1Paediatric Anaesthesia Unit, Geneva Childrens 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
Accepted for publication: June 4, 2001
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Abstract |
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Br J Anaesth 2001; 87: 6027
Keywords: anaesthetic techniques; inhalation; measurement techniques, pneumotachograph
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Introduction |
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Although the viscosities and densities of carrier gases (air, oxygen, and nitrous oxide) can be extracted from physical tables,5 and the density data for the volatile anaesthetic agents can be calculated based on their molecular structure, the viscosity values for carrier gases containing different concentrations of vaporized volatile anaesthetic agents are not available and cannot be readily deduced from theoretical physical equations. Consequently, it is not known how the commonly used volatile inhalation agents in different concentrations affect flow measurements with a resistive-type pneumotachograph. The aim of the present study was, therefore, to determine experimentally whether the different concentrations of clinically administered volatile anaesthetic agents affect the viscosities and densities of the gas mixtures commonly applied in anaesthetic respiratory management, and hence to estimate the extent to which the amounts of the various components affect flow measurements with a pneumotachograph.
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Methods |
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Measurement apparatus
We adopted the experimental method of Lutchen and colleagues to measure Zt.7 This technique allowed us to determine Zt in a wide frequency range in order to ensure the validity of the Poiseuille law, while it also permitted us to change the resident gas in the tube. The scheme of the measurement set-up is presented in Figure 1. A loudspeaker-in-box system generated a small-amplitude pseudorandom forcing signal in a rigid-walled plexiglass box (Vbox=1.6 litre). The oscillatory signal contained seven frequency components in the range 0.1176.04 Hz. The components in the forcing signal were selected according to the non-sum-non-difference rule8 to minimize the effects of non-linearities and harmonics cross-talk. In particular, the 2nd, 5th, 11th, 19th, 31st, 59th, and 103rd harmonics of the fundamental frequency (0.0586 Hz) were included in the signal with component amplitudes decreasing with increasing frequency. A rigid polyethylene plastic tube (l=29 cm, r=1 mm) was led through the front panel of the box, and two similar-sized plastic bags were attached to the internal and external ends of the tube. As the bags were always flaccid, for example, the pressure in the bags was atmospheric, their influence on the impedance measurement could be neglected.
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Procedure
All measurements were performed in the same climate-controlled laboratory under constant atmospheric conditions (970 hPa), with stable temperature (23°C) and humidity. Before each oscillatory measurement, the two bags were emptied by exposing them to direct vacuum until complete collapse. At the beginning of the experiment, the impedance of the box-tube system (Zc) was measured when the plastic bags and the tube were filled with air. Both plastic bags and the tube were then filled with different concentrations of various anaesthetic inhalation agents in turn (halothane, isoflurane, sevoflurane, and desflurane) in 100% oxygen, in a mixture of 50% oxygen+50% nitrogen, or in a mixture of 21% oxygen+79% nitrogen (air). The concentration of the volatile agents were varied between 0.75 and 4% for halothane, 1.25 and 5.4% for isoflurane, 2 and 9% for sevoflurane and 6 and 18% for desflurane by measuring four different concentrations of each within these ranges for each volatile gas. The highest concentrations were limited by the physical characteristics of the vaporizers. Gas mixtures were generated from an anaesthetic machine by the flow of 6 litres of carrier gas through the vaporizer; the outlet was sampled at a rate of 200 ml min1 by means of a Datex AS3 monitor until a steady-state measurement was achieved. Both plastic bags and the tube were then filled with the resulting mixture via a 3-way tap. Before oscillatory measurements, the concentration of each component in the blended gas in the bags was analysed with a Datex AS3 monitor, which was calibrated before each use. Four sets of measurements were made on each gas mixture, the four resulting curves were averaged and used for further analyses (see below). Following a set of measurements with each gas mixture, the external end of the tube was plugged, and the impedance of the closed box (Zb) was determined.
Data analysis and statistics
Fast Fourier transformation was used to compute Zc (Pb/V· with the tube open) and Zb (Pb/V· with the tube closed) from the 30-s long recordings by using a 17-s time window and 95% overlapping. As Zc consists of Zb and Zt in parallel, Zt was calculated by parallel removal of Zb from Zc: Zt = ZbZc/(Zb Zc).
Our data analysis assumes that density and viscosity of a gas mixture change linearly with the density and viscosity values of the pure component gases. Linearity of density necessarily applies to mixtures of ideal gases and it can be assumed that deviations from linearity do not play a significant role in estimating the viscosity. Therefore, multiple linear regression analysis was used to estimate the viscosity and density values of the pure component gases by considering either viscosity or density values of the gas mixtures as dependent, and the amount of component gases as independent variables. Uncertainties in the parameter estimates were expressed as SE values.
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Results |
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Discussion |
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Methodological issues
Before discussing the implications of the present findings, as concerns the measurement of respiratory mechanics, some methodological issues need to be considered. First, the parameter estimations assume that the measurements were made in the presence of laminar flow in the tube.6 To verify this assumption, we calculated the Reynolds number (Re=rv/µ) from our experiments, by calculating the axial velocity (v) from the measurements of flow on entry into the plastic box. This calculation revealed that the Reynolds numbers were lower (130140) than the critical value (1160). Thus, it can be concluded that laminar flow developed in the tube, and thus, the use of the Poiseuille equation to estimate viscosity was appropriate.
Second, on the basis of physical principles, R for a rigid tube increases with increasing oscillatory frequency.10 Thus, if this phenomenon exerts a significant effect, we should overestimate the viscosity, as the Poiseuille law is valid for low frequencies. In this study, the real part was always almost entirely frequency-independent (e.g. Fig. 2). Therefore, averaging the R values over the frequency range studied did not seem to introduce any systematic error in the estimation of viscosity. Additionally, the I of a rigid tube decreases slightly with increasing oscillatory frequency.10 In this study, we observed an almost perfect linear increase in X in the frequency range studied. Thus, the inertance can also be considered frequency-independent in our measurements and can be used to estimate the density of the resident gas mixture in the tube.
Validity and accuracy
As the analysis applied in the present study provides viscosity and density values for carrier gases (pure oxygen and nitrogen), comparing our values to those in reference tables9 (the latter being corrected to temperature of 23°C and atmospheric pressure of 970 hPa) gives information regarding the reliability of the technique used in the present study. Viscosity values for oxygen and nitrogen determined experimentally in the present study are very close to their reference values (differences are 1.7 and 0.9%, respectively) suggesting that viscosity values of other pure component gases are also likely to be reliable. However, density values are slightly underestimated for both nitrogen and oxygen (6.7 and 1.5%, respectively). This small underestimation is most likely because of the slight distortion of the parabolic velocity profile, particularly at the entrance and at the exit of the tube, which decreases the factor n from 4/3 towards 1 in the equation relating inertance to density6 (see above). Accordingly, a 17% underestimation can be expected in all density values reported in the present study.
The precision of estimates for oxygen and nitrogen is excellent in the present experiment with standard errors lower than 2%. Nevertheless, our estimates for the viscosity and density values of volatile inhalation agents display lower accuracy. In the present study, the concentration of volatile agents delivered was limited to the physical capacities of the vaporizers. Thus, extrapolation of the physical parameters for pure volatile gases was based on measurements when these agents were present only at very low concentrations. Indeed, the accuracy of our estimate increased markedly with increasing volatile agent concentration in the carrier gas, with lowest accuracy for halothane (maximum concentration 4%) and highest for desflurane (maximum concentration 18%).
Implications
Flow measurement is sensitive to the viscosity of the gas, as the pressure drop across the resistive element of a pneumotachograph (either screen or Fleisch) is linearly related to this parameter.3 Thus, a decrease in gas viscosity underestimates the real flow and overestimates the resistance and the elastance, and vice versa. Accordingly, without applying a correction factor, this phenomenon biases the results of studies on respiratory mechanics. In recent publications, following a change in the nature of the inhaled gas, no correction factor was applied to flow measurements, which may have biased their results to some extent.11 12
In the present study, as expected, the viscosity of the carrier gas increased significantly when the oxygen content increased. Therefore, calibrating a pneumotachograph with air and using a different carrier gas with high oxygen content will lead to an underestimation of respiratory mechanical parameters. However, for a given carrier gas, the presence of a volatile agent in low concentration is not likely to significantly affect the resulting viscosity. Thus, our results suggest that the effect of the viscosity alteration as a result of volatile agent administration may not be physiologically significant (approximately 4% at most) when a single reading of a respiratory mechanical parameter is made in a given clinical setting. However, when repeated measurements are performed in animal studies or under clinical circumstances in order to compare different agents or populations, it may be important to take into account viscosity changes in the different gas mixtures in the evaluation of respiratory mechanics under different experimental conditions.13 Given that we determined viscosity values of pure volatile agents, the impact of the altered viscosity on flow measurement can be estimated and in future studies the importance of applying a correction factor will depend on the particular experimental condition.
The density of the gas mixture was affected by the amount of both oxygen and the volatile agents (Table 4). Although the density of the gas in the lungs does not play a very significant role under most experimental conditions, this physical parameter may influence the measured mechanical parameters via the Bernoulli effect in lateral pressure measurements4 and/or in the event of turbulent flow.6 14 It may be noted that, as the measurement of flow with a pneumotachograph is based on the detection of a differential pressure between the two ports, the influence of density via the Bernoulli effect is cancelled out in such differential pressure measurements.3 However, the measurement of pressure at an airway opening is more likely to be affected by the Bernoulli effect and thus by density. Furthermore, density affects flow measurements under conditions of turbulence, when the Reynolds number exceeds a critical value.14 In this regard, Bates and colleagues4 proposed an experimental method for determination of the correction factor to be applied at high Reynolds numbers. Our measurements suggest the need for such a correction if either the oxygen content or the volatile agent concentration of the intrathoracic gas is changed during the experimental procedure.
In conclusion, we adopted a measurement technique7 to determine the physical properties of volatile agents commonly administered in anaesthetic management. The measurements were validated by comparing viscosity and density values of oxygen and nitrogen to their reference values.9 We conclude that volatile agents in commonly used clinical concentrations slightly affect the gas mixture viscosity. However, the oxygen content in the carrier gas and the nature of the volatile anaesthetic agent affect the density of the gas mixture significantly. Therefore, our results suggest that attention should be paid to the compositions of the commonly used inhalation anaesthetic gases during respiratory mechanical measurements; application of a gas-dependent correction factor may be necessary for accurate flow measurements with a resistive-type pneumotachograph.
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Acknowledgements |
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References |
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2 Johns DP, Pretto JJ, Streeton JA. Measurement of gas viscosity with a Fleisch pneumotachograph. J Appl Physiol 1982; 53: 2903
3 Bates JHT, Turner MJ, Lanteri CJ, Jonson B, Sly PD. Dynamic measurement of flow and volume. In: Stocks J, Sly PD, Tepper RS, Morgan WE, eds. Infant Respiratory Function Testing, 1st edn. New York: Wiley-Liss, 1996; 81109
4 Bates JH, Sly PD, Sato J, Davey BL, Suki B. Correcting for the Bernoulli effect in lateral pressure measurements. Pediatr Pulmonol 1992; 12: 2516[ISI][Medline]
5 Turner MJ, MacLeod IM, Rothberg AD. Effects of temperature and composition on the viscosity of respiratory gases. J Appl Physiol 1989; 67: 4727
6 Peslin R, Fredberg JJ. The respiratory system. In: American Physiologica Society, Handbook of Physiology Vol III. Mechanics of Breathing. Bethesda, MD: American Physiologica Society, 1986; 1467
7 Lutchen KR, Hantos Z, Petak F, Adamicza A, Suki B. Airway inhomogeneities contribute to apparent lung tissue mechanics during constriction. J Appl Physiol 1996; 80: 18419
8 Suki B, Lutchen KR. Pseudorandom signals to estimate apparent transfer and coherence functions of nonlinear systems: applications to respiratory mechanics. IEEE Trans Biomed Eng 1992; 39: 114251[ISI][Medline]
9 Weast RC, Astle MJ, Beyer WH. Handbook of Chemistry and Physics, 66th edn. Boca Raton, FL: CRC Press, Inc., 19851986; 6206
10 Franken H, Clement J, Cauberghs M, Van de Woestijne KP. Oscillating flow of a viscous compressible fluid through a rigid tube: a theoretical model. IEEE Trans Biomed Eng 1981: 28: 41620[ISI][Medline]
11 Katoh T, Ikeda K. A comparison of sevoflurane, enflurane, and isoflurane on bronchoconstriction caused by histamine. Can J Anaesth 1994; 41: 12149[Abstract]
12 Rooke GA, Choi JH, Bishop MJ. The effect of isoflurane, halothane, sevoflurane, and thiopental/nitrous oxide on respiratory system resistance after tracheal intubation. Anesthesiology 1997; 86: 12949[ISI][Medline]
13 Turner MJ, MacLeod IM, Rothberg AD. Calibration of Fleisch and screen pneumotachographs for use with various oxygen concentrations. Med Biol Eng Comput 1990; 28: 2004[ISI][Medline]
14 Cerra FJ, Gardner M, Amar. Viscous flow in ducts. In: White FM, ed. Fluid Mechanics, 4th edn. New York, NY: McGraw-Hill, 1998; 305399