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Biomedical subjects

C E Hahn

Publications and source records attributed to C E Hahn.

At least 19 recordsLinked to original sources
PubMed1992

In vitro performance test system for pulse oximeters.

An in vitro system was developed capable of testing the accuracy and reproducibility of pulse oximeter readings. The pulse oximeter probe receives signals through a pulsating blood cuvette. The development of the design of the cuvette is described. Using the final design (or 'model finger'), a comparison is made between readings from a Datex Satlite pulse oximeter (SpO2) and saturation values obtained by use of a multi-wavelength bench oximeter (SaO2). Linear regression analysis of the data gives SpO2 = 0.88 SaO2 + 11.2 (r = 0.979, p < 0.001).

Fingers↗
PubMed1992

Response of 10 pulse oximeters to an in vitro test system.

Pulse oximeters are often used in situations in which severe hypoxaemia may occur. We have developed an in vitro system to test the accuracy of pulse oximeter calibration. The probe of 10 different oximeters was attached to a model finger in an in vitro blood circuit, and pulse oximeter readings (SpO2) were compared with multi-wavelength in vitro oximeter readings (SO2) over a range of SO2 values from 50 to 100%. The oximeters tested varied widely in their accuracy and linearity. We conclude that the system can test the accuracy, reproducibility and linearity of response of pulse oximeter readings at low oxyhaemoglobin saturations.

Calibration↗
PubMed1989

A new microprocessor-controlled anaesthetic machine.

This paper describes the development of a microprocessor controlled anaesthetic machine comprising an integrated anaesthetic apparatus and monitoring system. Following prolonged reliability trials in the laboratory, changes have been made to major components which were described in earlier publications.

Anesthesiology↗
PubMed1988

Halothane interference at an amperometric oxygen electrode: the development of an oxygen/halothane sensor.

The role of electrode materials in the reduction of halothane and oxygen mixtures at rotating and stationary electrodes is considered. Halothane is found to be electroactive within the available electrode-potential range on silver and on gold. Double potential step experiments are considered in order to isolate the individual halothane and oxygen signals. A double integration, single potential step technique is evaluated for the quantitative simultaneous measurements of oxygen and halothane on gold. The influence of electrode purity is discussed in terms of a reaction mechanism and electrode output error.

Calibration↗
PubMed1988

An automated interferometer for the analysis of anaesthetic gas mixtures.

A microprocessor-controlled interferometer is described. The eyepiece of a conventional Jamin type interferometer has been replaced by an array of photocells which records the intensity across the interference pattern. Mathematical correlation procedures are used to locate the principal interference pattern maximum and, by sequential analysis of a fresh gas mixture followed by fresh gas plus vapour, it is possible to determine both oxygen and vapour concentrations. The instrument was used to analyse mixtures of oxygen and nitrous oxide and also oxygen, nitrous oxide plus halothane. It was found that the oxygen concentration could be determined to an accuracy of +/- 1% v/v and the vapour concentration to +/- 0.1% v/v. The instrument is suitable for monitoring concentrations delivered by an anaesthetic machine and may be included in a microprocessor-controlled anaesthetic machine.

Analog-Digital Conversion↗
PubMed1988

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↗
PubMed1988

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↗
PubMed1987

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↗
PubMed1986

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↗
PubMed1986

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↗
PubMed1985

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↗
PubMed1984

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↗
PubMed1984

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↗
PubMed1984

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↗