Department of Environmental and Occupational Medicine, University of Aberdeen, University Medical School, Foresterhill, Aberdeen AB25 2ZD, UK j.a.ross@abdn.ac.uk
Declaration of interest. Patented by the University of Aberdeen. If commercial production were to occur, the university and authors would benefit.
Accepted for publication: July 18, 2002
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Abstract |
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Methods. A sinusoidal pump set at stroke volume of 2 litres and a rate of 2022 bpm and cycling for 1 min in three was used to simulate breathing during the painful contractions of labour.
Results. The temperature inside a 10-litre capacity cylinder did not drecrease sufficiently to cause separation of the gas mixture. Temperature in the demand valve decreased to 15.5°C and this caused a small amount of liquid formation within the valve. Accordingly, the inspired concentration during the first breath of mixture in a cycle could be transiently as high as 0.55%. The concentration observed at the patient connection after the first breath varied between 0.17 and 0.28%.
Conclusions. The system delivered a clinically acceptable performance although further development to avoid liquid condensation is needed.
Br J Anaesth 2002; 89: 8204
Keywords: anaesthetics volatile, isoflurane; anaesthetics gases, nitrous oxide; anaesthetics, gases, premixed gases
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Introduction |
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Methods |
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Gas was sampled for analysis at both ends of the breathing hose and temperature in the gas flow was measured by thermistors at the same points. A third thermistor probe was placed on the surface of the cylinder at its mid-point and was shrouded by a block of polystyrene covered in turn by a 5 cm diameter patch of 4 mm closed cell neoprene. A fourth probe recorded the room temperature at workbench level. All thermistor probes (Yellow Springs Instruments series 400) were calibrated against a mercury thermometer (National Physical Laboratory). Gas sampling was intermittently switched between the patient end (hand-piece), and the valve end (elbow connector). The sinusoidal pump was set to a tidal volume of 2 litres and a rate of 21 strokes min1 and switched on for 1 min and off for 2 min by an automatic timer until the cylinder was empty. The experiment was conducted after the 12-litre cylinder had stood overnight and its external temperature was 20°C.
Internal cylinder temperature and isoflurane concentration
We used a 10-litre test cylinder, fitted with dip-tubes, as described previously.4 It contained 3528 g (14.20 MPa at 15°C) IN2O gas (containing 0.25% isoflurane). It was attached to the patient simulator via the EntonoxTM demand regulator and breathing hose and emptied by the sinusoidal pump as described above for the 12.2-litre cylinder, but at 22 strokes min1. The cylinder and its appendages rested on electronic weighing scales (Sartorius IB 31000P) and the weight of the cylinder assembly and the volume of expired gas were recorded for each 3-min cycle. Internal and external cylinder temperatures were continuously recorded.
Three thermistor probes (Yellow Springs Instruments series 400) were calibrated against a mercury thermometer (National Physical Laboratory) and used for temperature recording. One was placed in a 4.72 mm internal diameter dip tube with a closed end that reached to 6.5 mm above the cylinder bottom. This measured the gas temperature internally in the most dependent part of the cylinder. Another was placed on the external surface of the cylinder half way along its length. It was shielded with a block of polystyrene foam backed with a 9 cm diameter pad of closed cell 4 mm-thick neoprene foam. The remaining sensor was used to measure room temperature.
Minimum temperatures, at the end of each cycle, were measured from the recording chart. The isoflurane concentration was continuously recorded alternately, from the distal (patient) end of the breathing hose and from inside the bottom of the cylinder via the long dip-tube.
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Results |
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Simulated use experiments: measurement of internal temperature and isoflurane concentration during cylinder emptying experiment
The experiment was conducted after the special 10-litre cylinder had stood overnight and its internal temperature was 16.2°C. During the experiment, room temperature rose from 16.1°C to a maximum of 18.9°C. At the end of the experiment when the pump was labouring to obtain gas through the system the pump was switched off. There was sufficient residual pressure within the cylinder to release 7 litres of gas containing 0.25% isoflurane, confirming that there had been no change in the concentration of isoflurane within the cylinder from beginning to end. Excluding the residual 7 litres, the total expired gas volume was 2460 litres for 56 cycles of 3 min each, equivalent to 43.93 litres for every minute the pump was on, representing constant emptying rate of 14.64 litres min1. The rate of cylinder emptying was linear as was the weight reduction of the cylinder contents, and was not affected by the marked frosting of the demand regulator, which occurred during the first half of the experiment. From the twelfth cycle onwards, the falling internal temperature curve from the experiment is shown with the dew curve4 in Figure 4. It shows that the temperature fall during the experiment did not reach any point where phase separation could occur. This was confirmed by the consistency of the isoflurane sampling record throughout the experiment, both from the patient end of the breathing hose and from the bottom of the cylinder via the long dip tube. The concentration of isoflurane was 0.25% in the cylinder gas both at the start and at the end of the experiment.
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Discussion |
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We measured internal cylinder temperature during simulated use in order to see whether it would drop below the cricondentherm. (The cricondentherm is the maximum temperature of a multi-component system at which liquid and vapour can exist in equilibrium, and above which liquid cannot be formed regardless of pressure. The cricondenbar is the maximum pressure at which liquid and vapour can co-exist.6 At a temperature above its cricondentherm or at a pressure above its cricondenbar a mixture must be in a homogenous phase.) This did not happen and there was no separation of the isoflurane component of the gas mixture. Accordingly, we did not expect separation of the mixture during clinical use and residual gas analysis of cylinders returned from clinical use confirmed this.
The temperature decrease in the cylinder depends on a number of factors. Gas expansion in the cylinder causes an adiabatic decrease in temperature that is related to the reduction in pressure. Gas expansion through the demand valve causes local cooling and heat will be lost from the cylinder contents into the metal so cooled. On the other hand heat passes into the cylinder contents from the cylinder itself initially and then from the environment through the cylinder wall to offset these two factors. The cylinder used for this experiment was 10 litres nominal water capacity. Clearly the temperature changes in either larger or smaller cylinders will be different. In larger cylinders, the rate of pressure decrease will be less although the heat lost through a cooling demand valve will be the same. The heat available from the metal of the cylinder and through the cylinder wall will be greater because of its larger mass and surface area. As temperature decrease is related to pressure decrease this will be less in a larger cylinder. The converse is true for small cylinders and if breathing volumes are kept the same, the temperature in a small cylinder will decrease much more than in a large cylinder. Therefore, although our experiment indicates that condensation of isoflurane in a 10-litre cylinder is very unlikely, the same may not be true for smaller cylinders and further study would be needed if such cylinders were to be used clinically.
Saturated vapour pressure is temperature dependent and during the simulated use experiments the airflow temperature dropped to 15°C at the demand valve end of the administration circuit. During the use of the proprietary mixture EntonoxTM the temperature of the demand valve may decrease to less than 20°C without affecting the function of the valve or the composition of the mixture delivered.7 At 15°C the saturated vapour pressure of isoflurane is 3.5 kPa and that of nitrous oxide is 2112 kPa.8 The pressure in the first stage of the demand valve is 1461 kPa and so the partial pressure of 0.25% isoflurane is 3.65 kPa and 50% nitrous oxide is 730.5 kPa. At temperatures of the order of 15°C and less, therefore, there is likely to be some condensation of isoflurane in the demand valve. Also, because of the much higher pressure, there is certain to be small amounts of both liquid nitrous oxide and isoflurane in the part of the regulator before the first stage valve. This explains the varying concentration of isoflurane observed in the breathing circuit. During each stroke of the pump, the temperature in the demand valve is reduced. Once the saturated vapour pressure becomes less than the partial pressure of isoflurane in the mixture, some condensation occurs on the cold surfaces of the demand valve. During the 2 min rest phase, the demand valve temperature rises and, as the condensate vaporizes, the concentration of isoflurane in the gas sampled by the analyser rises. However, the maximum concentration seen as a result of this effect was 0.55% at the valve end. The peak concentration seen at the patient end of the hose was transient and affected the first breath of a cycle only.
After the transient peak at the start of a breathing period, the concentration at the patient end of the hose varied between 0.17 and 0.28% and this is comparable with the performance of an Oxford Miniature Vaporiser under similar circumstances.1
While the variation in delivered concentration identified here has caused no problem in the clinical use of IN2O, the condensation of isoflurane that causes it is undesirable. Although the present system had an acceptable performance for clinical work, an administration system that prevents this effect would be desirable.
This agent, which has been suggested for use in pain relief in labour,9 may be criticised for causing local atmospheric contamination. Although not the topic of this paper, the practical use of such agents should consider environmental control. The United Kingdom Control of Substances Harmful to Health Regulations (1999) place a statutory obligation on the users of anaesthetics to control the exposure of hospital staff within occupational exposure limits and lower if this is practicable. The addition of isoflurane to premixed 50% nitrous oxide in oxygen makes atmosphere control even more necessary than with the use of nitrous oxide alone as there is a requirement to consider the additive effects of such agent mixtures. We found it was possible to collect the expired gas from the expiratory valve of the breathing apparatus in a simple and convenient way using the wall suction equipment.9 Other active scavenging systems would be as effective. In addition, our risk assessment of the use of the agent in a maternity suite did not reveal any problem before the clinical use of this agent.10
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Acknowledgement |
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References |
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6 Ahmed TH. Hydrocarbon phase behavior. In: Chilingar GV, ed. Contributions in Petroleum Geology and Engineering. Houston, Texas: Gulf Publishing Company, 1989; 7: 135
7 Hill DW. Pressure gauges and regulators. In: Hill DW, ed. Physics Applied to Anaesthesia. London: Butterworths, 1972; 4769
8 Rodgers RC, Hill GE. Equations for vapour pressure versus temperature: derivation and use of the Antoine equation on a hand-held programmable calculator. Br J Anaesth 1978; 50: 41524[Abstract]
9 Ross JAS, Tunstall ME, Campbell DM, Lemon JS. The use of 0.25% isoflurane premixed in 50% nitrous oxide and oxygen for pain relief in labour. Anaesthesia 1999; 54: 116672.[ISI][Medline]
10 Ross JAS. Isoflurane Entonox mixtures for pain relief during labour. Anaesthesia 2000; 55: 7112[Medline]