Six-Minute Walk with Helium-Oxygen

We recently performed a 6-minute walk test with helium-oxygen (heliox) for a patient of one of the physicians that specializes in airway stenting. His reasons for the test weren’t particularly clear (and he hasn’t bothered to try to clarify them with me) but most probably it has to do with differentiating between central and peripheral airway obstruction. Interestingly, he predicted the patient would have a significant improvement in 6-minute walk distance and instead there was little difference between the heliox 6MWT and one performed with 3 LPM supplemental O2.

6MWT: SaO2: Distance:
80% Helium – 20% O2, by mask 95% 440 meters
3 LPM O2, by nasal cannula 98% 457 meters

Helium is an inert, insoluble, low mass gas and both its therapeutic use and its use in physiological measurements has to do with it’s low density (and the fact that it’s highly insoluble, but that’s for purposes different than those discussed here).

  Density (g/m3)
He 0.179
N2 1.251
O2 1.429
Air (78% N2, 21% O2) 1.293
Heliox (80% He, 20% O2) 0.429

A typical way to assess its effect is by comparing air and heliox flow-volume loops:

heo2_fvl

Interestingly, despite an apparent increase in flow rates there is usually no significant difference in FEV1 (one study showed a range of +2% to +7% in a group of over 1500 subjects). The most common heliox FVL measurements are the change in expiratory flow at 50% of the FVC (ΔMEF@50%) and the Volume of Isoflow (which is the point at which the air and heliox expiratory flows become equivalent). Many of the earlier studies with heliox also measured ∆MEF@75% and ∆FEF25-75, and a tiny handful of studies (particularly given the technical difficulties) have measured ∆RAW and ∆sGAW.
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Inspiratory Flow-Volume Loops

Recently I was discussing a flow-volume loop with one of our pulmonary physicians. His concern was whether the loop, which was from a patient with a cracked hyoid, was showing inspiratory obstruction or not. I had to point out that the inspiratory flows on the loop in question came from the pre-FVC inspiration that started at FRC, not RV, and that we don’t emphasize maximal inspiratory flow at that point in the test, just maximal inspiratory volume and for these reasons the flow-volume loop was not a reliable indicator of inspiratory obstruction.

IFVL

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