Viscosity and density of common anaesthetic gases: implications for flow measurements

Abstract

Although viscosity (mu) is a crucial factor in measurements of flow with a pneumotachograph, and density (rho) 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 mu and rho 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. Mu and rho were calculated from real and imaginary portions of Zt, respectively, using the appropriate physical equations. Multiple linear regression was applied to estimate mu and rho 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 mu for 2 MAC desflurane). In contrast, all of the volatile agents significantly affected rho 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.

Comparison of pneumotachography and anemometery for flow measurement during mechanical ventilation with volatile anesthetics

“…Previously, Habre et al [12] characterized changes in the physical properties of gas mixtures for various concentrations of volatile anesthetics, and concluded that clinically relevant concentrations significantly affected estimates of ρ ( i.e ., up to 48% deviation compared to room air), but only marginally affected estimates of μ ( i.e ., less than 4% deviation compared to room air). Following these principal findings, Miyaji et al [4] demonstrated that ventilation with sevoflurane and desflurane altered delivered V T for feedback-control conditions determined using a variable orifice flow sensor.…”

“…The PNT is commonly used in research settings for accurate flow measurement during mechanical ventilation [12, 17–19]. The PNT measures a differential pressure across a calibrated resistive element, and assumes that the relationship between ∆ P and V̇ is linear and dependent on μ, but not on other gas properties such as ρ, k , and c p [12, 17].…”

“…1), the addition of volatile anesthetics in clinically relevant concentrations will have a negligible influence on effective gas viscosity [12, 17]. By contrast, a more complicated relationship exists between V̇ HWA and several other properties of the gas [9, 10].…”

“…By contrast, a more complicated relationship exists between V̇ HWA and several other properties of the gas [9, 10]. Based on previous findings [4, 12], we assumed that changes in gas density were primarily responsible for changes in V̇ HWA during measurement with anesthetic gas mixtures. Thus we adjusted V̇ HWA using a compensation factor accounting for changes in gas mixture density ( ρ mix ) relative to the density of the gas used for the calibration of the HWA ( ρ cal ): $${{\dot{V}}^{\prime }}_{\mathrm{italicHWA}}=\left(\frac{{\rho }_{\mathrm{cal}}}{{\rho }_{\mathrm{mix}}}\right){\dot{V}}_{\mathrm{HWA}}$$where ${{\dot{V}}^{\prime }}_{\mathrm{italicHWA}}$ is the compensated estimate of HWA flow.…”

“…Given the differences in the physical principles of measurement between the PNT and the HWA, we hypothesized that mechanical ventilation with volatile anesthetics, which are known to differentially affect μ and ρ of a gas mixture at clinically relevant concentrations [12], would systematically bias estimates of measured V̇ along with flow-derived estimates of V T . Accordingly, the goal of this study was to characterize the differences between these two flow-measuring devices for three commonly used volatile anesthetic agents, over a range of clinically relevant concentrations, ventilator rates, and V T .…”