Family physicians are frequently asked to make recommendations to patients before they travel aboard commercial aircraft, and the need for such advice is also increasing. The need to understand basic aerospace physiology has been accelerated by the growing number of passengers who use commercial air travel and, in particular, the increasing number of elderly, disabled or chronically ill passengers. Although commercial air transportation is very safe compared with other forms of transportation, both environmental and health concerns must be considered when counseling patients who are about to travel by air.

The Air Carrier Access Act of 1986¹ required the U.S. Department of Transportation to develop regulations to ensure that persons with disabilities are treated without discrimination in any way, consistent with the safe carriage of all passengers. Because of Air Carrier Access rules, people with medical conditions who might not have selected airline travel in the past are now regularly attempting to use this mode of transportation. However, if legitimate medical acceptance issues arise, a medical certificate from the treating physician stating that the passenger is medically stable for air travel and will not require extraordinary medical assistance during the flight may be required. This request for a medical certificate is based on information provided by the passenger regarding a specific health condition or may be required when an arriving passenger is visibly ill. The Aerospace Medical Association (telephone: 703-739-2240) monograph “Medical Guidelines for Airline Travel”² is a useful guide for physicians providing these determinations.

In-Flight Resources

Federal Air Regulations (FARs) require all U.S.–based airlines to carry a basic emergency medical kit with specified contents (Table 1), as well as a first-aid kit for emergencies that may occur during flight. However, the contents of the kits are limited and are intended for basic emergency treatment only, not to sustain or treat critically ill passengers on extended flights. The medical kit may be opened during flight only when authorized by a physician, either on board or from the airline's medical department connected to the aircraft via air-to-ground communications. In addition, a number of airlines have installed automatic external defibrillators on board aircraft and have trained flight attendants in their use. The Aviation Medical Assistance Act of 1998³ requires the Federal Aviation Administration to study the additional medical equipment and training that should be required based on analyses of the frequency of medical incidences encountered and to issue a future Notice of Proposed Rule-making to modify the current FAR. Some airlines have begun installing enhanced medical kits containing a wide variety of acute cardiac life support drugs and equipment to aid in medical emergencies.

OXYGEN

Supplemental oxygen is available on an emergency basis during flights but often is limited to flow rates of 2 and 4 L per minute, and the supply is strictly limited. Passengers with stable medical conditions requiring low-flow oxygen cannot bring their own oxygen on board, according to FARs concerning hazardous cargo (empty oxygen containers are allowed to be transported as baggage). Most air carriers will provide oxygen, either with adjustable (2 to 8 L per minute) or nonadjustable (low flow at 2 L per minute or high flow at 4 L per minute) flow meters. There is a fee for this service, either charged per unit of oxygen used or per ticket coupon (one coupon per boarding), and a minimum notice of 24 to 48 hours or longer is required, along with a medical certificate from the passenger's physician certifying that the person is medically cleared to fly at a relative cabin altitude of 8,000 feet and specifying the flow rate, whether intermittent or continuous, and type of delivery mask (face mask or nasal cannula) to be used.

Oxygen is supplied as either large-cylinder (3,228 L) or small-cylinder (300 L) compressed gas.⁴ Passengers must arrange for oxygen to be available during airport layovers by contacting a local supplier in the layover city or through their home oxygen service, with advance notice of at least 24 hours to ensure delivery. Other types of medical respiratory equipment, such as nebulizers or pediatric mechanical ventilators, may sometimes be allowed to be used on board, but their usage must be pre-approved to prevent interference with sensitive aviation electronic equipment and must conform to applicable FARs specifications.

Environmental Factors

Although flying is generally a safe and comfortable method of transportation, several environmental and physiologic stresses may be encountered in modern commercial aircraft. These include but are not limited to preflight activities, lowered barometric pressure and partial pressure of oxygen, sustained periods of noise and vibration exposure, turbulence, variable air circulation, environmental temperature changes, low humidity, disruption of circadian rhythms, sustained periods of postural immobility and varying exposure to low-level radiation.⁵ In addition, smoking during flight is still allowed by many international carriers, although smoking is banned on all domestic flights in the United States. Factors such as length of flight and class of ticket purchased must be considered in assessing the impact of these environmental and physiologic stresses.

OXYGEN REQUIREMENTS

Commercial jet aircraft maintain a relative cabin altitude between 5,000 and 8,000 feet during routine flight, with the FARs specifying that an 8,000-foot environment be maintained even at the highest operating altitude. At this relative altitude, the barometric pressure (PB) decreases from a normal sea level value of 760 mm Hg to around 560 mm Hg, causing the normal baseline arterial partial pressure of oxygen (PaO₂) of 98 mm Hg at sea level to decrease to around 60 to 70 mm Hg in normal individuals.⁶ This corresponds to approximately a 90 percent oxygen saturation (SaO₂) on the oxyhemoglobin dissociation curve, a point below which there is a steep gradient of the pressure/saturation relationship. Using 50 to 55 mm Hg as a minimum acceptable PaO₂ level for a healthy person, passengers with cardiovascular, circulatory or pulmonary compromise associated with a reduced PaO₂ before flight could easily experience symptoms related to hypoxemia at normal cabin altitudes. An arterial blood gas determination is therefore recommended before travel for passengers with any symptomatic lung disorders.

Although compensatory hyperventilation enables many compromised passengers to increase their oxygenation, it has been recommended that a minimum preflight sea level PaO₂ measurement of 68 to 70 mm Hg be used to ensure adequate oxygenation without supplementation.8 Additional preflight evaluation for these patients may include pulmonary function testing, high-altitude simulation testing (measuring PaO₂ while breathing mixed gases of varying oxygen content, thus simulating an aircraft environment at altitude) or simply observing patients' ability to walk and climb stairs. In addition to lowered oxygen content, the air extracted from the outside environment at the high altitudes of commercial aviation is extremely dry, causing the inside cabin humidity to remain low, ranging anywhere from 10 to 20 percent. This low humidity affects certain medical conditions, most notably those involving the respiratory passages and skin.

Cardiovascular Disease

Physicians are often asked to advise patients who want to travel and have a recent history of myocardial infarction or other cardiovascular disorders. A recent study⁹ of 196 passengers who traveled aboard commercial jet aircraft following myocardial infarction suggests that most complications occurred in those traveling within the first two weeks after the acute event, and there was no substantiated reason that air travel be delayed beyond four weeks. Although recommendations vary, many experts agree that patients should not fly within three weeks of uncomplicated myocardial infarction. To help determine a passenger's potential for risk, a treadmill exercise stress test is useful to document absence of ischemic symptoms and to enable the prognosis and functional capacity to be estimated. Travel for patients with myocardial infarction complicated by arrhythmia or left ventricular dysfunction should be delayed even longer, until documentation of adequate control of medical complications is obtained.

Although the safety of air travel following percutaneous transluminal coronary angioplasty is not documented, it seems reasonable that these patients are at low risk unless complications relating to the procedure occur. Similarly, patients who have recently undergone coronary artery bypass grafting are not considered at high risk presuming a stable postoperative course. However, because the risk of barotrauma is inherent with any chest surgery and subsequent exposure to decreased atmospheric pressure, it is recommended that air travel be delayed for at least two weeks following this surgery. Other cardiac conditions, although not specifically a contraindication to air travel, require individual assessment of medical stability, symptoms and likelihood of completing the trip without complications. Table 2 lists some cardiovascular contraindications to commercial airline flight as recommended by the Aerospace Medical Association. General travel advice for patients with cardiac disease is listed in Table 3.

Pregnancy

Pregnant women are generally advised not to travel by air when approaching their expected delivery date, although the specific passenger acceptance guidelines vary among airlines. Many airlines require a medical certificate from a physician if the stated delivery date is within four weeks, and even brief domestic flights are not recommended within seven days of an expected delivery. These precautions are necessary to prevent a delivery during flight rather than to guard against harm to the fetus, since the fetal circulation and fetal hemoglobin protect the fetus against desaturation during routine air flight. Because of the increased affinity for oxygen of the fetal hemoglobin and the shape of the fetal oxygen hemoglobin dissociation curve, the fetus is able to maintain a greater degree of hemoglobin oxygen saturation than an adult at similar partial pressures of oxygen.¹⁰

Since the incidence of superficial and deep thrombophlebitis associated with increased levels of clotting factors and venous dilatation is increased, pregnant air travelers should request an aisle seat and walk about the cabin when possible. These passengers should stretch and perform isometric leg exercises, especially during long flights involving prolonged cramped seating arrangements. Unexpected air turbulence could place the fetus at risk, and pregnant travelers should wear seat belts when seated, preferably placed low around the pelvis to decrease the potential for fetal injury. Other factors, including dehydration associated with the decreased cabin humidity, motion sickness, intestinal gas expansion, and availability of care and insurance coverage at the destination should medical attention be required, should be discussed with pregnant patients before departure.¹¹ Although flying in commercial aircraft is generally not risky in uncomplicated pregnancies, pregnant patients with a history of significant anemia, prematurity, cervical incompetence, bleeding or other increased risks should be advised not to fly.

Deep Venous Thrombosis

Air travel is a risk factor for the development of deep venous thrombosis (DVT), a phenomenon known as “economy class syndrome.” Although immobility and edema of the lower extremity, orthostatic stress and compression of the popliteal vein at the edge of the seat are considered factors leading to the development of DVT, symptoms and complications such as pulmonary embolus often do not appear until several hours or even days after the journey and therefore are often not attributed to the recent air travel. Hemoconcentration associated with diminished fluid intake and insensitive water loss in the dry cabin atmosphere are contributing factors.

In addition, patients who are obese, smokers or using oral contraceptives and those who have other risk factors should be counseled about their increased risk before travel. Preventive measures include requesting bulkhead seating if available, wearing support stockings, taking periodic walks during flight, doing isometric calf exercises and maintaining adequate hydration.^12,13

Diabetes

Traveling long distances by air can be especially challenging for diabetic patients, especially those who travel across several time zones. Adjustments to mealtimes, glucose self-monitoring and timing of medications must be considered and are especially important in patients with type 1 (formerly known as insulin-dependent) diabetes. Diabetic passengers should be cautioned to carry all medications with them during the flight, as well as supplies of needles, syringes, blood glucose monitoring equipment, a glucagon emergency kit and sugared snacks. Special diabetic meals should be requested well in advance of the travel date. It is also wise to travel with a physician's letter specifying the diagnosis, usual medication dosages and the necessity of traveling with syringes, as well as a “Diabetes Alert Card” (available from the American Diabetes Association).

Insulin should be kept refrigerated (but never frozen) until it is to be used. Insulin may be kept unrefrigerated at room temperature if it will be used within a month.¹⁴ Insulin should not be exposed to temperature extremes such as might be encountered in the baggage compartment. Table 4 lists recommended supplies and advice for diabetic travelers.

Frequent self-monitoring of blood glucose during transmeridian travel may be more practical than complex calculated adjustments in insulin dosage. Other methods include slowly readjusting a patient's daily and nightly patterns to coincide with the time zone of the destination before departure or improvising a day-by-day schedule to allow a slow transition from the day of departure and continuing after the destination has been reached, with attention to mealtimes and physical exertion. Algorithms are available to guide insulin adjustments for eastward or westward travel.² Eastward travel means a relatively short day, and less insulin may be required, whereas westward travel means longer days and the potential need for more insulin. Stress associated with travel adds an additional variable to any “formula” considered, and patients with diabetes should be advised to monitor blood glucose as well as urinary ketone levels frequently.

Ear Conditions

Because of the rapid cabin pressure changes normally encountered even in commercial flight operations, any medical condition affecting the patency of the eustachian tube or sinus ostia could lead to complications during flight. Negative pressure in the middle ear created by blockage creates a partial vacuum, leading to pain and possibly tinnitus, vertigo, hearing loss or even rupture of the tympanic membrane. Failure to equilibrate pressures in the middle ear (barotitis media) or paranasal sinuses (barosinusitis)¹⁵ typically occurs on descent and may be caused by a variety of conditions, including middle ear infections, effusions, acute or chronic sinusitis, or allergies or infections creating nasal congestion.

Recent surgical procedures involving structures of the inner or middle ear may be affected by pressure changes and are a contraindication to flight. Recent tympanostomy tube insertion, however, usually prevents symptoms of barotitis media. Patients should be instructed in the use of topical and systemic decongestants and be able to demonstrate an effective Valsalva maneuver (closing the nose with thumb and index finger and exhaling gently with the mouth closed) by observing the movement of the tympanic membrane through an otoscope. Chewing gum and frequent swallowing may be helpful. Infants should be given a bottle or pacifier to suck on to prevent discomfort, particularly during ascent and descent.

Barodontalgia (toothache provoked by exposure to changing barometric pressure) is difficult to predict but does occasionally occur in patients with dental pulp disease. Motion sickness, which involves the vestibular sensory system, can be treated prophylactically in susceptible travelers with either oral diphenhydramine (Benadryl) or transdermal scopolamine (Transderm Scop).

Other Medical Conditions

Physicians may be asked to certify that passengers who have recently undergone a surgical procedure are stable enough to undertake air travel. Increasingly, patients are traveling specifically to have an outpatient procedure performed with the intent of traveling home as soon as possible. It should be remembered that although airlines are obligated by the Air Carrier Access Act¹ to allow boarding of passengers unless there is a chance that extraordinary medical assistance in-flight may be required, travel should be delayed as long as possible to decrease the chance of complications.

Passengers with recent abdominal, central nervous system, ophthalmologic or thoracic surgery are susceptible to problems associated with expansion of trapped gas at reduced cabin pressures. Intestinal gas may expand by 25 percent by volume at a cabin altitude of 8,000 feet,² which may cause tearing of suture lines, hemorrhaging or bowel perforation in passengers with recent abdominal surgery complicated by ileus or those with small or large bowel obstruction. Although specific recommendations vary, a waiting period of at least seven to 10 days after any laparoscopic or surgical procedure involving introduction of a gas into the body is reasonable, provided other patient stability factors are met. In particular, passengers with pneumothorax do not tolerate air travel well because of expansion of trapped gases. Immunosuppressed transplant patients are at increased risk of infection, although the concentration of microorganisms in airline cabin air is lower than in ordinary city locations.¹⁶

Patients with anemia (whether the acute type associated with recent surgery or trauma, or the chronic type associated with an intrinsic medical condition) should be further assessed or be advised to delay travel if possible. Supplemental oxygen is generally advised for anyone with a hemoglobin level below 8.5 g per dL (85 g per L), unless the anemia is known to be well compensated, and is particularly advisable for patients with sickle cell anemia. A passenger who has recently participated in scuba diving is at risk for developing decompression sickness, so divers should be advised to wait at least 12 hours before flying if making only one dive per day. For multiple dives or those requiring decompression stops, additional time up to 24 hours is recommended before flying.² Jet lag is a physiologic reaction to traveling long distances in relatively short periods of time, and several suggestions may be helpful in preventing symptoms (Table 5).

Any medical devices equipped with pneumatic components, such as urinary catheters or feeding tubes, pneumatic splints or other closed infusion devices, are affected by gas expansion. To prevent introduction of air into a hollow viscus, all feeding and infusion tubes must be capped off during the ascent and descent phases of flight. Generally, the length of a flight is not a concern for otherwise healthy passengers with a recent fracture and a properly applied cast or splint. However, recently applied casts made of plaster or fiberglass must be “bi-valved” to prevent circulatory problems and should be adequately designed to facilitate swelling in the affected extremity. Pneumatic splints are not allowed by many airlines but, if used, some air should be released before departure to permit anticipated gas expansion.

Guidelines for travelers with specific medical conditions are summarized in Table 6. Advising patients who travel by air of risks and necessary precautions before departure may ultimately prevent emergency flight diversions or medical complications, as well as make trips more comfortable and pleasant.