Stresses of Flight and Transport

Two types of stresses are associated with the transport environment and air medical transport: the stresses of flight and self-imposed stresses. These stresses are cumulative and may lead to significant emotional and physiologic compromise.

Several authors and organizations have identified various stresses of flight: barometric pressure, hypoxia, noise, vibration, thermal changes, decreased humidity, dehydration, gravitational forces, fluid leakage out of intravascular spaces (third spacing), fatigue, spatial disorientation, flicker vertigo, and exposure to fuel vapors and exhaust. Patients, transport team members, and pilots may all be affected by the stresses of flight. Vibration, noise, and turbulence are generally more severe in helicopters than in other forms of transportation. The stresses that may have the greatest effect on ground transport are noise, vibration, temperature, gravitational forces, and fatigue.

Any significant altitude change exposes the patient, pilots, and transport team members to additional physiologic stresses. There are 3 major factors that influence the incidence, onset, and severity of complications that can be experienced during air transport: rate of ascent (or descent), the altitude achieved, and the length of stay at that altitude. Varying severity of compli­cations occurs when any of these factors or a combination of them exceeds a person's ability to adapt to the new environment.

Infants and young children have many anatomical and physiologi­cal differences which make their responses to illness and stresses different from adults and are at greater risk for the development of many altitude- related illnesses.

Barometric Pressure

The effects of changing altitude during air medical transport may be related directly to physical gas laws. The effect of barometric pressure changes can affect the transport team, patient, and equipment in many ways.

There are 3 mechanisms by which barometric pressure affects the body. The first follows Boyle's law, dealing with gas within an enclosed space and changes in ambient pressure. If air is unable to escape, positive pressure develops that may result in a rupture or the compression of adjacent struc­tures. The second mechanism follows Henry's law, when gas dissolved in blood is released. The third mechanism applies to barometric changes in an underwater environment (ie, scuba diving) and addresses abnormal tissue concentrations of various gases.

In the setting of eustachian tube dysfunction, disturbances of the middle ear (ie, barotitis media) may result from barometric pressure changes. As altitude increases, gas expands in the middle ear behind the tympanic membrane. As altitude decreases, the gas within the middle ear contracts, pulling the tympanic membrane inward. Gas usually will pass through the eustachian tube (actively or passively), allowing for equalization of pressures; air escapes as expansion occurs on ascent, or air enters the middle ear on descent. However, if a person has allergies, an upper respiratory infection, or sinus problems or is a small infant, the eustachian tube may be obstructed and equalization may be restricted. Children with adenoidal hypertrophy and recurrent otitis media are at a greater risk of failure to equilibrate pres­sures in the middle ear.

Encouraging a small infant to suck a pacifier or older children to swal­low, drink liquids, and use the Valsalva maneuver during ascent and descent helps to maintain patency of the small eustachian tubes and to prevent pain. Nasal decongestants may also be helpful to prevent symptoms when used 1 to 2 hours before takeoff and 30 minutes before descent. If the patient is paralyzed, equalization of pressure requires active assistance during descent.

Normally, air can pass easily in and out through the air-filled sinus cavities. If a person has an upper respiratory or sinus infection, swelling of the mucous membrane lining may result. This trapped air expands as alti­tude increases, causing barosinusitis. Symptoms include severe sinus pain and epistaxis.

Special attention should be given to patients with suspected or docu­mented pneumothorax. It is optimal that pneumothorax be diagnosed and treated before transport, because pneumothorax is prone to further expan­sion at higher altitudes.

The stomach and the intestines normally contain a variable amount of gas (up to 1000 mL in an adult) at a pressure approximately equivalent to the surrounding atmospheric pressure. The stomach and large intestine contain considerably more gas than does the small intestine. On ascent, symptoms of bloating may develop. At 18 000 ft, the volume of gas in an enclosed expand­able space will double, but symptoms usually do not become severe until an altitude of 25 000 ft, when the volume of gas triples. Crying children and infants who are feeding tend to swallow a substantial amount of air. In addi­tion, eating large meals, ingesting a large amount of a carbonated beverage, chewing gum (and swallowing air), and preexisting gastrointestinal prob­lems may also increase the volume of gas in the intestines. As gas expansion occurs, a person may experience discomfort, nausea, vomiting, shortness of breath, and hyperventilation.

Changes in atmospheric pressure may affect any medical equipment with air enclosed in a given space. Endotracheal tube balloons should be evaluated to prevent rupture or excessive pressure on the tracheal wall dur­ing ascent and for an inadequate air seal on descent. Replacing the air in the endotracheal tube cuff with water eliminates this potential complication during air medical transport. The air in intravenous containers expands on ascent, resulting in an increased flow of the intravenous fluid. On descent, the flow of the intravenous fluid slows when the air volume is decreased. Pneumatic splints and pneumatic anti-shock garments (also known as MAST) also may be affected by pressure changes, resulting in hypotension on descent or distal circulation compromise during ascent. Ventilators not tested for use in the flight environment can also malfunction because of pressure changes.

Henry's law predicts that gases will move from an area of higher con­centration to that of lower concentration. Clinically, a drop in barometric pressure may result in the release of gas dissolved in blood. When a scuba diver ascends too quickly, nitrogen gas bubbles can form in the blood. Special precautions should be taken for decompression victims who must be transported by helicopter. In some cases, even a minimal altitude increase can cause significant gas bubble formation. It is advised that patients with a decompression illness be transported at an altitude of not greater than 1000 ft above the diver's ascent site in nonpressurized aircraft.

Hypoxia

During air medical transport, the most threatening aspect of hypoxia is its insidious onset. The transport team may be involved in patient care activities and may not notice the early onset of signs or symptoms in the patient or in themselves.

Serious effects of altitude hypoxia do not usually develop until atmo­spheric pressure drops to between 10 000 to 12 000 ft. No one is exempt from the effects of hypoxia, although the onset and severity of symptoms may vary among individuals.

The results of available research suggest that no significant risk is asso­ciated with air medical transport of a pregnant woman and her fetus. The arterial partial pressure of oxygen in the fetus is significantly lower than that of the mother. A healthy fetus at sea level has arterial oxygenation (PaO₂) of 32 mm Hg in the umbilical arterial circulation, whereas the PaO₂ of the mother will be approximately 100 mm Hg. At an altitude of 8000 ft, the PaO₂ of the mother will drop to 64 mm Hg, corresponding to an oxygen saturation of approximately 90%; the fetal PaO₂ will drop only from 32 to 25.6 mm Hg. In addition to the lower PaO2 in the fetus, the oxygen dissociation curve for fetal hemoglobin differs from that for mature hemoglobin. Consequently, fetal hemoglobin is more fully saturated at a lower PaO₂ than is the hemo­globin of the mother.

Neonates, especially preterm neonates, are more likely than adults to develop hypoxia as the partial pressure of alveolar oxygen falls during ascent. Although the usual alveolar-arterial oxygen difference in adults is approxi­mately 10 mm Hg, the difference in neonates is much larger (approximately 25 mm Hg). Therefore, a modest drop in partial pressure of alveolar oxygen will result in hypoxia in neonates.

Many factors may influence an individual’s susceptibility to hypoxia. Children and other patients with low tidal volumes and increased oxygen consumption are less able to respond to the hypoxic insult and, therefore, are more prone to the development of related complications. Many pediatric medical illnesses are exacerbated at altitude, including pneumonia, acute asthma, pneumothorax, shock, and blood loss. Numerous social factors also have an important role in susceptibility.

Although altitude-related hypoxia in patients is a concern, the routine use of pulse oximetry and supplemental oxygen minimizes this hazard. In the setting of hypoxemia, increasing Fio₂ levels and, in some circumstances, the addition of positive end-expiratory pressure (PEEP) easily compensates for the hypoxic effects of altitude. However, in rare patients already receiving maximal oxygen support, flight at lower altitudes may allow the artificial cabin pressure to approach that of sea level, resulting in an increase in the partial pressure of oxygen and maintaining an acceptable PO₂.

Hypoxia is also a concern for pilots and transport team members, who generally are not monitored. During air medical transport at high altitudes, it may be advantageous to check oxygen saturation values of the pilots and transport team members. In addition, the Federal Aviation Administration (FAA) has specific regulations addressing the use of oxygen. Federal Aviation Regulations (FARs) require pilots to use supplemental oxygen if they are flying at cabin altitudes above 10 000 ft for more than 30 minutes and at all times when above 12 000 ft. At cabin pressure altitudes above 15 000 ft, each occupant of the aircraft must use supplemental oxygen.

Noise

Noise and vibration may represent the most difficult and troublesome stresses encountered in the air and ground transport environments. Excessive noise may interfere directly with patient care.

During transport, it may be impossible to accurately auscultate the lungs or blood pressure. As a result, the transport team must rely on other means to monitor and assess patient condition. Close observation for alteration in the patient's respiratory rate, chest expansion, level of consciousness, discom­fort, and abdominal distention may detect a possible change in the patient's condition. Blood pressure can be monitored by using invasive or noninvasive devices. Pulse oximetry provides valuable information about the patient's oxygenation and respiratory status, and carbon dioxide detectors or monitors are helpful when assessing tracheal tube position and patency.

As with many of the stresses of flight, there is individual variation in tolerance and effect of noise. The longer the exposure and the more intense the noise, the greater the potential damage.

Prolonged and intense exposure to noise may generate discomfort, headaches, fatigue, nausea, visual disturbances, vertigo, temporary or perma­nent ear damage, and deterioration in performance of tasks. During aircraft operation, hearing protection (ie, ear plugs, headsets, or helmets) should be worn by the flight crew, transport team, and patient.

Vibration

Vibration is inherent to all transport vehicles and may interfere with patient assessment and some routine physiologic functions. In general, helicopters produce more stress from vibration and noise than fixed-wing aircraft. The most common sources of vibration during air medical transport are the air­craft engines and air turbulence. During helicopter transport, vibration is most severe during transition to a hover or during turbulent weather condi­tions. In fixed-wing transport, vibration increases during high-speed, low- level flight and during cloud penetration in turbulent weather. In ground ambulances, poor road conditions, tight vehicle suspensions, narrow wheel­bases, and high centers of gravity predispose to rough and unstable rides that may be detrimental or excessively painful to patients with spinal cord injury, intracerebral hemorrhage, and orthopedic injuries.

Exposure to moderate vibration results in a slight increase in metabolic rate and can cause fatigue, shortness of breath, motion sickness, chest pain, and abdominal pain. Vibration from the aircraft also may interfere with normal body thermoregulation and with the operation of some invasive and noninvasive electronic patient monitoring equipment.

Little can be done by pilots or flight crew members to eliminate or decrease the amount of vibration in the aircraft. This also is true of the ambulance drivers and ground transport personnel in ground vehicles. To minimize the effects of vibration, efforts should be made to avoid or reduce direct contact with the vehicle's frame. Padding should be placed on any part of the frame that may come in contact with people on board. Adequate padding in the form of cushioned seats and stretcher pads should be used. Direct contact with the bulkhead of the vehicle should be avoided by plac­ing blankets or other cushions appropriately. Patients and transport team members should be properly restrained at all times to minimize the effects of vibration. In ground transport vehicles, careful attention also should be given to correct loads, tire pressures, appropriate shock absorbers, and over­all vehicle maintenance.

Thermal Considerations

During helicopter, fixed-wing, and ground transports, the patient and trans­port team may be exposed to a significant temperature variation that may result in clinical and operational complications. These temperature changes may be attributable to inherent seasonal changes, geographic factors, and altitude variation.

Exposure to extremes in temperature can result in increased metabolic rate, oxygen demand and consumption. This may further compromise an already hypoxic patient. Prolonged exposure also can result in motion sick­ness, headache, disorientation, fatigue, discomfort, irritability, impaired per­formance, and reduced ability to cope with other stresses, such as hypoxia.

Many factors can exacerbate or mitigate exposure to temperature varia­tion, such as air circulation, duration of exposure, condition and type of clothing, and physical condition. Whenever possible, the transport team should take steps to prevent potential complications related to thermal stress. The cabin should be kept at a comfortable temperature, minimiz­ing exposure to ambient environmental extremes. To prevent hypothermia, appropriate layers of clothing or blankets should be used to limit heat loss. In addition, wet clothing or moist dressings should be removed. Prolonged exposure to high temperatures may require increased oral or intravenous fluids to prevent dehydration. The use of increased ventilation, cool water mist, or moist dressings may be of benefit.

Regardless of which medical transport vehicle is used, it is recommended that the transport team members be “dressed for the weather.” In the sum­mer, it may be appropriate for transport team members to undertake a transport wearing scrubs and a short, nonflowing hospital lab coat, as long as these articles can be safely worn in that environment. As the temperatures get colder, however, this would not be adequate, because team members should be prepared for prolonged, unexpected exposure to the elements in case of an accident, vehicle breakdown, remote location, or change in envi­ronmental controls. In the winter, appropriate attire includes a winter coat, gloves, hat, appropriate footwear, and layered clothing.

Humidity and Dehydration

As altitude increases and the air cools, the amount of moisture in the air drops significantly. Therefore, a pressurized aircraft that draws its fresh air from the outside dry atmosphere results in a pressurized cabin with an extremely low humidity level, and dehydration becomes another con­cern. In addition, dry medical oxygen will further predispose the patient to dehydration.

The decrease in humidity is particularly important as it relates to patient airway secretions. Dried airway secretions can lead to airway obstruction, atelectasis, and hypoxemia. Providing humidified medical oxygen helps to prevent airway obstruction attributable to dried secretions.

To prevent dehydration, fluid intake (oral or intravenous) should be monitored carefully, and all patients should receive humidified medi­cal oxygen. These recommendations are especially important during long transports.

Gravitational Forces

During routine flight operations, gravitational forces (g-forces) will not significantly affect the patient, pilots, or transport team. However, an under­standing of the relevance of gravitational forces to positioning of the trans­port team and patient within the aircraft and to their safety and survival is needed. One “g” represents the force that a person exerts when seated and is a result of gravitational force imposed on the body. Gravitational forces are applied to the body during ascent and descent and during a change in speed or direction.

During any sudden or excessive change in direction or speed, a per­son or object is subjected to the effects of gravitational forces. During deceleration of an air or ground vehicle, an unrestrained or improperly restrained person in a forward-facing seat may be injured or ejected from the seat. In contrast, a rear-facing seat may provide better restraint during crash deceleration.

In theory, patient positioning within the aircraft may enhance or mini­mize the effect of gravitational forces during takeoff (acceleration) and landing (deceleration). For patients with cardiac disease, myocardial perfu­sion is improved during acceleration by positioning the patient with the head toward the back (aft) of the aircraft. As negative gravitational forces increase, pooling of blood occurs in the upper part of the body. In head- injured patients or patients with fluid overload, augmentation of positive gravitational forces, which would pool blood in the lower extremities, may be desirable. This is accomplished by positioning the patient with the head toward the front (fore) of the aircraft. In a head-injured patient, positioning the head toward the front of the aircraft may reduce the risk of a transient increase in intracranial pressure during takeoff.

Fluid Dynamics

Long-distance or high-altitude air medical transport may precipitate third spacing of fluid. A decrease in barometric pressure may cause this leakage that also may be aggravated by temperature extremes, vibration, and gravi­tational forces. Signs and symptoms include edema, dehydration, increased heart rate, and decreased blood pressure.

Other stresses of flight or preexisting medical problems, such as preex­isting capillary leak, cardiac conditions, and nephrotic disease, may aggravate the onset and complications of third spacing.

Fatigue

Although fatigue is considered a stress of transport, it also may be considered an end-product of the other contributing factors that make up the stresses of flight and the self-imposed stresses. Hypoxia, gravitational forces, baromet­ric changes, and dehydration all contribute to fatigue that may compromise both the crew and patient. By understanding the elements that cause and contribute to fatigue, the team member may be able to mitigate the effect of this stressor.

Fatigue is a state or condition that follows a period of excessive mental or physical activity or inactivity. The emotional and physical stress of pro­longed patient care in the transport environment may result in fatigue and it is important to minimize the factors that can contribute to fatigue, especially the self-imposed stresses, which should be within their direct control.