Design of a test system for fast time response fibre optic oxygen sensors.

A test system has been developed that can be used to calibrate and determine the time response, linearity and temperature sensitivity of a fibre optic oxygen sensor. The simple system obviates the need for precision gas standards and the requirement to generate a true square wave step response, which is seldom achievable. The sensor is mounted in a small chamber containing air or a known fraction of oxygen. By means of a computer-controlled switch, the absolute pressure within the chamber can be changed rapidly to a new steady state value. The partial pressure of oxygen changes in direct proportion to the absolute pressure, and so the accuracy and linearity and response time of the PO(2) calibration are limited only by those of the absolute pressure sensor. The temperature sensitivity of a commercial sensor and a means of correction are also described.

A real-time algorithm to improve the response time of a clinical multigas analyser.

OBJECTIVE: An algorithm to improve the response time of a clinical respiratory multigas analyser is presented. METHODS: The algorithm involves the application of a second order differential equation to the analyser gas output signals in real-time. The adjusted analyser output signals are compared with those of a quadrupole respiratory mass spectrometer sampling and analysing simultaneously. RESULTS: Our results show a close correlation between the adjusted clinical gas analyser and the mass spectrometer signals. Lung volumes derived from a non-invasive sinusoidal inert gas forcing technique, in a model test lung, using the adjusted clinical gas analyser and the mass spectrometer signals demonstrated comparable results. CONCLUSIONS: The algorithm provides an improvement on the relatively slow response times of the clinical gas analyser for breath-by-breath time-dependent applications. The same algorithm can also be applied to other instruments which have slow response times.

Overfill testing of anaesthetic vaporizers.

We tested six anaesthetic vaporizers with keyed filler adaptors to see if it was possible to overfill them. For those vaporizers which could be overfilled, the maximum level of overfill was determined and the effect of overfilling on the vaporizer output concentration was measured. Three of the vaporizers, the TEC 4, PPV Mk 1 and MIE Vapamasta 5, could be overfilled. In the case of the TEC 4 and PPV vaporizers, overfilling by more than 100 ml caused a large increase in the vaporizer output concentration. Overfilling the Vapamasta 5 by this amount caused the output concentration to decrease.

The effect of dyshemoglobins on pulse oximetry: Part I, Theoretical approach and Part II, Experimental results using an in vitro test system.

Pulse oximeters are known to be inaccurate in the presence of elevated concentrations of carboxyhemoglobin and methemoglobin. This paper attempts to alleviate some of the confusion that exists between fractional and functional saturation, and to clarify the comparison of each with SpO2. A series of theoretical relationships between pulse oximeter reading (SpO2) and actual oxygen saturation (both fractional and functional) is derived using simple absorption theory. The theoretical relationships are checked using an experimental in vitro test system. This consists of a blood circuit containing a model finger, capable of simulating the pulsatile transmission signals through a real finger. Theoretical predictions and experimental results are compared and are found to agree well in the presence of carboxyhemoglobin, but less well with methemoglobin. Possible reasons are discussed.

An evaluation of the Brüel and Kjaer monitor 1304.

A laboratory evaluation was performed on the Brüel and Kjaer multigas monitor 1304, incorporating a pulse oximeter. The instrument was tested for accuracy, stability, response and delay times, frequency response and the effects of water vapour, alcohol, cyclopropane and sevoflurane. The instrument's performance was found to be within or very close to the manufacturer's specifications for accuracy, stability and response and delay times. It was unaffected by water vapour and alcohol and the effect of cyclopropane on the vapour channel was lower than has been reported for other analysers. The response to sevoflurane was of the same order as that of the other vapours. A 90% response to square wave changes of gas composition was maintained up to 60 breaths.min-1 for CO2, O2, and N2O and up to 40 breaths.min-1 for the vapours when the nafion sampling tube was used.

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.

Evaluation of a multigas anaesthetic monitor: the Datex Capnomac.

A laboratory investigation was carried out to evaluate the performance of the Datex Capnomac multigas anaesthetic agent analyser, with particular emphasis on accuracy, response and delay times, zero and gain stability and interference from water vapour. The analysis of anaesthetic vapours was less accurate than the analysis of CO2, O2 and N2O, but acceptable for clinical use. The response to square wave changes in gas composition was accurate at frequencies up to 60 per minute for CO2 and 30 per minute for O2, but with N2O and the anaesthetic vapours there was a decrease in accuracy at frequencies above 20 breaths/minute. The instrument appeared to be unaffected by water vapour.

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.

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.

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.

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.

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.