Desflurane vapor pressure depression |
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| Authors: | RJ Brosnan BH Pypendop |
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| Institution: | University of California, Davis, CA, USA |
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| Abstract: | Due to its high vapor pressure and low boiling point, desflurane requires a specially designed, electronically controlled, temperature and pressure compensated vaporizer to regulate agent delivery to the anesthetic circuit. However, if the vapor pressure and boiling point were decreased, desflurane could be used in any conventional variable bypass vaporizer. Raoult's Law states that the vapor pressure of a liquid is proportional to its molar fraction in a solution. Accordingly, propylene glycol was used as a solvent for desflurane, and the physical characteristics of this mixture were evaluated at various molar concentrations and temperatures. Desflurane boiling point increased and vapor pressure decreased as a nonlinear function of dilution, but these changes were less than predicted by Raoult's Law. Using a circle system with a breathing bag attached at the patient end and a mechanical ventilator to simulate respiration, an in‐circuit, nonprecision vaporizer containing 40% desflurane and 60% propylene glycol achieved a 11.5 ± 1.0% (mean ± SD) circuit desflurane concentration with a 5.2 ± 0.4 (0 = off, 10 = maximum) vaporizer setting. This experiment was repeated with a dog attached to the breathing circuit under spontaneous ventilation with a fresh gas flow of 0.5 L min–1. Anesthesia was maintained for over two hours at a mean vaporizer setting of 6.2 ± 0.4, yielding mean inspired and end‐tidal desflurane concentrations of 8.7 ± 0.5% and 7.9 ± 0.7%, respectively. Within 5 minutes after cessation of anesthesia, the dog was awake, extubated and standing. In clinical practice, propylene glycol may not prove an ideal solvent for desflurane due to its instability in solution and substantial positive deviation from Raoult's Law. However, rather than alter the vaporizer to suit physical properties of anesthetic agents, this study demonstrates that it may also be possible to alter anesthetic agents to suit physical properties of the vaporizer. |
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