When you travel by air, your immediate concern may be about weather, getting bumped or making your connecting flight. But what happens if you have a medical emergency at 30,000 feet?

The answer depends largely on the airline and what provisions it has made for in-flight emergencies. Although rare, the final decision to divert an airplane and make an unscheduled landing for such an emergency is up to the captain. Flight attendants usually handle in-flight medical emergencies, often with the help of medically trained passengers who happen to be on board. Many airlines also have arrangements for their flight crews to communicate with expert medical assistance on the ground, if necessary.

It's difficult to determine precisely how many people become seriously ill while flying. But many of these illnesses are related to cardiac and pulmonary diseases, such as a heart attack or blood clot in the lungs (pulmonary embolism). If you have a medical condition, especially one that involves your heart or lungs, check with your doctor before you fly. Here's what else you should know.

If you have certain conditions, it may be best not to fly. Here's a look at some potential problems:

• Cold or upper respiratory infection. When you have a cold, the tiny tube that connects your throat and middle ear is often blocked. Normally this tube, called the eustachian tube, equalizes air pressure in your middle ear with the increasing outside air pressure. Blockage in the tube leaves a vacuum in your ear when the airplane is descending, leading to pain and discomfort. This condition may impair your hearing and cause ringing in your ears, dizziness and considerable pain.

  If possible, avoid flying when you have a cold or upper respiratory infection. If you must fly, take an antihistamine several hours before your flight. Check with your doctor about whether to take an antihistamine combined with a decongestant. Your doctor may advise you to take a nasal spray containing a vasoconstrictor. Using this 30 minutes before descent may prevent ear blockages. Avoid alcoholic beverages and drink plenty of fluids while aboard the aircraft to ensure adequate hydration and keep secretions thin and easy to clear. If your ear or sinus blockage doesn't resolve after 24 hours on the ground, talk to your doctor.

• Heart attack. Most experts advise against air travel for at least three weeks after a heart attack, even if you've had no complications. Ask your doctor. If you do travel, check the expiration date of your nitroglycerin tablets and get a new supply if they're more than six months old. Bring a supply of your regular medications along with you in your carry-on luggage. Be sure they're in their prescription bottles with their appropriate label. Keep an easily accessible list of your medications with you during your trip for easy reference by a health care provider during an emergency. Also, consider carrying a copy of your most recent electrocardiogram (ECG). If you have a pacemaker, have your doctor check the battery before you go.
• Broken bones. A rigid cast can cause problems as expanded air gets trapped between the cast and your skin as your limb swells. Make sure the cast is either removable or bivalved, which allows for expansion. Keep your limb elevated as much as possible to prevent blood clot formation.
• Surgery. Gases inside your body expands 25 percent at cabin pressures of 800 feet, which causes swelling that may affect how wounds heal. Consider postponing your trip if you've had recent abdominal, central nervous system, eye or chest surgery. This is especially true if you have had a procedure that typically leads to a large amount of intestinal gas, such as a colonoscopy with polypectomy. Check with your doctor before you make this decision.
• Diabetes. In general, if you plan ahead, traveling with diabetes isn't a problem. But heightened security at airports makes it much more difficult to travel with diabetic needles. Carry your medications, medical supplies and evidence of a valid prescription with you, as well as a letter from your doctor detailing your diagnosis and medication dosages. Request special meals in advance. Wear a medical ID tag or bracelet, or carry a diabetes alert card, available from the American Diabetes Association. Trips may cross several time zones, and you may need to adjust your insulin requirements. It may be useful to carry additional short-acting insulin — typically in the form of an injector pen — in case you need to supplement your regular dosage.

If you do experience an emergency related to these or other conditions, know that plans exist to respond to such emergencies. The Federal Aviation Administration requires that major U.S. air carriers have an enhanced emergency medical kit and basic first-aid kit on board. The medical kit includes a stethoscope, a blood pressure device, syringes, needles, a CPR mask, breathing tubes, a self-inflating manual resuscitation device and a few medications. The kit may be opened only when authorized by a doctor, either on board or by telephone.

Commercial air carriers participating in scheduled passenger-carrying activities with more than one flight attendant on board must carry an automated external defibrillator (AED). This portable, computerized device treats sudden cardiac arrest or heart rhythm abnormalities, which may cause inadequate blood flow to your brain and other vital organs.

An AED can be used by someone without formal medical training because the device detects the heart rhythm and the likelihood that the arrhythmia can be corrected by electric shock. If it can be corrected, the device automatically delivers the shock. In such cases, response time is critical.

The medical training of cabin crews varies but usually includes basic first aid, cardiopulmonary resuscitation (CPR) and AED instruction. If you have certain health conditions, some airlines require either a medical certificate from your doctor verifying that you're medically stable enough for air travel or a medical chart review by the airline's medical director.

Airlines based in other countries have different regulations. Currently there are no universal guidelines for handling in-flight medical emergencies.

Each year, only a very small number of passengers who travel on airplanes require emergency assistance. To help reduce your risk, talk with your doctor before you travel if you have any significant medical problems. In addition, the Aerospace Medical Association offers these tips:

• Keep all medication with you in your carry-on luggage, and be sure it's in its original container.
• Carry an abbreviated copy of your medical records.
• Alert airlines in advance of special requirements.
• Consider buying insurance that includes provisions for air evacuation home in the event of any medical condition.

On long trips remember to stay hydrated and stretch and move around, both on the plane and in the terminal. This will keep blood from pooling in your legs, which can cause fainting or blood clots, sometimes referred to as deep vein thrombosis (thrombophlebitis).

If you have an existing illness or injury, it doesn't necessarily have to ground you. Take proper precautions, talk to your doctor, and plan ahead to help avoid potential problems.