ADF Health September 2002 - Volume 3 Number 2

Letters to the Editor

The Western Front pilgrimage for World War I veterans, 1993

Lieutenant Colonel Roger K A Allen, MB BS, PhD, FRACP, FCCP, RAAMC
Suite 299, St Andrew's Place, 33 North Street, Spring Hill, QLD.
Correspondence: Lieutenant Colonel Roger K A Allen, Suite 299 St Andrew's Place, 33 North Street, Spring Hill, QLD 4000. rogerallen@ozemail.com.au

TO THE EDITOR: In the previous issue of ADF Health, Capps and Killer described the preflight work-up of a group of aged veterans soon to travel to Europe. 1 An altitude chamber was used to assess the physiological response to mild hypoxia expected in aircraft.

As a respiratory physician who has never seen, let alone needed, a barochamber, I am reassured that I have never yet lost a patient par avion. I recommend the subject undergo a case-specific assessment, which should include investigating the possibility of undiagnosed obstructive sleep apnoea or central sleep apnoea, as the most dangerous period inflight is during recumbent rapid-eye-movement sleep. Although the elderly spend considerable time asleep during flight, Capps and Killer did not mention sleep apnoea or Cheyne-Stokes breathing.

For the vast majority of travellers, regardless of age, a forced expiratory volume in one second greater than one litre and resting oxygen saturation greater than 90% bodes well for air travel (no need for measurement of arterial blood gases). I have never been let down once by this maxim. For smokers or those with basal crackles, I measure diffusing capacity (CO uptake). This takes five minutes and, with a few added extras, it is possible to work out the five causes of hypoxaemia:

Low ambient oxygen tension (eg, low cabin pressure)
Hypoventilation (eg, sleep apnoea and too much wine)
Ventilation/perfusion mismatch (eg, chronic obstructive airway disease)
Diffusion block (eg, emphysema)
Shunt (eg, intracardiac and intrapulmonary).

The alveolar-arterial oxygen gradient is normal in 1 and 2, but widened in 3, 4 or 5. Rebreathing 100% oxygen solves 5. Diffusing capacity (KCO) is lowered in 4, but not in 3.

Hypercapnoea is caused by only one thing: hypoventilation.

A high altitude simulation test is easier and cheaper than a barochamber test and less problematic. An E-sized gas cylinder (volume 3800 L) costs about $500 and is sufficient to test a battalion of aged veterans.

Ingredients. (1) a wooden step, two steps high; (2) a Hudson mask; (3) an air cylinder containing 17% oxygen, instead of the normal 20.95% (this simulates the cabin pressure in a jetliner, which is usually about 5000-7000 ft); (4) a finger oximeter; (5) an observer; (6) a veteran or other test subject; (7) a testing room near to sea level.

Recipe. Mix all together gently for five minutes, observing the subject's oximetry, pulse and general appearance.

As a cost-saving measure, the wooden step can be skipped, and the subject can be invited to walk on the level, as if going to the bathroom on a plane. The step is really for first class passengers only, who may have to negotiate a spiral staircase in flight. An ECG lead can be added to taste.

Result. A large desaturation (eg, to 75%) during the test, with hyperventilation and ECG changes, should make the ears prick up.

This is a safe, cheap, reliable test with minimal risk, even for claustrophobic subjects. In our current litigious climate it may prove difficult to justify the use of a hypobaric chamber should any adverse event occur.

Finally, a diagnostic sleep study in suspects may avert inflight surprises. Please consider …

References

Capps RA, Killer GT. The Western Front pilgrimage for World War I veterans, 1993. Health care planning for a successful outcome. ADF Health 2002; 3: 31-35.