility of beefing up the ground medical care, which in addition to the fire rescue squads includes two first-aid stations staffed by nurses. The stations operate daily from 6 a.m. to 11 p.m.

Officials at O'Hare have ordered a feasibility study, scheduled to be completed next month, to consider different types of medical facilities ranging from a 24-hour-a-day clinic to an organ-transplant center for Illinois patients who will be receiving organs flown in from other cities.

"Our goal is to have a facility that is staffed and equipped appropriately but can support itself financially," says Robert Threatte, deputy commissioner for airport planning and development.

Despite the relatively fast response times that airport ambulances have, more lives could be saved and ill and injured patients could be made much more comfortable by having on-site medical care available at airports, some experts believe. For example, tissue plasminogen activator, a drug awaiting federal approval, is one of a number of new drugs that can break up clots during a heart attack, allowing blood to get through to the heart. But the drugs must be administered within about four hours. Other common airport emergencies that should be treated immediately include severe nosebleeds, which may require expert packing and cauterizing; pulmonary embolisms (clots in the lung), which should be treated immediately with oxygen and anticoagulants; and administration of painkillers such as morphine for severe accident victims and heart attack patients. Only one of the Washington area airports, Washington National, has a medical clinic on its premises. George Washington University has operated a clinic at National for three years, staffed by a doctor and nurse. The clinic is open Monday through Friday from 8:30 a.m. until 5 p.m.

Like some other airports, the medical unit at National augments its income by doing FAA medical tests and by providing care such as allergy shots for airport personnel and immunizations for travelers going to foreign countries that require them.

According to Deborah Miller, the clinic's manager, about 4,000 people came to the clinic last year; about half of those were emergencies. The clinic is located on the lower level of the main passenger terminal, across from the Eastern Airlines shuttle baggage claim area. Cost of an emergency visit is $35; tests are extra.

Both Baltimore-Washington International and Dulles are served by emergency medical technicians on ambulances stationed at the airports. If an ambulance is in use, an outside service is called as a backup.

San Francisco International Airport, the country's seventh-largest airport, operates an around-the-clock clinic every day of the year staffed by doctors, nurses and X-ray and lab technicians. Last year, according to Dr. Lawrence Smookler, the clinic's medical director, the clinic treated close to 14,000 emergencies, including 40 to 50 heart attacks and an equal number of false alarms.

"We also see quite a few pulmonary embolisms," says Smookler. These dangerous clots are often caused by sitting in one place for an extended period, an occupational hazard at airports.

Because the emergencies alone can't sustain the clinic financially, about half of the work done there includes annual FAA medical tests for pilots and air traffic controllers and medical care for the 45,000 airport employes -- including orthopedic visits for many airport workers who break arms, legs and fingers on the job. Smookler says that the clinic is an integral part of the emergency set-up and that in order to stay prepared, the clinic staff holds monthly drills. "They're lucky to have us here," says Smookler.

As the population gets older and emergency medical intervention gets better, it becomes more crucial to get medical help quickly. And as millions of passengers continue to spend as much time on the ground as in the air while waiting for a flight, demand for care at the terminal is likely to grow.

Fran Lunzer is a free-lance writer in New York.