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A688
October 19, 2008 2:00 PM - 4:00 PM Room Hall E2-Area M, |
| Heliox as Carrier Gas for Isoflurane Wash-Out |
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Gang Zheng, M.D., Wilhelm Schwab, Ph.D., Nikolaus Gravenstein, M.D., Timothy E. Morey, M.D., Samsun Lampotang, Ph.D. Anesthesiology, University of Florida, Gainesville, Florida |
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Background: Helium-oxygen mixtures (Heliox) have been used in healthcare since 1934 and are generally used for airway obstruction. Assuming that heliox behaves like a gaseous compound, Graham's Law indicates that, due to its lower “molecular weight” than O2, diffusion through heliox should be faster than through O2. We compared heliox against O2 as a carrier gas during wash-out of isoflurane in a test lung. Methods: An anesthesia machine (Aestiva/5, GE Healthcare, Madison, WI) with a SmartVent ventilator (with “Heliox mode” software that accounts for the physical properties of heliox) and a heliox flow meter was used to deliver either 100% O2 or heliox (25% O2, 75% He2) to a mechanical test lung (TTL, Michigan Instruments, Inc., Grand rapids, MI). Settings were VT 500 ml, 8 bpm, I:E 1:2 and 2 L/min of fresh gas flow rate. The test lung was set at a compliance of 0.1 L/cm H2O. Crumpled medical gauze was loosely inserted (ten 4x4s in each bellows) into each test lung bellows cavity. One ml of liquid isoflurane was injected onto the gauze. Room temperature was 24oC. Isoflurane concentrations at the Y-piece were measured using a calibrated, side-stream multi-gas analyzer (Capnomac Ultima, Datex-Ohmeda, Madison, WI) and recorded every 10 seconds for 15 minutes using an internally developed recordkeeper. The experiment was repeated 7 times (n=7) for each carrier gas and the average isoflurane value was plotted. Following peak concentrations, the wash-out phase was analyzed using single phase exponential decay (Prism 4.03, GraphPad Software, San Diego, CA). Results: Figure 1 plots average isoflurane concentration on the Y-axis and time (in increments of 10 seconds) on the x-axis and indicates that heliox under identical conditions with a mechanical test lung enhances wash-out of isoflurane compared to O2. During wash-out, the rate constants for isoflurane in heliox or oxygen were 0.00238±0.00062 and 0.00090±0.0011 s-1, respectively, with corresponding half-lives of 4.9 and 12.9 min (P=0.011).[figure1]Conclusions: Our in-vitro results suggest that, as a carrier gas, heliox may provide a clinically relevant improvement in inhaled anesthetics clearance compared to O2. The in-vitro data will be used for an IRB application for a follow-up clinical study designed to establish if there is a clinical benefit to using heliox to speed clearance of potent inhaled agents. Anesthesiology 2008; 109 A688 |
Figure 1
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