British Journal of Anaesthesia, 2001, Vol. 87, No. 4 602-607
© 2001 The Board of Management and Trustees of the British Journal of Anaesthesia
Laboratory Investigations |
Viscosity and density of common anaesthetic gases: implications for flow measurements
1Paediatric Anaesthesia Unit, Geneva Children’s Hospital, 6 rue Willy Donze, CH-1205 Geneva, Switzerland, 2Department of Medical Informatics and Engineering, University of Szeged, Szeged, Hungary and 3Division of Clinical Sciences, Institute for Child Health Research and Department of Paediatrics, University of Western Australia, Perth, Australia*Corresponding author
Although viscosity (µ) is a crucial factor in measurements of flow with a pneumotachograph, and density (
) also plays a role in the presence of turbulent flow, these material constants are not available for the volatile anaesthetic agents commonly administered in clinical practice. Thus, we determined experimentally µ and of pure volatile anaesthetic agents. Input impedance of a rigid-wall polyethylene tube (Zt) was measured when the tube was filled with various mixtures of carrier gases (air, 100% oxygen, 50% oxygen+50% nitrogen) to which different concentrations of volatile anaesthetic inhalation agents (halothane, isoflurane, sevoflurane, and desflurane) had been added. µ and were calculated from real and imaginary portions of Zt, respectively, using the appropriate physical equations. Multiple linear regression was applied to estimate µ and of pure volatile agents. Viscosity values of pure volatile agents were markedly lower than those for oxygen or nitrogen. Clinically applied concentrations, however, did not markedly affect the viscosity of the gas mixture (maximum of 3.5% decrease in µ for 2 MAC desflurane). In contrast, all of the volatile agents significantly affected even at routinely used concentrations. Our results suggest that the composition of the carrier gas has a greater impact on viscosity than the amount and nature of the volatile anaesthetic agent whereas density is more influenced by volatile agent concentrations. Thus, the need for a correction factor in flow measurements with a pneumotachograph depends far more on the carrier gas than the concentration of volatile agent administered, although the latter may play a role in particular experimental or clinical settings.
Br J Anaesth 2001; 87: 602–7