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