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