Air travel can cause or exacerbate certain medical problems. There are few medical conditions that strictly prohibit air travel; however, some patients must make a good plan and follow precautions. Medical assistance may be requested in the event of in-flight illness; general assistance and limited medical supplies are available on domestic commercial flights in the United States. Physiological effects Changes in atmospheric pressure Some small aircraft flying at altitudes <3050m are not pressurized. Modern jet aircraft have equal cabin pressure and atmospheric pressure at 1525~2440m altitude, regardless of altitude. In this pressure situation, the free gas in the human body cavity can expand about 25%; this expansion may aggravate certain medical problems. Upper respiratory tract inflammation or allergic reactions can cause obstruction of the eustachian tube and sinus openings leading to aviation otitis media or aviation sinusitis. Odontogenic facial pain can occur with changes in air pressure. Frequent yawning or swallowing with the nose closed during descent, nasal decongestants, or the use of antihistamines before and during the flight can often prevent or relieve these conditions. Children are particularly susceptible to aviation otitis media and should be given fluids and food during descent to encourage swallowing. Occasional sudden and unexpected cabin decompression can also cause other problems. Air travel is contraindicated in people who have or are likely to have a pneumothorax (e.g., people with large alveoli or cavities), people with retained air or gas (e.g., clamped closed bowel, chest or abdominal surgery within 10 days or intraocular gas injection), because even mild gas expansion can cause pain or irritation of tissue. Patients who have had a colostomy should have a large fecal bag with them and should expect increased bowel movements. Oxygen pressure drop The cabin pressure is equivalent to the pressure at an altitude of 2640m, so that the partial pressure of arterial oxygen reaches about 70mmHg, healthy passengers tolerate the partial pressure of oxygen well. Anyone who can generally walk 46m or climb a flight of stairs, as well as people with stable medical conditions, can tolerate normal cabin conditions without the need for oxygen. However, problems may occur in patients with moderate or severe lung disease (e.g., asthma, emphysema, cystic fibrosis), heart failure, anemia with hemoglobin below 8.5 g/dl, severe angina, sickle cell disease (but not hereditary) and certain congenital heart defects. These patients can usually fly safely if they have access to special continuous oxygen equipment provided by the flight; 72 hours advance notice is required. Those who are stable after recovery from myocardial infarction, often within 8 to 10 days, may also be able to fly. Mild ankle edema can often occur after a long flight due to venous stasis and should not be confused with heart failure. Smoking can aggravate mild hypoxia, do not smoke before flying. Hypoxia and fatigue can exacerbate the effects of alcohol. Turbulence Turbulence can cause airsickness or trauma. Once seated, passengers should always fasten their seat belts. Physiological rhythm disorders (jet lag) High-speed air travel across multiple time zones can disrupt normal physiological rhythms. Because bright sunlight can excise the body's biological clock, bright evening light can delay the start of normal sleep, while early morning sunlight can advance the biological clock (sleep earlier than the usual sleep time). Melatonin is a hormone secreted by the pineal gland that provides a hint of the time of night; if a traveler traveling east across several time zones takes 0.5 to 5 mg of melatonin the night he arrives at his destination, sleep time is often earlier. The effect of melatonin depends on the timing of its administration to the destination. To compensate for physiological rhythm disturbances, certain therapeutic measures need to be altered; for example, the dose and timing of insulin administration should be adjusted according to the number of time zones crossed, the time spent at the destination, the food available, and the activity; glucose must be monitored frequently. Adjustment of treatment should be based on the time of fading, not on local time. Psychological tension states Fear of flying and claustrophobia are psychological in nature and are not influenced by reason. Hypnosis and behavioral changes may reduce the fear of flying in some individuals. Pre-flight and in-flight administration of weak sedatives can be helpful for timid travelers. Hyperventilation can often be mistaken for a heart attack and can cause tetanus-like symptoms or impaired consciousness. Psychotic tendencies may be exacerbated during the flight and add to the trouble. Patients with violent and accidental tendencies must be accompanied by a caregiver and given proper sedation. Precautions Prolonged sitting still, especially in pregnant women and those with venous disease, can lead to deep vein embolism and also to pulmonary embolism. Walk around the cabin every 1-2 hours or do some simple gymnastic activities on the spot when you need to sit. Drinking enough water and avoiding alcohol can prevent dehydration because of the low humidity in the cabin. People who wear corneal contact lenses should always inject artificial tears to prevent corneal irritation caused by low humidity in the cabin. Infectious diseases can endanger other passengers in a crowded cabin. International immunization regulations change frequently; information can be obtained from local or state health departments. Travelers should carry enough medical supplies with them to continue treatment in case of lost luggage, delayed flight arrival, theft from hotels or unavailability locally. When it is necessary to carry narcotics or rare and large quantities of medications, a letter from a doctor should be available to avoid problems during security and customs checks. If the patient becomes ill after leaving home, a summary of the patient's medical records (including an electrocardiogram) can be very helpful. Patients with seizures that may be disabling (e.g., epilepsy) or patients with chronic illnesses should wear a medical identification tag around the neck or wrist. For recent dental exams, it is wise to have spare glasses and hearing aid batteries. Wire fixation of maxillofacial trauma during flight is contraindicated unless a special quick release device is available, as airsickness can lead to accidental aspiration of vomit. New pacemakers have safety devices from all kinds of disturbances. The metal content of the pacemaker and the prosthetic brace can trigger the safety alarm; a doctor's certificate should be brought to avoid troubles during the safety check. Pregnant women within 36 weeks are not a contraindication to air travel; high-risk pregnant women must be individually examined and evaluated. Air travel for pregnant women in their ninth month of pregnancy usually requires a written permit from a physician no later than 72 hours prior to departure, indicating the expected date of delivery. The seat belt for pregnant women should be fastened under the abdomen across the femur. It is particularly prone to thrombophlebitis. Flying is contraindicated for children under 7 days of age. Children with chronic medical conditions (e.g. congenital heart disease, chronic lung disease, anemia) are subject to the same precautions as adults. If notified in advance, flights will make every effort to accommodate patients with disabilities, including wheelchairs and stretchers. If commercial aircraft cannot provide such accommodations to patients, air ambulance suits must be available. Some airlines may accept patients requiring special equipment (e.g., intravenous rehydration devices and ventilators), provide appropriate escorts, and make advance arrangements. Special foods, including low sodium, low fat and diabetic diets, are available upon prior request. Further information on air travel can be obtained from the medical departments of the major airlines or from the Federal Aviation Administration's regional aviation corpsmen. Special arrangements (e.g., oxygen, wheelchairs) can be accommodated by prior reservation, but should be communicated at least 72 hours in advance.