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Transferring Fixed Wing Air Medical Patients With Intracranial Hemorrhages

      Air medical transport, including by fixed wing aircraft, has become increasingly frequent as health systems become regionalized to absorb costs and enhance catchment areas. Speed of travel is perceived as 1 of the most notable advantages of air medical transport, thus improving patient access to once unobtainable resources offered at destination locations. Despite these advantages, risks must be considered before transporting. These risks are unique to each scenario, especially in patients with intracranial hemorrhage (ICH).
      When considering fixed wing transport, one must consider the challenges that may present simply because of the mode of transportation. Space limitations, turbulence, and restricted special equipment are some challenges that may present, but with proper planning and training, these may be mitigated. Additionally, flying at sea-level cabin pressure presents with challenges of delayed transport and increased costs. Some risks are unique to the type of patient at hand, such as the ICH patient. There exists a paucity of research regarding the pathophysiology of air medical transport in the setting of a patient with ICH. The environmental changes at altitude and the body's adaptation to its surrounding environment present the greatest risks to evaluate when interhospital transport (IHT) is being considered.
      The transport of patients with ICH, either pre- or postneurosurgical intervention, is a decision that must be made on a relatively frequent basis because the worldwide incidence of spontaneous nontraumatic ICH is 24.6 per 100,000 persons per year, with approximately 40,000 to 67,000 cases in the United States per year.
      • Caceres J
      • Goldstein J
      Intracranial hemorrhage.
      A large number of these patients present initially to their respective nearby medical centers and are unable to obtain the type of surgical intervention they require.
      • Weyhenmeyer J
      • Guandique C
      • Leibold A
      • et al.
      Effects of distance and transport method on intervention and mortality in aneurysmal subarachnoid hemorrhage.
      With a mortality rate ranging from 35% to 52% and with the majority of fatalities occurring within the first 4 days, timely IHT is paramount in these patients.
      • Caceres J
      • Goldstein J
      Intracranial hemorrhage.
      • Weyhenmeyer J
      • Guandique C
      • Leibold A
      • et al.
      Effects of distance and transport method on intervention and mortality in aneurysmal subarachnoid hemorrhage.
      • Beseoglu K
      • Holtkamp K
      • Steiger H
      • Hänggi D
      Fatal aneurysmal subarachnoid haemorrhage: causes of 30-day in-hospital case fatalities in a large single-centre historical patient cohort.
      Patients with ICHs have a high risk of recurrent bleeding after the initial bleed in a relatively short period of time. A common commercial aircraft cruises at an altitude of 30,000 to 40,000 feet with the cabin pressurized to approximately 8,000 feet (0.74 atm).
      • Brändström H
      • Sundelin A
      • Hoseason D
      • et al.
      Risk for intracranial pressure increase related to enclosed air in post-craniotomy patients during air ambulance transport: a retrospective cohort study with simulation.
      ,
      • Kouliev Richardson
      • Glushak
      Intracranial hemorrhage during aeromedical transport and correlation with high altitude adaptations in the brain.
      The decision to fly at the assigned altitude versus sea-level cabin pressure is routinely considered in air medical flights without clear scientific evidence of the best mode of travel based on ICH specifics. At sea level, the partial pressure of oxygen is 98 mm Hg, which leads to a PaO2 of 98 mm Hg and O2 saturations of 98% to 100% in healthy individuals. At 8,000 feet, the partial pressure of oxygen is 55 mm Hg, which leads to a PaO2 of 90 mm Hg. The maintenance of sea-level cabin pressure is not common practice all over the world, and the criteria supporting the choice of pressurization during transport are not evidence based.
      As altitude is increased, the partial pressure of oxygen decreases, which induces hypoxia.
      • Kouliev Richardson
      • Glushak
      Intracranial hemorrhage during aeromedical transport and correlation with high altitude adaptations in the brain.
      This hypoxic state results in cerebrovascular dilation, thus increasing intracranial pressure (ICP), which ultimately may lead to disruption of the blood-brain barrier.
      • Basnyat B
      • Murdoch D.
      High-altitude illness.
      ,
      • Gallagher S
      • Hackett P.
      High-altitude illness.
      In the setting of ICH, these changes increase the risk of recurrent bleeding. The Brain Oxygen Optimization in Severe Traumatic Brain Injury Phase II (BOOST-II) Trial and The Excellence in Prehospital Injury Care Traumatic Brain Injury (EPIC-TBI) Trial both show the importance of the avoidance of hypoxia in TBI.
      • Okonkwo D
      • Shutter L
      • Moore C
      • et al.
      Brain oxygen optimization in severe traumatic brain injury phase-II.
      ,
      • Spaite D
      • Hu C
      • Bobrow B
      • et al.
      The effect of combined out-of-hospital hypotension and hypoxia on mortality in major traumatic brain injury.
      The Brain Trauma Foundation Guidelines for the Management of Severe Traumatic Brain Injury propose keeping peripheral capillary oxygen saturation > 90% so that the corresponding PaO2 is > 60 mm Hg.
      Brain Trauma Foundation
      Guidelines for the management of severe traumatic brain injury third edition.
      We propose that the threshold should be higher during air transport so that there is a greater safety margin in the avoidance of hypoxia. In the absence of data, we propose peripheral capillary oxygen saturation > 94% during flight.
      There are little data on increased ICP in healthy subjects during flight because direct monitoring of ICP is invasive, with an intraparenchymal probe being the gold standard. However, magnetic resonance imaging studies have shown that people with no cerebral injury at altitude experience cerebral edema of some volume of unknown etiology.
      • Mórocz I
      • Zientara G
      • Gudbjartsson H
      • et al.
      Volumetric quantification of brain swelling after hypobaric hypoxia exposure.
      Cerebral edema is a leading cause of death in patients with ICH who do not die immediately after the initial insult.
      • Beseoglu K
      • Holtkamp K
      • Steiger H
      • Hänggi D
      Fatal aneurysmal subarachnoid haemorrhage: causes of 30-day in-hospital case fatalities in a large single-centre historical patient cohort.
      We do have the ability to identify increased ICP in a noninvasive fashion by transocular ultrasound, and this method can even be used to detect rapid fluctuations in ICP as described by Maissan et al,
      • Maissan I
      • Dirven P
      • Haitsma I
      • Hoeks S
      • Gommers D
      • Stolker R
      Ultrasonographic measured optic nerve sheath diameter as an accurate and quick monitor for changes in intracranial pressure.
      who were able to detect sudden increases in ICP by transocular ultrasonography by measuring optic nerve sheath diameters in intensive care patients during tracheal manipulation.
      Do the seemingly associated altitude-related hypoxia and downstream effects of increased ICP secondary to vasodilation and blood-brain barrier disruption lead to increased mortality or does transporting to a center for definitive management provide the patient with the greatest chance of survival and favorable neurologic outcomes? Unfortunately, there are little data regarding these questions because most of the data we have come from helicopter IHT. Applying ICH data from rotary wing transports to fixed wing ICH patients could be detrimental to patient care because these are 2 very different means of transport. Rotary wing transports are typically at much lower altitudes, meaning lower risk for ICP elevation secondary to hypoxia.
      Martinique may provide us with the best data thus far. Martinique is an island in the Caribbean that transfers patients from Martinique to Paris (approximately 4,300 miles) for neurosurgical intervention because there are daily flights scheduled, no foreign country policy or regulatory issues such as visas or payment, and no language barriers.
      • Mejdoubi M
      • Schertz M
      • Zanolla S
      • Mehdaoui H
      • Piotin M
      Transoceanic management and treatment of aneurysmal subarachnoid hemorrhage.
      During a 10-year case series, 119 patients were identified with aneurysmal subarachnoid hemorrhage, and 91 of the 119 were transferred to Paris with a median delay of 32 hours 13 minutes because of the trans-Atlantic flight.
      • Mejdoubi M
      • Schertz M
      • Zanolla S
      • Mehdaoui H
      • Piotin M
      Transoceanic management and treatment of aneurysmal subarachnoid hemorrhage.
      None of the 91 patients transferred experienced a recurrence of bleeding or a significant complication during the flight.
      • Mejdoubi M
      • Schertz M
      • Zanolla S
      • Mehdaoui H
      • Piotin M
      Transoceanic management and treatment of aneurysmal subarachnoid hemorrhage.
      In rebuttal, a case report published by Kouliev et al
      • Kouliev Richardson
      • Glushak
      Intracranial hemorrhage during aeromedical transport and correlation with high altitude adaptations in the brain.
      discusses the case of a patient with ICH of unknown etiology who was transported on day 10 postoperatively who subsequently developed a recurrent bleed of unknown etiology and developed symptoms of increased ICP after being in the air approximately 4 hours. The current guidelines give no specific recommendations for patients being transported after ICH. Granted, a patient who is being transported for immediate intervention is not comparable with a patient being transported after intervention who is now deemed stable or improving.
      The postoperative ICH patients may be the patients at greatest risk of complications secondary to fixed wing transport. Boyle's law states that as pressure decreases, the volume of a gas expands at a given temperature.
      • Brändström H
      • Sundelin A
      • Hoseason D
      • et al.
      Risk for intracranial pressure increase related to enclosed air in post-craniotomy patients during air ambulance transport: a retrospective cohort study with simulation.
      This can become problematic because pneumocephalus is a common finding in postneurosurgical patients, which can take weeks to reabsorb. This expansion of gas can cause problems of obstruction, increased ICP, and reduced cerebral perfusion.
      Brändström et al
      • Brändström H
      • Sundelin A
      • Hoseason D
      • et al.
      Risk for intracranial pressure increase related to enclosed air in post-craniotomy patients during air ambulance transport: a retrospective cohort study with simulation.
      found that one third of postcraniotomy patients secondary to subdural hematoma had pneumocephalus. They looked at intracranial gas volume expansion versus cabin pressures at varying altitudes up to 8,000 feet and intracranial hypertension via a recognized simulation. It was found that intracranial gas volumes of ≤ 11 mL with transport at a pressurized cabin of 8,000 feet would not result in intracranial hypertension. However, at intracranial volumes > 11 mL, the chance to develop intracranial hypertension existed when flying at altitude.
      • Brändström H
      • Sundelin A
      • Hoseason D
      • et al.
      Risk for intracranial pressure increase related to enclosed air in post-craniotomy patients during air ambulance transport: a retrospective cohort study with simulation.
      Using computer modeling, Anderson et al
      • Andersson N
      • Grip H
      • Lindvall P
      • et al.
      Air Transport of Patients with Intracranial Air: Computer Model of Pressure Effects.
      found under normal altitude the intracranial air volume will increase by approximately 30% at normal maximum cabin altitude (8,000 feet). The increase in ICP depends on both the initial air volume and the rate of change in cabin altitude. For an intracranial air volume of 30 mL, the estimated worst-case increments of ICP from sea level to maximum altitude would be from 10 mm Hg to 21 mm Hg. They recommend the use of sea level for patients with 30 mL intracranial air.
      With what evidence is available, decreased mortality and improved neurologic outcomes are greatest at definitive neurologic care centers. ICHs cause significant morbidity and mortality, and these are both improved at specialized treatment centers. Further research is needed on this subject, specifically investigating morbidity, mortality, and outcomes of ICH patients transported at various altitudes, both with and without pneumocephalus. For now, patients with the greatest risk seem to be those who are postoperative who do not require an immediate intervention because of postoperative pneumocephalus. Great thought must be given to the decision on when and how to transport these patients. The volume of intracranial gas can be accurately determined by computed tomographic scanning and should be calculated closest to the time of transport because the data derived from Brändström et al
      • Brändström H
      • Sundelin A
      • Hoseason D
      • et al.
      Risk for intracranial pressure increase related to enclosed air in post-craniotomy patients during air ambulance transport: a retrospective cohort study with simulation.
      are the best data we have at this time. Patients with intracranial gas volumes > 11 mL should be transported in air ambulances that are able to pressurize the cabin greater than 0.74 atm and ideally transported at sea level to minimize adverse outcomes secondary to increased ICP.
      Furthermore, consideration should be given for the altitude at which city the patient is located. If this altitude is higher than sea level, then this altitude should be the cabin altitude that should be maintained. Additional consideration during transport should be given to the destination airport's altitude. If this airport is higher than sea level, the cabin pressure should change at stepped intervals to avoid a drastic increase upon landing at the final airport.

      Conclusion

      The literature does not provide an evidence-based algorithm to determine when ICH patients should be transferred by fixed wing or indications for the use of sea-level flights. As such, the following considerations should be made for every fixed wing ICH flight: 1) the ability of the sending facility to perform emergent neurosurgical intervention, 2) the ability of the sending facility to treat the emergent needs of multisystem trauma, 3) the amount of pneumocephalus measured from a computed tomographic scan, and 4) the risk of prolonged flight if traveling at sea-level cabin pressure.

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