Aeromedical retrieval

This chapter is directly relevant to Section 2.1.1 of the CICM Second Part General Exam Syllabus (First Edition), which specifically lists “transport of the critically ill patient” as core knowledge for the intensive care specialist. Transport of the critically ill patient is the subject of a CICM official guideline document (IC-10 Guidelines for Transport of Critically Ill Patients) which contains a lot of detail about the specific perils of air transport.  This issue has appeared a few times:

  • 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.

Principles and practice/implementation

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

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

Relevant guidelines and evidence

The IC-10 Guidelines for Transport of Critically Ill Patients is the only resource worth spending precious pre-exam minutes on, but the interested reader with an abundant attention span will benefit from comparing the CICM document to the weirdly similar ANZCA "PG52(G) Guideline for transport of critically ill patients 2015 to think about who plagiarised who. For actual guidelines, i.e specific recommendations of what to do, the best local resource would have to be the RFDS Clinical Manuals Part 1, 2, 3 & 4 from the Royal Flying Doctors Service. Additionally, Milligan et al (2011) is an excellent plain language narrative review.

Controversies and risks

Risks of aeromedical transport are well documented and seem easy to reconstruct from first principles to anybody who has ever seen an aircraft disaster movie. In brief, from the IC-10 Guidelines document, these risks are as follows:

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

Milligan, J. E., et al. "The principles of aeromedical retrieval of the critically ill." Trends in Anaesthesia and Critical Care 1.1 (2011): 22-26.