Introduction
For humans to fly, they must adapt to a very dynamic environment. The Wright brothers were successful because they understood that stability was not possible. To stay in the air, the pilot and aircraft had to be able to adjust to the changing conditions [1]. To care for critically ill and injured patients in this setting requires a basic knowledge of both the forces affecting an aircraft and the forces that affect humans within that aircraft. There are the classic forces of aerodynamics: lift, gravity, thrust, and drag. The forces affecting humans also include vibration, barometric pressure, acceleration, spatial disorientation, and thermal stresses, among others.
Aerodynamic forces
In order to understand how the flight environment affects patients and air medical providers, the EMS physician must have a basic understanding of aerodynamic forces and terminology.
For an aircraft to fly, there must be a source of lift. For the fixed wing airplane, that source is the wing. In the helicopter, the rotor blade supplies the lift. In both cases, the wing or rotor passing through the air encounters two phenomena. Bernoulli’s principle states that when air is accelerated, it has a lower pressure. A wing with a curved upper surface and a straight lower surface causes air traveling over the upper surface to speed up to catch its counterpart moving beneath the wing. The pressure on the upper surface is reduced compared to that of the lower surface, producing lift.
Helicopters often have essentially symmetrical airfoils for their rotors; thus the speed of the air relative to the rotor is the same for the upper and the lower surface. For these rotors, and for the wings of many aerobatic aircraft, lift depends on the angle of attack. This is the angle that develops between the chord line (the imaginary line formed between the most forward point in the leading edge and the farthest aft point in the trailing edge) and the direction of the air. Children who put their hands outside the car window and feel the air move their arm up and down take advantage of this phenomenon.
The density of air determines how much lift a given wing or rotor can generate. Hot air does not have as much density as does cold air. Air pressure also decreases with altitude. The combination of the actual altitude above sea level and the effect of the temperature is expressed as the density altitude. Thus an aircraft that might be able to generate enough lift to take off in the winter at JFK airport (at sea level) might not be able to take off in Denver, the Mile High City, in August.
Gravity, or weight, is the force that opposes lift. For aircraft, the weight of the aircraft, its fuel, and its passengers or load determines the effects of gravity. An aircraft that is too heavily loaded cannot overcome the effects of gravity with the effects of lift. Aircraft are tested to determine their useful load, which is the remainder when the weight of the aircraft and its necessary supplies (e.g. fuel, oil) is subtracted from the amount of lift that can be generated.
Thrust is the ability of the engine, or the main rotor, to move the aircraft through the air. A propeller, or a jet engine, provides the thrust for airplanes. For helicopters, the main rotor provides this thrust.
The Wright brothers were among the first to understand that an aircraft propeller is essentially a rapidly spinning wing. The term airscrew has been used to describe how a propeller pulls the aircraft through the air. Just as a screw pulls itself through a piece of wood, the propeller bites into the air and pulls the airplane forward through the air. This pulling allows the wing to pass through the air and generate lift.
Drag is the force that opposes the aircraft’s movement through the air. Thin, smooth, gradually curved shapes move through a fluid more easily than boxy shapes. If one has rowed a jon boat, with its squared bow and flat bottom, then paddled a long, slender kayak, it is easy to appreciate the effects of drag. A slender, tapered business jet experiences much less drag than a biplane.
Effects on humans
The first recorded ascents of humans into the air occurred in the 1780s, with the French balloonists, Montgolfier, Charles, and de Rosier. Charles, using a hydrogen-filled balloon, was the first to note that when he ascended rapidly he became exceptionally cold and that when he descended he developed ear pain. Others ascended even higher, with Glaisher and Coxwell noting the effects that occurred when they climbed to 9,450 meters, nearly perishing in the attempt. Even before powered flight was achieved, the effects of altitude on the organism had become apparent [2].
Atmospheric effects
The atmosphere has a significant effect on humans in flight. Temperature decreases with altitude at a rate of about 2 ºC (3.5 ºF) per 1,000 feet – the adiabatic lapse rate [3]. For patients and medical crews this phenomenon can become important, as even in helicopters an ascent to 5,000 feet above ground level is not uncommon, resulting in an uncomfortable temperature change. While the Commission on Accreditation of Medical Transport Systems (CAMTS) standard 02.05.15 requires “climate control [4],” knowing the extent of the changes that may occur during a flight is an important consideration in patient packaging.
Barometric pressure at 18,000 feet/5,500 meters is half of that found at sea level, resulting in a doubling of the gas volume, in accordance with Boyle’s law [5]. The most familiar manifestation of this phenomenon is the ear discomfort that many people experience as an airliner descends for landing. Barotitis media and barosinusitis may occur because of these gas volume changes and should be taken seriously as they can incapacitate crew members during critical phases of flight [6]. Although the middle ear and the sinuses are dramatic examples, any gas-filled structure or device can be affected. A pneumothorax or an endotracheal balloon may expand or contract depending on the pressure/altitude change.
While common sense might suggest that any patient transported by air with a pneumothorax should have a thoracostomy, the research is not as clear. A case series from Somalia is illustrative: two patients, treated with needle thoracostomy, survived a trip at 3,000 meters without difficulty. A third patient, who was transported at a lower cabin altitude, also survived his trip but later succumbed to his wounds. The latter patient had extensive adhesions secondary to tuberculosis, making thoracic drainage more difficult [7]. Although they caution the reader against air transport of a pneumothorax, the writers of another case report note that the patient underwent a 2-hour airplane flight without complication, the pneumothorax being discovered incidentally after her arrival at the receiving burn center [8].
Placing a needle or a tube in a patient’s chest is not without risk in itself. In an elegant study from the University of Oklahoma, an experimental model of pneumothorax was flown in a helicopter and the volume changes measured at 1,000 and 1,500 feet above ground level (AGL), altitudes commonly encountered in helicopter EMS (HEMS) transport. The authors noted a 1.5% increase in pneumothorax size per 500 ft increment. They also suggest that the use of oxygen may mitigate some of the effects [9]. It appears that prophylactic placement of a tube, even in a known pneumothorax, may not be needed. Pneumocephalus and penetrating eye trauma also produce worries regarding pressure changes, but there is little literature surrounding the effects of flight on these [10].
Concerns have been raised about endotracheal tube cuff pressures exceeding 30 cmH2O and producing tracheal mucosal injury. A Swiss group adjusted the pressure of the cuff prior to departure, and then measured the pressures during flight, noting that almost all of their patients’ cuff pressures exceeded 30 cmH2
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