Overfill testing of anaesthetic vaporizers

We tested six anaesthetic vaporizers with keyed filler adaptorsto see if it was possible to overfill them. For those vaporizerswhich could be overfilled, the maximum level of overfill wasdetermined and the effect of overfilling on the vaporizer outputconcentration was measured. Three of the vaporizers, the TEC4, PPV Mk 1 and MIE Vapamasta 5, could be overfilled. In thecase of the TEC 4 and PPV vaporizers, overfilling by more than100 ml caused a large increase in the vaporizer output concentration.Overfilling the Vapamasta 5 by this amount caused the outputconcentration to decrease.     

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.     

An evaluation of the Brüel and Kjær 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.     

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.     

A MICROPROCESSOR-CONTROLLED ANAESTHETIC VAPORIZER

A microprocessor-controlled anaesthetic vaporizer is described.Fresh gas is mixed in the correct proportions using two pulsedsolenoid valves and a proportion of this passes through a thirdpulsed solenoid valve and is bubbled through liquid halothane.The temperature of the liquid agent is measured and the pulsefrequency is modified to give the correct vapour concentrationfor the set flow rate and measured temperature. Initially, thevapour was produced by bubbling fresh gas through the agentin a conventional halothane bottle. However, because of thelarge liquid volume available, nitrous oxide was found to dissolvein large quantities in the halothane. A small volume vaporizerwhich was continually replenished from a reservoir was designed.Measurements of the vapour concentrations emerging from sucha vaporizer were made and were found to agree with the set values±0.1% v/v.     

A NEW MICROPROCESSOR-CONTROLLED ANAESTHETIC MACHINE

This paper describes the development of a microprocessor controlledanaesthetic machine comprising an integrated anaesthetic apparatusand monitoring system. Following prolonged reliability trialsin the laboratory, changes have been made to major componentswhich were described in earlier publications.     

AN AUTOMATED INTERFEROMETER FOR THE ANALYSIS OF ANAESTHETIC GAS MIXTURES

A microprocessor-controlled interferometer is described. Theeyepiece of a conventional Jamin type interferometer has beenreplaced by an array of photocells which records the intensityacross the interference pattern. Mathematical correlation proceduresare used to locate the principal interference pattern maximumand, by sequential analysis of a fresh gas mixture followedby fresh gas plus vapour, it is possible to determine both oxygenand vapour concentrations. The instrument was used to analysemixtures of oxygen and nitrous oxide and also oxygen, nitrousoxide plus halothane. It was found that the oxygen concentrationcould be determined to an accuracy of ±1% v/v and thevapour concentration to ±0.1% v/v. The instrument issuitable for monitoring concentrations delivered by an anaestheticmachine and may be included in a microprocessor-controlled anaestheticmachine.     

EFFECTS OF CARRIER GAS COMPOSITION ON THE OUTPUT OF SIX ANAESTHETIC VAPORIZERS

The effect of carrier gas composition on the output of six anaestheticvaporizers was studied using oxygen, nitrous oxide, helium andargon as the carrier gases. Vaporizer output was measured withan MGA 200 mass spectrometer and a Riken refractometer and,in addition, the pressure decrease across each vaporizer wasdetermined simultaneously. A change in carrier gas compositionproduced both a transient and a steady state change in vaporizeroutput. The possible reasons for the changes in steady stateoutput are discussed in relation to the construction of eachvaporizer. The addition of nitrous oxide to the carrier gasproduced changes of clinical significance only when the vaporizerswere used at extreme dial settings and flow rates.     

A MICROPROCESSOR-CONTROLLED GAS MIXING DEVICE

This paper describes the use of solenoid valves for the productionof binary gas mixtures. The system is controlled by a microprocessorand is capable of delivering accurate flows and concen-trationsof gases over the ranges commonly used in anaesthesia. Sincethe flow produced by the valves is pulsatile, a system for mixingthe gases and smoothing the pulses is described. A back pressureregulator is fitted downstream of each of two mixing/dampingchambers and a method of using this as a flow transducer isdescribed. The advantages of this system over conventional rotametersare discussed.