- Email: [email protected]
Up, Up, and Away: Aeromedical Transport Physiology
Abstract: Initially developed by the military through wartime experiences, aeromedical transport has become the cornerstone of many pediatric and neonatal transport programs. Expedited transport of critically ill pediatric patients via rotor- or fixed-wing aircraft may improve outcomes when clinical conditions warrant the rapid delivery of patients to qualified medical centers. However, air transport provides many unique problems for practitioners. Confined space, vibration, noise, and physiologic derangements from high altitude and low cabin pressure are all unique variables encountered during air transport. A sound understanding of these effects with their potential consequences is of paramount importance. Anticipating complications from these physiologic changes is necessary for management and appropriate equipment utilization during transfer. This review explores the history of air transport, associated physiologic changes, and specific equipment needs for transporting this unique patient population.
Keywords: pediatric transport; neonatal transport; atmosphere; pressure; barometric *Department of Pediatrics, Division of Critical Care Medicine, Yale University School of Medicine, New Haven, CT; †Department of Pediatrics, Division of Critical Care Medicine, Yale University School of Medicine, New Haven, CT. Reprint requests and correspondence: John S. Giuliano, Jr, MD, Division of Pediatric Critical Care Medicine, Yale University School of Medicine, 333 Cedar Street, LCI 305, New Haven, CT 06520-8064. [email protected] (L.A. Polikoff), [email protected] (J.S. Giuliano)) 1522-8401/$ - see front matter © 2013 Elsevier Inc. All rights reserved.
Up, Up, and Away: Aeromedical Transport Physiology Lee A. Polikoff, MD*, John S. Giuliano Jr., MD†
A
s early as 1784, the French physician Jean Francois Picot was using hot air balloons to treat patients. He is credited with thinking that the purer air at altitude was more beneficial to his patients and used balloons as a means to transport them. 1 French physicians further expanded transporting critically ill patients by air in the late 19th century during the Franco-Prussian War of 1870 to 1871. Historians note, “during the German siege of Paris, observation balloons flew out of the city with many bags of mail, a few high-ranking officials, and 160 casualties.” By 1912, the French Army was training to use aircraft to transport causalities to field hospitals. The following year, a French military officer reported, “we shall revolutionize war surgery if the aeroplane can be adopted as a means of transport for the wounded.” By 1915, French flight crews had begun to actively transport patients. This service continued to expand throughout the First World War. 2 Nationally, the period between World Wars I and II saw increased research and experimentation with both rotor- and fixed-wing aircraft. However, it was not until the Second World War that evacuation of critically ill patients became more widespread among the United States military. By 1943, the United States Army had begun developing helicopters specifically for the purpose of medical transport. The following year, Lt Carter Harman first used a new Sikorsky helicopter specifically outfitted for medical transport to evacuate wounded soldiers in Burma.
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This mission is thought to be the first helicopter medical evacuation ever undertaken. During the Korean War, air evacuations continued to increase. The US Army established designated air transport teams and created the Mobile Army Surgical Hospital. However, it was not until the inception of the Vietnam War that helicopters and medical transport became almost synonymous. The rugged terrain, progressive escalation, and rising causalities necessitated expansion of the program. It was during this period that helicopter transport became widely viewed as an essential means of casualty transportation. Concurrent with the military progress, civilian programs using aeromedical transport (primarily helicopters) began evolving. From his work as an Army surgeon in France following the Second World War, R. Adams Cowley, MD, used his experience to convince the Maryland Legislature to create a helicopter-driven civilian casualty evacuation program. The Maryland State Police flew patients to the nearest trauma center rather than the nearest emergency department. He noted that, for both military and civilian victims of trauma, rapid delivery to a qualified trauma center was of utmost importance. He coined the term Golden Hour for trauma management. 3 The confluence of both military and civilian experiences in the 1960s and 1970s ultimately gave rise to the current model for both adult and pediatric aeromedical transport. Through local municipalities, private aeromedical companies, and health care facilities, transport with rotor- and fixed-wing aircraft makes up significant components of many transport services.
level to approximately 1200 miles. These concentric gaseous rings are held in place by gravity. The Earth's atmosphere has 2 primary gaseous components: 78.08% nitrogen and 20.95% oxygen. Smaller components include approximately 1.25% water vapor, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen and helium. These relative gas concentrations are maintained up to approximately 70 000 ft above sea level. Atmospheric pressure, which is synonymous with barometric pressure, is used to describe pressure changes imparted on the Earth's surface. Atmospheric pressure is the force exerted by the atmosphere at any given point. At sea level, atmospheric pressure is 760 mm Hg. As altitude increases, atmospheric pressure decreases. A lower atmospheric pressure reduces the alveolar partial pressure of, most importantly, oxygen according to the alveolar gas equation: pA O2 ¼ FI O2 ðPATM −pH2 OÞ−
pa CO2 RQ
where pAO2 is the alveolar partial pressure of oxygen, FIO2 is the fraction of inspired oxygen, PATM is the atmospheric pressure, pH2O is the partial pressure of water vapor, and paCO2 is the arterial partial pressure of carbon dioxide. Finally, RQ is the respiratory quotient; for most conditions, the RQ is estimated to be 0.8 for the purposes of alveolar gas equation calculations. Because oxygen diffuses across the alveolar-capillary membrane based on pressure gradients, a reduction in available alveolar oxygen tension lowers the arterial oxygen content and may impact cellular function.
THE ATMOSPHERE The human body has primarily evolved at sea level. For this reason, it possesses very few compensatory mechanisms for maintaining normal homeostasis when acutely at high altitudes. The Earth's atmosphere is a complex environment made up of different layers of gaseous components, variable partial pressures, and temperature changes, which convey various physiologic effects on both patients and flight crews. Understanding these relationships will allow physicians to anticipate and prepare for additional physiologic derangements in already unstable patients.
COMPOSITION The atmosphere is the environment composed of varying concentrations of gasses arranged in concentric layers around the Earth. It extends from sea
PHYSIOLOGIC ZONES The atmosphere can also be described by partial pressures of gas and the physiologic effects imparted on the human body. It can be divided into 3 physiologic zones based upon these changes. The first zone is called the physiologic efficient zone, which extends from sea level to approximately 12 500 ft. Although there is a reduction in atmospheric pressure, only minor impairments occur to normal gas exchange and oxygenation at the upper ends of this zone (Table 1). The major issues associated with transport at this level relate to gas expansion according to Boyles Law, discussed in more detail below. As pressures around gas molecules are reduced, each molecule travels further away from each other, resulting in gas expansion with decreased density. The gas expansion phenomenon may cause mild discomfort
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TABLE 1. Physiologic zones of the Earth's atmosphere. Zone
Altitude
Pressure
Characteristics Minor trapped gas problems within tracts, such as middle ear and Eustachian tube, sinuses, and GI tract. Shortness of breath, dizziness, headache, and fatigue if prolonged exposure. Major physiological problems such as hypoxia and decompression sickness. Pressurized environment needed for survival. “Armstrong's line”—unprotected exposure above this level can cause body fluids to vaporize.
Physiologic efficient zone
Sea level to 12 500 ft
760-523 mm Hg
Physiologic deficient zone Space equivalent zone
12 500-50 000 ft
523-87 mm Hg
N 50 000 ft
87-0 mm Hg
Data from Federal Aviation Administration Civil Aerospace Medical Institute, “Introduction to Aviation Physiology”. 16 GI indicates gastrointestinal.
and manifest as mild pain in the middle ear, sinuses, or gastrointestinal tract but can have significant implications for the transport of critically ill children. In particular, devices dependent upon inflated cuffs, such as endotracheal tubes, laryngeal mask airways, and Foley catheters, generally will expand beyond desired insufflation pressures, which may result in iatrogenic trauma. Moreover, patients with a pneumothorax, pneumomediastinum, or any other condition with resultant intrathoracic or abdominal air require appropriate drainage devices to prevent the effects of air expansion. These techniques will be discussed in more detail later in this review. The second zone is the physiologic deficient zone. There is a significant reduction in both atmospheric pressure and temperature in this zone, which results in profound impairment of normal physiologic function without either immediate intervention or use of protective equipment (Table 1). Of note, this is the zone where most commercial aviation occurs with cabins typically pressurized to approximately 7000 ft to counteract the effects of the low atmospheric pressure. The space equivalent zone extends from approximately 50 000 ft to 1000 miles above sea level. Extremely low temperatures and low pressures make this zone incompatible with life without an artificial atmosphere. Sixty-three thousand feet above sea level is a delineating altitude called Armstrong's line. The atmospheric pressure at this altitude is approximately 47 mm Hg, which is equivalent to the partial pressure of water in the human body. At this pressure, water within the body turns into vapor, causing body fluids to essentially boil (Table 1).
GAS LAWS To fully anticipate potential complications encountered in aeromedical transport, one must comprehend gas and pressure relationships at altitude. Dalton's law states that the total pressure of a gas mixture is equal to the sum of the partial pressures of each gas contributing to the mixture: Ptotal ¼ p1 þ p2 þ … þ pn where Ptotal is the total pressure, p1 is the pressure of the first gas, p2 is the pressure of the second gas, and so on. As an unpressurized aircraft gains altitude, the total atmospheric pressure as well as each of the partial pressures comprising the total atmospheric pressure decreases. During flight transport at altitude, pressurizing the cabin and adding oxygen are 2 ways to counteract the hypoxia experienced due to the lower partial pressure of oxygen. Boyle's law states that a volume of gas is inversely proportional to its pressure as long as temperature remains constant: P1 V1 ¼ P2 V2 or P1 =P2 ¼ V2 =V1 where P1 is the pressure of a gas and V1 is the volume of that gas. As discussed above, Boyle law explains why gases expand and contract in body cavities as patients ascend and descend, respectively.
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Henry's law states that the amount of gas dissolved in solution at a given temperature varies directly with the pressure of that gas over the solution: C ¼ kH Pgas where C is the solubility of a gas, kH is Henry law constant for that gas with units in molar per atmospheric pressure, and Pgas is the pressure of that gas. Henry's law explains decompression sickness, colloquially known as the “bends.” Similar to deep sea divers, patients can experience decompression sickness as an unpressurized aircraft's altitude increases quickly. The pressure exerted on its occupants will decrease and nitrogen can come out of solution (ie, the plasma). However, this is not typically a problem in the modern era of pressurized cabins.
ALTITUDE PHYSIOLOGY Atmospheric Pressure Changes Many factors including severity of illness, travel distance, vehicle availability, personnel/equipment needs, weather, logistics, and cost will dictate which mode of transportation to be used. Specifically addressing aeromedical transport, fixed-wing aircrafts allow for rapid transport over long distances but confer different potential physiologic challenges as a result of the need to create an artificial atmosphere. The cabin pressure is maintained at a pressure significantly lower than the cruising altitude but significantly higher than sea level. A report prepared by the Civil Aviation Safety Subcommittee of the Aviation Safety Committee of the Aerospace Medical Association noted that adverse physiological effects of flight, caused by ascent to altitude and its associated reduction in atmospheric pressure, have been known since the first manned balloon flights in the 19th century. 4 Both commercial and aeromedical fixed-wing transport aircraft have used 8000 ft as the maximum operational cabin altitude for pressurization, to ensure adequate alveolar oxygen partial pressure to allow for appropriate tissue oxygenation in healthy travelers. Previous research has demonstrated that prolonged air travel in commercial airlines with fixed cabin pressure by otherwise healthy pediatric patients had significant saturation declines due to the reduced cabin partial pressure of oxygen (Po2). 5 Thus, physicians transporting critically ill children at altitude in these environments may expect greater degrees of desaturation and potentially more hemodynamic compromise. They should be prepared to
increase oxygen supplementation and possibly ventilator support when necessary. Conversely, most rotor-wing aircraft have unpressurized cabins resulting in cabin pressures identical to their cruising altitude. Physiologic aberrations due to changes in altitude may be similar to fixedwing aircraft because most helicopters fly at the artificially pressurized cabin altitudes but may also vary significantly with altitude changes. Moreover, rapid ascent may cause symptoms of acute mountain sickness (ie, headache, nausea, vomiting, and insomnia) or more severe symptoms related to highaltitude pulmonary or cerebral edema. 6 In addition, recent research assessing cerebral oxygenation using near-infrared spectroscopy (NIRS) technology compared patients on room air, supplemental oxygen, and mechanical ventilation before takeoff and at cruising altitude. Patients transported at greater than 5000 ft above sea level had a statistically significant decline in NIRS readings whether on invasive ventilation or not. 7 Healthy pilots also experienced similar declines to their systemic and cerebral saturations when cabin pressure was suddenly lost. 8 These data support the supposition that critically ill pediatric patients may have significant changes in cerebral oxygenation as helicopter cruising altitudes increase. Moreover, NIRS may be an effective tool in monitoring cerebral oxygenation during both fixed-wing and rotor-wing transport.
Counteracting Atmospheric Pressure Changes It is well known that gases expand at altitude. For example, 100 mL at sea level expands to 130 mL at 6000 ft and to 400 mL at 32 800 ft. 9 When this occurs inside body cavities, such as the pleural space, gut, or middle ear, the consequences can be problematic in certain populations. This phenomenon is called dysbarism, the syndrome resulting from the effects of a pressure differential between ambient atmospheric pressure and the pressure of gases in the body. As previously described, gas readily expands with any decrease in pressure in accordance with Boyle's law. The ultimate manifestation of this phenomenon is experienced during ascent, with a net reduction of pressure giving rise to an increase in gas volume. The converse also holds true that the increase in atmospheric pressure during descent results in a decrease in the volume of gas. These pressure and volume fluctuations have significant implications during the transport of critically ill children. When occurring in hollow viscera and sinuses, a patient may experience pain, further manifestations of
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TABLE 2. Counteracting atmospheric pressure changes. Complications
Suggested Solutions
Inner ear obstruction GI obstruction or ileus
Have patient drink, suck on pacifier, or perform Valsalva maneuver. Place or open NG, OG, gastrostomy, or surgical drainage tube. Suction frequently. Place chest tube or needle decompress. Have the pilot pressurize to lower altitude. Deflate cuff or balloon. Fill with sterile water or saline. Suction frequently.
Air leak syndromes Device air
NG indicates nasogastric; OG, orogastric.
obstruction or rupture of already diseased tissue. For patients with illnesses that impair Eustachian tube air efflux, children can be asked to swallow liquids, infants to suck on pacifiers, and adolescents to perform Valsalva maneuvers, if their underlying condition allows. Gastrointestinal air also increases during ascent. The gastrointestinal tract contains approximately 1000 mL of gas at atmospheric pressure, with most residing in the stomach and large intestines. This necessitates the placement or opening of drainage devices including naso/orogastric tubes, gastrostomy tubes, or surgical drains in patients with pretransport distension or obstruction to the flow of intra-abdominal air (Table 2). In addition, devices placed to support critically ill children need to be carefully monitored. These include cuffed endotracheal tubes, Foley catheters, and laryngeal mask airways. If possible, the cuff or balloon should be deflated before takeoff. Many describe airway device cuff pressures significantly changing within cabin pressure ranges used by medical transport aircraft. 10,11 This suggests that the risk of potential complications associated with airway device cuff overinflation may be reached sooner in children than adults. Transmural cuff monitoring during ascent and descent with appropriate inflation adjustments can minimize these fluctuations. If deflation is not possible, the cuff or balloon can be filled with saline or water before departure because liquids do not follow Boyle's law. To prevent detrimental pressure fluctuations within a patient's lungs, the internal aspects of endotracheal tubes also need to be assessed for patency throughout the flight. The patient's airway should be suctioned before and during transport (Table 2). Pressure changes can also make seemingly insignificant pneumothoraces evolve into significant
space-occupying lesions causing hemodynamic instability, especially in an unstable child. Therefore, transporting crews should have illumination or other ways of identifying the presence of extrapulmonary air. 12 Chest tubes should be placed before departure in patients with a pneumothorax, and all teams should carry needle thoracentesis kits for rapid decompression at altitude. Patients with other air leak syndromes that cannot be evacuated before transport can be flown at lower altitudes or have the cabin pressurized to sea level to prevent expansion of trapped air or worsening of tamponade physiology. This is especially true in patients with simple pneumocephalus to prevent progression to tension pneumocephalus and herniation. A study of 21 patients with pneumocephalus evacuated by air did not reveal clinical neurologic deterioration. 13 However, the retrospective study design, lack of inflight imaging, and lack of objective neurologic testing make global conclusions difficult (Table 2). Finally, changes in atmospheric pressure may also impact the microvasculature giving rise to third-spacing of fluid. These changes are typically only encountered with prolonged exposure to altitude and not during the relatively short exposures of aeromedical transport. However, it is still prudent to remain vigilant when evaluating vital sign derangements such as tachycardia, hypotension, or falling urine output. Certain patient populations prone to capillary leak may require higher vasoactive support or additional intravenous fluids.
Gravitational Forces Critically ill patients with unstable hemodynamics may also become susceptible to changes in gravitational forces exerted during air travel. In a
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supine child with head positioned toward the nose of the aircraft, positive gravitational forces may cause venous pooling of blood in the lower extremities during ascent. 14 Such pooling can impair venous return and further compromise cardiac output in certain circumstances. It has been shown that positive airway pressure may exacerbate the effects of these forces. However, placing the patient in a perpendicular position or with the patient's head to the rear of the aircraft may reduce these effects. 9 Others have suggested that patient positioning can also play a part in raising or lowering intracranial pressure during air transport. 15 However, in a case report of a patient with cerebral edema and head facing the tail, the intracranial pressure elevation was less than 5 mm Hg during takeoff, which appeared to be less clinically significant. 13
In-Flight Monitoring The dynamic environment of aeromedical transport makes clinical assessment challenging. Auscultation of the chest and abdomen may be impaired due to noise and vibration. Cardiorespiratory and pulse oximetry monitoring may be unreliable due to motion artifact. Monitoring equipment should have a functioning battery back-up, which charges during flight. Military medics have shown that, in addition to standard cardiorespiratory monitoring, other devices such as capnography and point-of-care testing, such as glucometers and i-STAT analyzers (Abbott Labs, Abbott Park, IL), are important adjuncts to help identify correctable derangements during transport. 16 Previous data have documented that defibrillation can be accomplished during fixedwing transport. 17 However, transport teams should notify pilots before discharging the defibrillator should any unforeseen events arise that could potentially compromise the electrical systems of the aircraft.
Medications Given the unpredictable nature of aeromedical transport, teams should have a full pharmacologic armamentarium available to them. Unfortunately, the spatial confines of the cabin become the limiting factor. Inotropic and vasoactive medications are important, in the event hemodynamic changes are experienced at altitude. For example, arrhythmias have been reported in patients with fragile cardiac function presumably through the up-regulation of their sympathetic nervous system as a result of flight stress. 18 In addition, appropriate anxiolysis and analgesia are essential for safe transport. Judicious use will help reduce anxiety and ultimately decrease
metabolic demand. Moreover, it will prevent an unstable patient from deteriorating further by reaching for and removing critical access or monitoring devices. Finally, muscle relaxation is necessary when transporting mechanically ventilated patients to reduce the risk of unplanned extubation.
Hypoxia/Hypoxemia Hypoxia is a state of oxygen deficiency in blood (hypoxemia), tissues, or cells, sufficient to cause an impairment of bodily functions. The physiologic response to decreased arterial oxygen content is chemoreceptor-induced hyperventilation with the primary goal of increasing minute ventilation. Increasing cardiac output, primarily through tachycardia in children, attempts to compensate for any residual hypoxia or hypoxemia. 19 The development of hypoxia is often subtle but can be easily addressed in most transport situations. According to the alveolar gas equation, the fractional inspired oxygen concentration can be increased to account for lower alveolar oxygen partial pressure at altitude, which contributes to hypoxemia. In patients who are mechanically ventilated, it may also be feasible to increase the positive end-expiratory pressure to help oxygenation. These techniques should be discussed with medical control before air transport, especially in hypoxic patients already requiring maximum levels of fractional inspired oxygen.
Vibration and Noise Both rotor- and fixed-wing aircraft have problems relating to excessive noise and vibration during transport. These aspects of flight directly impact patient care and create a potentially stressful working environment for flight crews. Tests conducted at the Federal Aviation Administration's Civil Aeromedical Institute demonstrate that all propeller-driven aircraft and all helicopters are sources of damaging noise intensities for the flight crew. The protection afforded by the cockpit is not enough to keep most active pilots from being overexposed. Thus, all flight crews should wear appropriate ear protection during transports when possible. 20 Many programs provide ear protection for the patient as well. In addition, excessive cabin noise also poses considerable challenges to the medical team charged with assessment and management. Previous studies have documented that sick and premature infants are exposed to high levels of sound during all modes of standard neonatal transportation, with the highest exposure occurring during aircraft transport. 21 Helicopters tend to be the worst culprits by producing more noise and
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vibration from the mode of propulsion than fixedwing aircraft. As previously mentioned, significant noise in the cabin may preclude proper auscultation, making subtle exam findings, such as accessory muscle use, respiratory rate, nasal flaring, and capillary refill, imperative when assessing these patients. Again, point-of-care testing has become the standard of care in pediatric transport medicine to aid the flight clinician. Although rotor-wing aircrafts have considerably more noise and vibration for a given trip, fixed-wing aircraft can experience vibration, particularly when traveling through turbulent air. Vibration alone may increase metabolic demand, worsen respiratory status, worsen hemodynamic instability, and increase pain. Significant vibration can also cause monitoring equipment to malfunction. Efforts to reduce vibration, such as increased padding on the transport stretcher and, if possible, to transport at altitudes with less turbulence, are ideal.
of secretions. 12 As mentioned in a previous section, thick secretions can have significant clinical implications if not expeditiously cleared. Finally, the administration of appropriate intravenous fluids is essential to ensure adequate hydration. Ongoing losses can be exacerbated in the dry environment of aeromedical transport. Some situations may require more than maintenance fluid rates to compensate for additional losses.
SUMMARY Through innovation, research, and experience, aeromedical transport of critically ill children has evolved into a highly specialized branch of pediatrics. Given the unique environment and physiologic changes experienced during flight, transport crews should be able to anticipate and treat complications quickly. In many circumstances, aeromedical transport may be the only viable option for critically ill children when rapid escalation of care is necessary.
Thermoregulation Because ambient temperature decreases as altitude increases, maintaining a comfortable cabin temperature is essential to prevent hypothermia. Temperature regulation should be maintained, as hypothermia and shivering increase oxygen consumption and may aggravate metabolic acidosis and hypoglycemia. 22 Moreover, previous research has shown that hypothermia in pediatric trauma patients causes alterations in their basal metabolic rate, urine output, acid/base status, and cardiac rhythm and increases mortality risk. 23 This is particularly important in neonates and young infants who, due to their large surface area to volume ratio, have difficulty with thermal regulation. Transport teams can minimize the risk of hypothermia by minimizing the amount of exposure to the external environment; prewarming isolettes; using incubator covers, double-walled incubators, internal heat shields, exothermic mattresses, and/or warm swaddling wraps; and keeping warm ambient temperatures in the aircraft cabin. 24
Humidity and Dehydration Pressurized aircraft uses air extracted from the external environment to fill their cabins. Unfortunately, this air has very low water vapor content, making the circulating air very dry. As the humidity drops with altitude, additional sources of humidification should be provided to inspired oxygen. Warming the added water vapor will also help with temperature control, fluid balance, and the viscosity
REFERENCES 1. Varon J, Fromm Jr RE, Marik P. Hearts in the air: the role of aeromedical transport. Chest 2003;124:1636–7. 2. Dorland P, Nanney J. Dust off: Army aeromedical evacuation in Vietnam. Center of Military History, United States Army 1982. Available at: http://history.amedd.army.mil/booksdocs/ vietnam/dustoff/default.html. Accessed 3-1-2013. 3. Cowley RA, Hudson F, Scanlan E, et al. An economical and proved helicopter program for transporting the emergency critically ill and injured patient in Maryland. J Trauma 1973; 13:1029–38. 4. Affleck J, Angelici A, Baker S, et al. Cabin cruising altitudes for regular transport aircraft. Aviat Space Environ Med 2008; 79:433–9. 5. Lee AP, Yamamoto LG, Relles NL. Commercial airline travel decreases oxygen saturation in children. Pediatr Emerg Care 2002;18:78–80. 6. Theis MK, Honigman B, Yip R, et al. Acute mountain sickness in children at 2835 meters. Am J Dis Child 1993;147:143–5. 7. Stroud MH, Gupta P, Prodhan P. Effect of altitude on cerebral oxygenation during pediatric interfacility transport. Pediatr Emerg Care 2012;28:329–32. 8. Muehlemann T, Holper L, Wenzel J, et al. The effect of sudden depressurization on pilots at cruising altitude. Adv Exp Med Biol 2013;765:177–83. 9. Samuels MP. The effects of flight and altitude. Arch Dis Child 2004;89:448–55. 10. Miyashiro RM, Yamamoto LG. Endotracheal tube and laryngeal mask airway cuff pressures can exceed critical values during ascent to higher altitude. Pediatr Emerg Care 2011;27:367–70. 11. Smith RP, McArdle BH. Pressure in the cuffs of tracheal tubes at altitude. Anaesthesia 2002;57:374–8. 12. American Academy of Pediatrics Section on Transport Medicine. Guidelines for air and ground transport of neonatal and pediatric patients (3rd ed). Elk Grove Village, IL: American Academy of Pediatrics; 2007.
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13. Donovan DJ, Iskandar JI, Dunn CJ, et al. Aeromedical evacuation of patients with pneumocephalus: outcomes in 21 cases. Aviat Space Environ Med 2008;79:30–5. 14. Milligan JE, Jones CN, Helm DR, et al. The principles of aeromedical retrieval of the critically ill. Trends Anaesth Crit Care 2011;1:22–6. 15. Morris M. Transport considerations for the head-injured patient: are we contributing to secondary injury? J Air Med Transp 1992;11:9–13. 16. Venticinque SG, Grathwohl KW. Critical care in the austere environment: providing exceptional care in unusual places. Crit Care Med 2008;36:S284–92. 17. Page RL, Joglar JA, Kowal RC, et al. Use of automated external defibrillators by a U.S. airline. N Engl J Med 2000; 343:1210–6. 18. Tyson Jr AA, Sundberg DK, Sayers DG, et al. Plasma catecholamine levels in patients transported by helicopter for acute myocardial infarction and unstable angina pectoris. Am J Emerg Med 1988;6:435–8.
19. Gong H. Air travel and oxygen therapy in cardiopulmonary disorders. Chest 1992;101:1104–13. 20. Federal Aviation Administration, Civil Aerospace Medical Institute, Aeromedical Education Division, AAM-400. FAA aeromedical training programs for civil aviation pilots. Available at: http://www.faa.gov/pilots/training/airman_ education/media/IntroAviationPhys.pdf . Accessed on 3-1-2013. 21. Buckland L, Austin N, Jackson A, et al. Excessive exposure of sick neonates to sound during transport. Arch Dis Child Fetal Neonatal Ed 2003;88:F513–6. 22. Corneli HM. Accidental hypothermia. Pediatr Emerg Care 2012;28:475–80. 23. Sundberg J, Estrada C, Jenkins C, et al. Hypothermia is associated with poor outcome in pediatric trauma patients. Am J Emerg Med 2011;29:1019–22. 24. Macnab AJ, Schweers D, Kendall MD, et al. Improved transport incubator temperature control with insulating thermal cover. Air Med J 1995;14:65–8.
Keywords: pediatric transport; neonatal transport; atmosphere; pressure; barometric *Department of Pediatrics, Division of Critical Care Medicine, Yale University School of Medicine, New Haven, CT; †Department of Pediatrics, Division of Critical Care Medicine, Yale University School of Medicine, New Haven, CT. Reprint requests and correspondence: John S. Giuliano, Jr, MD, Division of Pediatric Critical Care Medicine, Yale University School of Medicine, 333 Cedar Street, LCI 305, New Haven, CT 06520-8064. [email protected] (L.A. Polikoff), [email protected] (J.S. Giuliano)) 1522-8401/$ - see front matter © 2013 Elsevier Inc. All rights reserved.
Up, Up, and Away: Aeromedical Transport Physiology Lee A. Polikoff, MD*, John S. Giuliano Jr., MD†
A
s early as 1784, the French physician Jean Francois Picot was using hot air balloons to treat patients. He is credited with thinking that the purer air at altitude was more beneficial to his patients and used balloons as a means to transport them. 1 French physicians further expanded transporting critically ill patients by air in the late 19th century during the Franco-Prussian War of 1870 to 1871. Historians note, “during the German siege of Paris, observation balloons flew out of the city with many bags of mail, a few high-ranking officials, and 160 casualties.” By 1912, the French Army was training to use aircraft to transport causalities to field hospitals. The following year, a French military officer reported, “we shall revolutionize war surgery if the aeroplane can be adopted as a means of transport for the wounded.” By 1915, French flight crews had begun to actively transport patients. This service continued to expand throughout the First World War. 2 Nationally, the period between World Wars I and II saw increased research and experimentation with both rotor- and fixed-wing aircraft. However, it was not until the Second World War that evacuation of critically ill patients became more widespread among the United States military. By 1943, the United States Army had begun developing helicopters specifically for the purpose of medical transport. The following year, Lt Carter Harman first used a new Sikorsky helicopter specifically outfitted for medical transport to evacuate wounded soldiers in Burma.
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This mission is thought to be the first helicopter medical evacuation ever undertaken. During the Korean War, air evacuations continued to increase. The US Army established designated air transport teams and created the Mobile Army Surgical Hospital. However, it was not until the inception of the Vietnam War that helicopters and medical transport became almost synonymous. The rugged terrain, progressive escalation, and rising causalities necessitated expansion of the program. It was during this period that helicopter transport became widely viewed as an essential means of casualty transportation. Concurrent with the military progress, civilian programs using aeromedical transport (primarily helicopters) began evolving. From his work as an Army surgeon in France following the Second World War, R. Adams Cowley, MD, used his experience to convince the Maryland Legislature to create a helicopter-driven civilian casualty evacuation program. The Maryland State Police flew patients to the nearest trauma center rather than the nearest emergency department. He noted that, for both military and civilian victims of trauma, rapid delivery to a qualified trauma center was of utmost importance. He coined the term Golden Hour for trauma management. 3 The confluence of both military and civilian experiences in the 1960s and 1970s ultimately gave rise to the current model for both adult and pediatric aeromedical transport. Through local municipalities, private aeromedical companies, and health care facilities, transport with rotor- and fixed-wing aircraft makes up significant components of many transport services.
level to approximately 1200 miles. These concentric gaseous rings are held in place by gravity. The Earth's atmosphere has 2 primary gaseous components: 78.08% nitrogen and 20.95% oxygen. Smaller components include approximately 1.25% water vapor, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen and helium. These relative gas concentrations are maintained up to approximately 70 000 ft above sea level. Atmospheric pressure, which is synonymous with barometric pressure, is used to describe pressure changes imparted on the Earth's surface. Atmospheric pressure is the force exerted by the atmosphere at any given point. At sea level, atmospheric pressure is 760 mm Hg. As altitude increases, atmospheric pressure decreases. A lower atmospheric pressure reduces the alveolar partial pressure of, most importantly, oxygen according to the alveolar gas equation: pA O2 ¼ FI O2 ðPATM −pH2 OÞ−
pa CO2 RQ
where pAO2 is the alveolar partial pressure of oxygen, FIO2 is the fraction of inspired oxygen, PATM is the atmospheric pressure, pH2O is the partial pressure of water vapor, and paCO2 is the arterial partial pressure of carbon dioxide. Finally, RQ is the respiratory quotient; for most conditions, the RQ is estimated to be 0.8 for the purposes of alveolar gas equation calculations. Because oxygen diffuses across the alveolar-capillary membrane based on pressure gradients, a reduction in available alveolar oxygen tension lowers the arterial oxygen content and may impact cellular function.
THE ATMOSPHERE The human body has primarily evolved at sea level. For this reason, it possesses very few compensatory mechanisms for maintaining normal homeostasis when acutely at high altitudes. The Earth's atmosphere is a complex environment made up of different layers of gaseous components, variable partial pressures, and temperature changes, which convey various physiologic effects on both patients and flight crews. Understanding these relationships will allow physicians to anticipate and prepare for additional physiologic derangements in already unstable patients.
COMPOSITION The atmosphere is the environment composed of varying concentrations of gasses arranged in concentric layers around the Earth. It extends from sea
PHYSIOLOGIC ZONES The atmosphere can also be described by partial pressures of gas and the physiologic effects imparted on the human body. It can be divided into 3 physiologic zones based upon these changes. The first zone is called the physiologic efficient zone, which extends from sea level to approximately 12 500 ft. Although there is a reduction in atmospheric pressure, only minor impairments occur to normal gas exchange and oxygenation at the upper ends of this zone (Table 1). The major issues associated with transport at this level relate to gas expansion according to Boyles Law, discussed in more detail below. As pressures around gas molecules are reduced, each molecule travels further away from each other, resulting in gas expansion with decreased density. The gas expansion phenomenon may cause mild discomfort
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TABLE 1. Physiologic zones of the Earth's atmosphere. Zone
Altitude
Pressure
Characteristics Minor trapped gas problems within tracts, such as middle ear and Eustachian tube, sinuses, and GI tract. Shortness of breath, dizziness, headache, and fatigue if prolonged exposure. Major physiological problems such as hypoxia and decompression sickness. Pressurized environment needed for survival. “Armstrong's line”—unprotected exposure above this level can cause body fluids to vaporize.
Physiologic efficient zone
Sea level to 12 500 ft
760-523 mm Hg
Physiologic deficient zone Space equivalent zone
12 500-50 000 ft
523-87 mm Hg
N 50 000 ft
87-0 mm Hg
Data from Federal Aviation Administration Civil Aerospace Medical Institute, “Introduction to Aviation Physiology”. 16 GI indicates gastrointestinal.
and manifest as mild pain in the middle ear, sinuses, or gastrointestinal tract but can have significant implications for the transport of critically ill children. In particular, devices dependent upon inflated cuffs, such as endotracheal tubes, laryngeal mask airways, and Foley catheters, generally will expand beyond desired insufflation pressures, which may result in iatrogenic trauma. Moreover, patients with a pneumothorax, pneumomediastinum, or any other condition with resultant intrathoracic or abdominal air require appropriate drainage devices to prevent the effects of air expansion. These techniques will be discussed in more detail later in this review. The second zone is the physiologic deficient zone. There is a significant reduction in both atmospheric pressure and temperature in this zone, which results in profound impairment of normal physiologic function without either immediate intervention or use of protective equipment (Table 1). Of note, this is the zone where most commercial aviation occurs with cabins typically pressurized to approximately 7000 ft to counteract the effects of the low atmospheric pressure. The space equivalent zone extends from approximately 50 000 ft to 1000 miles above sea level. Extremely low temperatures and low pressures make this zone incompatible with life without an artificial atmosphere. Sixty-three thousand feet above sea level is a delineating altitude called Armstrong's line. The atmospheric pressure at this altitude is approximately 47 mm Hg, which is equivalent to the partial pressure of water in the human body. At this pressure, water within the body turns into vapor, causing body fluids to essentially boil (Table 1).
GAS LAWS To fully anticipate potential complications encountered in aeromedical transport, one must comprehend gas and pressure relationships at altitude. Dalton's law states that the total pressure of a gas mixture is equal to the sum of the partial pressures of each gas contributing to the mixture: Ptotal ¼ p1 þ p2 þ … þ pn where Ptotal is the total pressure, p1 is the pressure of the first gas, p2 is the pressure of the second gas, and so on. As an unpressurized aircraft gains altitude, the total atmospheric pressure as well as each of the partial pressures comprising the total atmospheric pressure decreases. During flight transport at altitude, pressurizing the cabin and adding oxygen are 2 ways to counteract the hypoxia experienced due to the lower partial pressure of oxygen. Boyle's law states that a volume of gas is inversely proportional to its pressure as long as temperature remains constant: P1 V1 ¼ P2 V2 or P1 =P2 ¼ V2 =V1 where P1 is the pressure of a gas and V1 is the volume of that gas. As discussed above, Boyle law explains why gases expand and contract in body cavities as patients ascend and descend, respectively.
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Henry's law states that the amount of gas dissolved in solution at a given temperature varies directly with the pressure of that gas over the solution: C ¼ kH Pgas where C is the solubility of a gas, kH is Henry law constant for that gas with units in molar per atmospheric pressure, and Pgas is the pressure of that gas. Henry's law explains decompression sickness, colloquially known as the “bends.” Similar to deep sea divers, patients can experience decompression sickness as an unpressurized aircraft's altitude increases quickly. The pressure exerted on its occupants will decrease and nitrogen can come out of solution (ie, the plasma). However, this is not typically a problem in the modern era of pressurized cabins.
ALTITUDE PHYSIOLOGY Atmospheric Pressure Changes Many factors including severity of illness, travel distance, vehicle availability, personnel/equipment needs, weather, logistics, and cost will dictate which mode of transportation to be used. Specifically addressing aeromedical transport, fixed-wing aircrafts allow for rapid transport over long distances but confer different potential physiologic challenges as a result of the need to create an artificial atmosphere. The cabin pressure is maintained at a pressure significantly lower than the cruising altitude but significantly higher than sea level. A report prepared by the Civil Aviation Safety Subcommittee of the Aviation Safety Committee of the Aerospace Medical Association noted that adverse physiological effects of flight, caused by ascent to altitude and its associated reduction in atmospheric pressure, have been known since the first manned balloon flights in the 19th century. 4 Both commercial and aeromedical fixed-wing transport aircraft have used 8000 ft as the maximum operational cabin altitude for pressurization, to ensure adequate alveolar oxygen partial pressure to allow for appropriate tissue oxygenation in healthy travelers. Previous research has demonstrated that prolonged air travel in commercial airlines with fixed cabin pressure by otherwise healthy pediatric patients had significant saturation declines due to the reduced cabin partial pressure of oxygen (Po2). 5 Thus, physicians transporting critically ill children at altitude in these environments may expect greater degrees of desaturation and potentially more hemodynamic compromise. They should be prepared to
increase oxygen supplementation and possibly ventilator support when necessary. Conversely, most rotor-wing aircraft have unpressurized cabins resulting in cabin pressures identical to their cruising altitude. Physiologic aberrations due to changes in altitude may be similar to fixedwing aircraft because most helicopters fly at the artificially pressurized cabin altitudes but may also vary significantly with altitude changes. Moreover, rapid ascent may cause symptoms of acute mountain sickness (ie, headache, nausea, vomiting, and insomnia) or more severe symptoms related to highaltitude pulmonary or cerebral edema. 6 In addition, recent research assessing cerebral oxygenation using near-infrared spectroscopy (NIRS) technology compared patients on room air, supplemental oxygen, and mechanical ventilation before takeoff and at cruising altitude. Patients transported at greater than 5000 ft above sea level had a statistically significant decline in NIRS readings whether on invasive ventilation or not. 7 Healthy pilots also experienced similar declines to their systemic and cerebral saturations when cabin pressure was suddenly lost. 8 These data support the supposition that critically ill pediatric patients may have significant changes in cerebral oxygenation as helicopter cruising altitudes increase. Moreover, NIRS may be an effective tool in monitoring cerebral oxygenation during both fixed-wing and rotor-wing transport.
Counteracting Atmospheric Pressure Changes It is well known that gases expand at altitude. For example, 100 mL at sea level expands to 130 mL at 6000 ft and to 400 mL at 32 800 ft. 9 When this occurs inside body cavities, such as the pleural space, gut, or middle ear, the consequences can be problematic in certain populations. This phenomenon is called dysbarism, the syndrome resulting from the effects of a pressure differential between ambient atmospheric pressure and the pressure of gases in the body. As previously described, gas readily expands with any decrease in pressure in accordance with Boyle's law. The ultimate manifestation of this phenomenon is experienced during ascent, with a net reduction of pressure giving rise to an increase in gas volume. The converse also holds true that the increase in atmospheric pressure during descent results in a decrease in the volume of gas. These pressure and volume fluctuations have significant implications during the transport of critically ill children. When occurring in hollow viscera and sinuses, a patient may experience pain, further manifestations of
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TABLE 2. Counteracting atmospheric pressure changes. Complications
Suggested Solutions
Inner ear obstruction GI obstruction or ileus
Have patient drink, suck on pacifier, or perform Valsalva maneuver. Place or open NG, OG, gastrostomy, or surgical drainage tube. Suction frequently. Place chest tube or needle decompress. Have the pilot pressurize to lower altitude. Deflate cuff or balloon. Fill with sterile water or saline. Suction frequently.
Air leak syndromes Device air
NG indicates nasogastric; OG, orogastric.
obstruction or rupture of already diseased tissue. For patients with illnesses that impair Eustachian tube air efflux, children can be asked to swallow liquids, infants to suck on pacifiers, and adolescents to perform Valsalva maneuvers, if their underlying condition allows. Gastrointestinal air also increases during ascent. The gastrointestinal tract contains approximately 1000 mL of gas at atmospheric pressure, with most residing in the stomach and large intestines. This necessitates the placement or opening of drainage devices including naso/orogastric tubes, gastrostomy tubes, or surgical drains in patients with pretransport distension or obstruction to the flow of intra-abdominal air (Table 2). In addition, devices placed to support critically ill children need to be carefully monitored. These include cuffed endotracheal tubes, Foley catheters, and laryngeal mask airways. If possible, the cuff or balloon should be deflated before takeoff. Many describe airway device cuff pressures significantly changing within cabin pressure ranges used by medical transport aircraft. 10,11 This suggests that the risk of potential complications associated with airway device cuff overinflation may be reached sooner in children than adults. Transmural cuff monitoring during ascent and descent with appropriate inflation adjustments can minimize these fluctuations. If deflation is not possible, the cuff or balloon can be filled with saline or water before departure because liquids do not follow Boyle's law. To prevent detrimental pressure fluctuations within a patient's lungs, the internal aspects of endotracheal tubes also need to be assessed for patency throughout the flight. The patient's airway should be suctioned before and during transport (Table 2). Pressure changes can also make seemingly insignificant pneumothoraces evolve into significant
space-occupying lesions causing hemodynamic instability, especially in an unstable child. Therefore, transporting crews should have illumination or other ways of identifying the presence of extrapulmonary air. 12 Chest tubes should be placed before departure in patients with a pneumothorax, and all teams should carry needle thoracentesis kits for rapid decompression at altitude. Patients with other air leak syndromes that cannot be evacuated before transport can be flown at lower altitudes or have the cabin pressurized to sea level to prevent expansion of trapped air or worsening of tamponade physiology. This is especially true in patients with simple pneumocephalus to prevent progression to tension pneumocephalus and herniation. A study of 21 patients with pneumocephalus evacuated by air did not reveal clinical neurologic deterioration. 13 However, the retrospective study design, lack of inflight imaging, and lack of objective neurologic testing make global conclusions difficult (Table 2). Finally, changes in atmospheric pressure may also impact the microvasculature giving rise to third-spacing of fluid. These changes are typically only encountered with prolonged exposure to altitude and not during the relatively short exposures of aeromedical transport. However, it is still prudent to remain vigilant when evaluating vital sign derangements such as tachycardia, hypotension, or falling urine output. Certain patient populations prone to capillary leak may require higher vasoactive support or additional intravenous fluids.
Gravitational Forces Critically ill patients with unstable hemodynamics may also become susceptible to changes in gravitational forces exerted during air travel. In a
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supine child with head positioned toward the nose of the aircraft, positive gravitational forces may cause venous pooling of blood in the lower extremities during ascent. 14 Such pooling can impair venous return and further compromise cardiac output in certain circumstances. It has been shown that positive airway pressure may exacerbate the effects of these forces. However, placing the patient in a perpendicular position or with the patient's head to the rear of the aircraft may reduce these effects. 9 Others have suggested that patient positioning can also play a part in raising or lowering intracranial pressure during air transport. 15 However, in a case report of a patient with cerebral edema and head facing the tail, the intracranial pressure elevation was less than 5 mm Hg during takeoff, which appeared to be less clinically significant. 13
In-Flight Monitoring The dynamic environment of aeromedical transport makes clinical assessment challenging. Auscultation of the chest and abdomen may be impaired due to noise and vibration. Cardiorespiratory and pulse oximetry monitoring may be unreliable due to motion artifact. Monitoring equipment should have a functioning battery back-up, which charges during flight. Military medics have shown that, in addition to standard cardiorespiratory monitoring, other devices such as capnography and point-of-care testing, such as glucometers and i-STAT analyzers (Abbott Labs, Abbott Park, IL), are important adjuncts to help identify correctable derangements during transport. 16 Previous data have documented that defibrillation can be accomplished during fixedwing transport. 17 However, transport teams should notify pilots before discharging the defibrillator should any unforeseen events arise that could potentially compromise the electrical systems of the aircraft.
Medications Given the unpredictable nature of aeromedical transport, teams should have a full pharmacologic armamentarium available to them. Unfortunately, the spatial confines of the cabin become the limiting factor. Inotropic and vasoactive medications are important, in the event hemodynamic changes are experienced at altitude. For example, arrhythmias have been reported in patients with fragile cardiac function presumably through the up-regulation of their sympathetic nervous system as a result of flight stress. 18 In addition, appropriate anxiolysis and analgesia are essential for safe transport. Judicious use will help reduce anxiety and ultimately decrease
metabolic demand. Moreover, it will prevent an unstable patient from deteriorating further by reaching for and removing critical access or monitoring devices. Finally, muscle relaxation is necessary when transporting mechanically ventilated patients to reduce the risk of unplanned extubation.
Hypoxia/Hypoxemia Hypoxia is a state of oxygen deficiency in blood (hypoxemia), tissues, or cells, sufficient to cause an impairment of bodily functions. The physiologic response to decreased arterial oxygen content is chemoreceptor-induced hyperventilation with the primary goal of increasing minute ventilation. Increasing cardiac output, primarily through tachycardia in children, attempts to compensate for any residual hypoxia or hypoxemia. 19 The development of hypoxia is often subtle but can be easily addressed in most transport situations. According to the alveolar gas equation, the fractional inspired oxygen concentration can be increased to account for lower alveolar oxygen partial pressure at altitude, which contributes to hypoxemia. In patients who are mechanically ventilated, it may also be feasible to increase the positive end-expiratory pressure to help oxygenation. These techniques should be discussed with medical control before air transport, especially in hypoxic patients already requiring maximum levels of fractional inspired oxygen.
Vibration and Noise Both rotor- and fixed-wing aircraft have problems relating to excessive noise and vibration during transport. These aspects of flight directly impact patient care and create a potentially stressful working environment for flight crews. Tests conducted at the Federal Aviation Administration's Civil Aeromedical Institute demonstrate that all propeller-driven aircraft and all helicopters are sources of damaging noise intensities for the flight crew. The protection afforded by the cockpit is not enough to keep most active pilots from being overexposed. Thus, all flight crews should wear appropriate ear protection during transports when possible. 20 Many programs provide ear protection for the patient as well. In addition, excessive cabin noise also poses considerable challenges to the medical team charged with assessment and management. Previous studies have documented that sick and premature infants are exposed to high levels of sound during all modes of standard neonatal transportation, with the highest exposure occurring during aircraft transport. 21 Helicopters tend to be the worst culprits by producing more noise and
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vibration from the mode of propulsion than fixedwing aircraft. As previously mentioned, significant noise in the cabin may preclude proper auscultation, making subtle exam findings, such as accessory muscle use, respiratory rate, nasal flaring, and capillary refill, imperative when assessing these patients. Again, point-of-care testing has become the standard of care in pediatric transport medicine to aid the flight clinician. Although rotor-wing aircrafts have considerably more noise and vibration for a given trip, fixed-wing aircraft can experience vibration, particularly when traveling through turbulent air. Vibration alone may increase metabolic demand, worsen respiratory status, worsen hemodynamic instability, and increase pain. Significant vibration can also cause monitoring equipment to malfunction. Efforts to reduce vibration, such as increased padding on the transport stretcher and, if possible, to transport at altitudes with less turbulence, are ideal.
of secretions. 12 As mentioned in a previous section, thick secretions can have significant clinical implications if not expeditiously cleared. Finally, the administration of appropriate intravenous fluids is essential to ensure adequate hydration. Ongoing losses can be exacerbated in the dry environment of aeromedical transport. Some situations may require more than maintenance fluid rates to compensate for additional losses.
SUMMARY Through innovation, research, and experience, aeromedical transport of critically ill children has evolved into a highly specialized branch of pediatrics. Given the unique environment and physiologic changes experienced during flight, transport crews should be able to anticipate and treat complications quickly. In many circumstances, aeromedical transport may be the only viable option for critically ill children when rapid escalation of care is necessary.
Thermoregulation Because ambient temperature decreases as altitude increases, maintaining a comfortable cabin temperature is essential to prevent hypothermia. Temperature regulation should be maintained, as hypothermia and shivering increase oxygen consumption and may aggravate metabolic acidosis and hypoglycemia. 22 Moreover, previous research has shown that hypothermia in pediatric trauma patients causes alterations in their basal metabolic rate, urine output, acid/base status, and cardiac rhythm and increases mortality risk. 23 This is particularly important in neonates and young infants who, due to their large surface area to volume ratio, have difficulty with thermal regulation. Transport teams can minimize the risk of hypothermia by minimizing the amount of exposure to the external environment; prewarming isolettes; using incubator covers, double-walled incubators, internal heat shields, exothermic mattresses, and/or warm swaddling wraps; and keeping warm ambient temperatures in the aircraft cabin. 24
Humidity and Dehydration Pressurized aircraft uses air extracted from the external environment to fill their cabins. Unfortunately, this air has very low water vapor content, making the circulating air very dry. As the humidity drops with altitude, additional sources of humidification should be provided to inspired oxygen. Warming the added water vapor will also help with temperature control, fluid balance, and the viscosity
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