The Science Journal of the American Association for Respiratory Care
BACKGROUND/HYPOTHESIS: Helium is an inert gas that is 1/8 as dense as nitrogen. Mixed with oxygen (heliox), the density of helium reduces the resistance factor in gas delivery. This property has been shown to augment tidal volume delivery in patients breathing spontaneously who have increased airway resistance. Heliox also reduces the driving pressure required for a given tidal volume delivery with a volume targeted breath. With a pressure targeted breath, however, the reduction in gas density would be expected to increase tidal volume for a given set inspiratory pressure.
Methods: A bench study was designed with the Siemens Servo 900c (Siemens Medical Systems, Danvers, MA), a system in which volume measurement remains accurate in the presence of helium (Gentile et al, Respiratory Care abstract, Nov 98). Pressure assist control was used with the following settings: Inspiratory pressure of 14cmH2O, PEEP 0 cmH2O, inspiratory time percent 25%, respiratory rate 10. The ventilator was attached to an IngMar test lung and PMG 3000 graphics module (IngMar Medical, LTD, Pittsburgh, PA). The lung resistance setting on the test lung was set at 3 (corresponding to a resistance of 70 cmH2O at 30 lpm). Tidal volumes were then measured using a conventional oxygen mixture and then with using increasing concentration of helium in oxygen.
Results: The results summarized in the table below show progressive increase in delivered tidal volume as gas density is reduced with helium.
|Mixture||50% Oxygen||40% Oxygen||30% Oxygen||21% Oxygen|
|Balance Air||265 ml||262 ml||260 ml||259 ml|
|Balance H2||327 ml||340 ml||350 ml||367 ml|
Conclusion: Heliox increases delivered tidal volume in a ?dose dependent? fashion when using pressure assist control ventilation.