This issue has appeared a few times, and in recent years it has resurfaced as a question about safely transporting the patient with a cerebral arterial gas embolism. Overall, the SAQs involving aeromedical retrieval have been:

  • Question 21 from the first paper of 2017 (gas embolism)
  • Question 7 from the second paper of 2010 (problems specific to aeromedical transport of critically ill patients)
  • The identical Question 14 from the first paper of 2007
  • Question 15 from the second paper of 2001 (essential features of a helicopter-transported ventilator).

The much more exciting topic of expanding gas bubbles at altitude is explored in the chapter dedicate to gas embolism. The following is a very brief summary of aeromedical retrieval issues.

Limitations of the aircraft

  • Little space for large equipment (eg. ECMO, IABP)
  • Little space for gas reserve (O2)
  • Less equipment available
  • Little room for CPR
  • Changes in aircraft tilt place the patient in Trendelenberg and reverse Trendelberg positions
  • Hypothermia can develop in the cold cabin
  • The aircraft is noisy
  • The lighting is sub-optimal
  • Turbulence can cause injuries to the poorly restrained patient

Dangers of altitude: changes in the behaviour of gases

  • ETT cuffs expand
  • Gas-filled cavities expand (eg. bowel, pneumothorax, pneumoencephalus)
  • Partial pressures of gas mixtures is lower (100% FiO2 at 2100m, the standard "cabin altitude" of commercial aircraft, is only 597mmHg)
  • Evacuation by air of those who have bee deep-sea diving is best avoided for about 24 hours- decompression sickness may result.

Dangers of altitude: changes in fluid behaviour

  • Decreased boiling point at altitude increases the rate of evaporative loss

Dangers of aircraft operation

  • Avionics may interfere with pacemakers
  • Noise may interfere with equipment alarms
  • Vibration interferes with examination of the pulse
  • Auscultation is practically impossible
  • Motion sickness may cause vomiting and aspiration (it would suck to be intubated purely because of motion sickness)

Essential features of a helicopter-transported ventilator

(straight from Table 4.2 (page 32) from Oh's Manual, "Features of an Ideal Transport Ventilator".)

  • Small, light, robust and cheap
  • Independent of an external power source
  • Easy to use and clean
  • Economical with gas consumption
  • Suitable for patients of all sizes, from neonates to huge adults
  • Totally variable FiO2
  • Able to deliver a variety of modes of ventilation
  • Able to ventilate with variable I:E ratios
  • Integrated monitoring and alarm functions
  • Alarms should be visual and auditory
  • Altitude compensated ventilation

References

Parsons, Chris J., and Walter P. Bobechko. "Aeromedical transport: its hidden problems." Canadian Medical Association Journal 126.3 (1982): 237.

ANZCA "Guidelines for Transport of Critically Ill Patients

CICM "Minimum Standards for Transport of Critically Ill Patients" (IC-10, 2010)

"Commercial Airliner Environmental Control System: Engineering Aspects of Cabin Air Quality".

Oh's Intensive Care manual: Chapter 4 (pp.27)    Transport  of  critically  ill  patients   by Evan  R  Everest  and  Matthew  R  Hoope