Dynamic models and solutions for evaluating ventilation, perfusion, and mass transfer in the lung--Part II: Analog solutions.
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Computer Simulation↗
Biomedical subjects
C E Hahn
Publications and source records attributed to C E Hahn.
A digital system for generating dynamic sinusoidal gas concentration signals.
A computer-controlled gas-mixing system is presented. It is capable of mixing four gases, the concentration of three of which will follow a path to be determined by the user. For our purposes the output O2 fraction is maintained constant and the levels of Ar and N2O vary sinusoidally and independently, with periods between 0.25 and 30 min. A fourth gas, N2 is necessary to make the sum of the individual fractions 100%. The system uses banks of between one and four solenoid valves each linked via a sonic choke to a common mixing chamber. A regime of pulse frequency modulation is employed. All calculations and timing of valve switching are performed by a dedicated microcomputer built for the purpose. The device has been used to provide respiratory gas forcing functions for a program of research in respiratory monitoring.
Humans↗
A compensation method for membrane-covered (Clark) electrodes.
Membrane-covered electrodes (Clark electrodes) are widely used for monitoring blood gases, particularly PO2. A method of compensating for the inherently limited speed of response of Clark electrodes is presented. The theoretical response in the time domain is related to that in the frequency domain, and the latter is deduced from measurement of the former. Although the response functions are both infinite series, both responses are nevertheless completely defined by a single time parameter Te characteristic of the electrode under given measurement conditions. Practical verification was performed using electrodes in the double-pulsed mode, but the theory is applicable equally to direct-current-polarized and simply pulsed electrodes.
Algorithms↗
Performance of a miniaturised O2-Hb dissociation curve analyser on dog's blood.
Oxygen-haemoglobin dissociation curves were determined on dogs' blood using a modified and miniaturised dissociation curve analyser. The Bohr factor describes the way in which pH varies the PO2 corresponding to a particular oxyhaemoglobin concentration. The factor was similar for three saturations and was little affected by whether the pH was changed by changing PCO2, or by adding fixed acid or alkali. The haemoglobin saturations in the mid-range tended to be lower than those predicted by the equation of Rossing and Cain (1966).
Animals↗
A microprocessor-controlled anaesthetic vaporizer.
A microprocessor-controlled anaesthetic vaporizer is described. Fresh gas is mixed in the correct proportions using two pulsed solenoid valves and a proportion of this passes through a third pulsed solenoid valve and is bubbled through liquid halothane. The temperature of the liquid agent is measured and the pulse frequency is modified to give the correct vapour concentration for the set flow rate and measured temperature. Initially, the vapour was produced by bubbling fresh gas through the agent in a conventional halothane bottle. However, because of the large liquid volume available, nitrous oxide was found to dissolve in large quantities in the halothane. A small volume vaporizer which was continually replenished from a reservoir was designed. Measurements of the vapour concentrations emerging from such a vaporizer were made and were found to agree with the set values +/- 0.1% v/v.
Anesthesia, Inhalation↗
A microprocessor-controlled gas mixing device.
This paper describes the use of solenoid valves for the production of binary gas mixtures. The system is controlled by a microprocessor and is capable of delivering accurate flows and concentrations of gases over the ranges commonly used in anaesthesia. Since the flow produced by the valves is pulsatile, a system for mixing the gases and smoothing the pulses is described. A back pressure regulator is fitted downstream of each of two mixing/damping chambers and a method of using this as a flow transducer is described. The advantages of this system over conventional rotameters are discussed.
Anesthesia, Inhalation↗
Nitrous oxide solubility in halothane and its effect on the output of vaporizers.
The absorption of nitrous oxide in halothane was studied by bubbling nitrous oxide and nitrous oxide/oxygen gas mixtures through a halothane bottle, using 100% oxygen as a control. The gas volume emerging from the halothane bottle was measured each minute, over a period of up to 15 minutes. When oxygen was used as a control gas, the averaged flow rate dropped slightly over the experimental period, due to the cooling of the halothane. However, in the presence of nitrous oxide, the initial flow rate of the gas emerging from the halothane bottle was greatly diminished, but then accelerated rapidly to reach that obtained with oxygen. The results suggested that nitrous oxide dissolved in large quantities in halothane, and the data are consistent with an Ostwald coefficient in excess of 4.0.
Absorption↗
Measurement of lung volume by multiple indicator dilution. Differences in apparent volumes of distribution of oxygen, nitrogen and argon.
A variant of a rebreathing/indicator-dilution technique was used to measure lung volume. Two or three indicators, which should have essentially the same pulmonary volume of distribution, were used. Gas compositions were measured by respiratory mass spectrometry and an extrapolation was used to allow for oxygen consumption. Carbon dioxide output, for which the extrapolation did not allow, caused considerable discrepancies between the apparent volumes of distribution of the three indicators: oxygen, nitrogen and argon. The discrepancies varied with the change in concentration imposed between the control and rebreathing periods. An iterative arithmetical correction is described, with its limitations. The approach offers a method of lung volume measurement which seems more applicable to anaesthesia than existing methods.
Argon↗
Measurement of lung volume by multiple indicator dilution. A simple model and its ramifications.
A simple, idealized rebreathing manoeuvre was simulated numerically on a Hewlett-Packard 9825A desk-top calculator. Approximate imitations were obtained of real-life observations of differences between apparent volumes of distribution of different indicators in the same rebreathing manoeuvre. The simulated patterns of discrepancy depended heavily on the uptake or output profiles which were assumed for carbon dioxide and nitrous oxide during the hypothetical rebreathing manoeuvres. The simulations pointed to the analytical algebraic expressions which explained the simulated patterns of discrepancy. A procedure was devised to correct for the effects of these uptakes or outputs. It depended on considering the mixing of oxygen, nitrogen and argon as if it were occurring in a separate part of the rebreathing system occupied only by these gases. The volume of this notional part would decrease linearly with time at a rate equal to the oxygen consumption.
Argon↗
Measurement of lung volume by multiple indicator dilution. Testing a modelled hypothesis with on-line computation.
Respiratory mass spectrometry and on-line computation were used to estimate lung volume in normal subjects by a rebreathing technique, using oxygen, nitrogen and argon as indicators. The computation had previously been tested in a numerical model. In real normal lungs, it performed well enough for practical purposes, despite recognizable oversimplifications in the modelling assumptions. The differences in estimate to be expected from different composition changes between control and rebreathing were less than 100 ml, and those from different indicators within a composition change were less than 50 ml. The use of two indicators in lung volume measurement allows a correction for volume changes attributable to carbon dioxide and nitrous oxide, and a third indicator provides a degree of quality control.
Humans↗
Effects of carrier gas composition on the output of six anaesthetic vaporizers.
The effect of carrier gas composition on the output of six anaesthetic vaporizers was studied using oxygen, nitrous oxide, helium and argon as the carrier gases. Vaporizer output was measured with an MGA 200 mass spectrometer and a Riken refractometer and, in addition, the pressure decrease across each vaporizer was determined simultaneously. A change in carrier gas composition produced both a transient and a steady state change in vaporizer output. The possible reasons for the changes in steady state output are discussed in relation to the construction of each vaporizer. The addition of nitrous oxide to the carrier gas produced changes of clinical significance only when the vaporizers were used at extreme dial settings and flow rates.
Anesthesia, Inhalation↗
Pulse polarography and intravascular oxygen electrodes.
The membranes covering standard polarographic in vivo PO2 electrodes were modified with a view to altering their response times. Their performance was examined under constant and pulsed polarization. Pulse polarization diminished the under-reading of the PO2 signal which occurred at low flow rates. The stability of the electrode signal was greatly improved by integrating the electrode current over the pulse period. The combination of thin membranes and pulsed polarography will provide a PO2 electrode with a fast time response without under-reading the true in vivo PO2.
Electrodes↗
A sandwich electrode for multi-gas analysis: a prototype.
A prototype composite electrode consisting of a metallized membrane "outer" cathode, and a conventional "inner" silver disc cathode, both sharing the same electrolyte, is described. Preliminary results indicate that the metallized membrane will act as an oxygen filter, allowing anaesthetic gases such as nitrous oxide, to pass through to the second (inner) cathode, where they may be reduced. Linear relationships exist for oxygen concentration and metallized membrane current; and for nitrous oxide concentration and the second cathodic current. Extension of this technique to measure both halothane and nitrous oxide on the second cathode, simultaneously with oxygen on the metallized membrane, is discussed.
Electrodes↗
Techniques for measuring the partial pressures of gases in the blood. Part II- in vivo measurements.
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Blood Gas Analysis↗
The polarographic measurement of halothane.
The polarographic reduction of halothane on silver cathodes was studied. Silver catalyses the reduction of halothane: two electrons are exchanged in the reduction process. Experiments using a rotating disc electrode showed a linear relationship between observed current and halothane concentration. A Silastic membrane-covered polarographic electrode was constructed using a silver cathode which gave a good linear response to changes in halothane concentration over the range 0.5% v/v halothane in nitrogen. Oxygen interferes with the measurement of halothane, but can be removed from a gas sample before analysis.
Electrodes↗
Techniques for measuring the partial pressures of gases in the blood. Part I--in vitro measurements.
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Blood Gas Analysis↗
On-line PO2 and PN2O analysis with an in vivo catheter electrode.
A technique was developed for the in vivo determination of PO2 and PN2O with a catheter electrode using double-pulse polarography. The method was evaluated in dog studies comparing readings from the electrode with those from a mass spectrometer employing an in vivo probe. The oxygen readings obtained from the catheter electrode were also compared with values obtained by conventional blood-gas analysis. Good agreement was observed between the electrode and the mass spectrometer for both PO2 and PN2O. Similar agreement was found between the electrode readings and blood-gas analysis for PO2. In the presence of halothane, the electrode over-read for both PO2 and PN2O; a remedy is suggested. The in vivo electrode provides an effective, less expensive, alternative to the mass spectrometer for the on-line measurement of PO2 and PN2O in vivo.
Animals↗