The Development of Gas Mixtures

John A S Ross

 University of Aberdeen Medical School

The early history of gas mixtures is that of the introduction of nitrous oxide into anaesthetic practice. First synthesised by Joseph Priestley in the 18 century, nitrous oxide became established in anaesthetic practice in the middle of the 19 . From the first, it was clear that the potency of nitrous oxide was such as to necessitate the induction of hypoxia as part of the anaesthetic technique. Early attempts to produce pressurised mixtures of oxygen and nitrous oxide in order to make equipment more portable and to avoid hypoxia were abandoned because of this. Stanislaus Klikowitsch, however, found that nitrous oxide was a useful pain relieving agent in labour at concentrations which allowed the administration of adequate oxygen and he used uncompressed premixed nitrous oxide mixtures. His observation and practice was never widely applied and it was only in the 1960s that it was perceived that the hypoxia induced by the use of nitrous oxide in dentistry and maternity was unacceptable. This coincided with Tunstall's suggestion that 50% nitrous oxide in oxygen premixed at high pressure would be both possible and useful in the provision of pain relief.

 

The saturated vapour pressure of nitrous oxide is 4.97 MPa at room temperature. But the partial pressure in Entonox at 137 bar it is 6.8 MPa. If ideal gas behaviour is assumed, the nitrous oxide component of Entonox should be a liquid at this pressure and temperature. Under these conditions both nitrous oxide and oxygen exists near or above their critical pressure and have behaviour that departs from ideal. The situation is very complex but approximates to the concept of nitrous oxide dissolving in the. oxygen gas phase. Entonox is stable in the gas phase down to -7°C.

Used in labour, 50% nitrous oxide in oxygen fails to provide adequate pain relief for a number of cases in whom 70% is more effective. 70% nitrous oxide in oxygen is unstable at 10°C and so is not a suitable combination for premixing. A mixture of 0.25% isoflurane in Entonox provides the desired increase in potency and it is possible to dispense this mixture, Isoxane, in high pressure cylinders. It is self-administered using the standard Entonox demand valve.

Entonox is also used to relieve pain in short surgical procedures such as wound dressings. However, the repeated administration of nitrous oxide is toxic. The same is not true for the volatile anaesthetic agents. A mixture of 0.25% isoflurane, 1% desflurane, 60% oxygen in nitrogen has a MAC value of 0.3 and seems to produce a useful degree of sedation ofrelativie rapid onset. This mixture is stable at room temperature and down to 0°C for a full cylinder. The saturated vapour pressure for desflurane at room temperature and pressure is 93.3 kPa and for isoflurane 31.7 kPa. Therefore both agents have an appreciably higher saturated vapour pressure in this compressed mixture and behave like nitrous oxide in this respect.. In clinical use the mixture has provided a significant degree of relief from pain and anxiety during repeated painful procedures.

 

 

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Automatic ventilator Driven, Fully Closed Breathing System

J Dingley

Cardiff

- Initial design for measurement of circulating blood volume by the inhaled carbon monoxide method on the ITU.

- Possible further development as part of a fully closed anaesthesia system.

A device is described in the lecture, which can be placed between a mechanical ventilator and a tracheal tube for ventilation of the lungs. It functions as a fully closed system, with CO; removed and oxygen automatically added to match uptake.

It was first used in critically ill ventilated patients to make the carbon monoxide (CO) method for measuring circulating blood volume more user friendly. We currently estimate this from pressures, urine output, haematocrit etc. However these correlate poorly with blood volume when this is measured by indicator dilution techniques.

Basis of CO method as used in research study:

- Measure baseline carboxyhaemoglobin, weight and haemoglobin concentration.

- Calculate CO mass needed to produce approximate increase in carboxyhaemoglobin of 2.5%. Convert this to a volume of gas correcting for the barometric pressure and the temperature of the ITU

- Inject this volume of CO into closed breathing system. Rebreathe until full uptake.

- Know mass of CO given and rise in carboxyhaemoglobin produced, so calculate mass of Hb in circulation. Knowing Hb concentration allows estimation of blood volume. Advantages of method: No isotopes, co-oximeters commonplace, small dose of CO.

Design: Simultaneous blood volume estimation by CO method and -⁵¹Cr labelled red cell method (established method) Both label red cells so should give similar results.

Results: 15 measurements in 14 patients.  

Median age	60yrs (22-87)
Median Apache II	18    (10-26)
Median weight	62.5kg (50-100)

 

Results compared by Bland and Altman method. Mean blood volume by both methods was 53 19ml. Bias (mean difference between the blood volume measurement by the two methods) was 284.2ml ± 359ml; the limits of agreement (mean difference ± 2SD) were -433.8ml and 1002.2ml. So 95% of expected differences will lie between those limits. The CO method gives slightly larger volume estimates than labelled red cells.

Descriptions are given of various forms of the device. It could be linked to a circle anaesthetic system as a fully closed Oxygen/Air/Volatile delivery system. Volatile agent would be delivered by closed loop computer controlled liquid injection. Such an arrangement is described. It can also volumetrically measure 02 consumption. Would be much cheaper than Drager Physioflex closed system, and easier to understand.

Cost per hour (1996)  1 MAC 

Fresh gas flow:	0.2 l.min-1	l.0 l.min-1	4.0 l.min-1
Isoflurane	£2.46	£3.74	£8.70
Sevoflurane	£2.41	£5.36	£16.24
Desflurane	£1.96	£5.06	£16.62

Sevoflurane and Desflurane are ideal agents for such a system on grounds of cost!

 

 

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New concepts in low flow anaesthesia - the Humphrey ADE/Circle system

David Humphrey

Durban,  South Africa

Introduction:

The Humphrey ADE system as a semi-closed system combines the advantages of Mapleson A, D and E systems into one easy-to-use system. It requires low fresh gas flows (FGF) in all modes of use for spontaneous, manual and controlled ventilation in adults and children. Such FGFs are typically an average of 3-4 1/min for adults and 2-3 1/min for children. These FGFs are much lower than those used with the Bain and T-piece for spontaneous respiration, allowing significant savings to be made. These low FGFs also make the savings to be achieved by recycling gases through a soda-lime canister minimal especially during the first hour of anaesthesia. However with the new insoluble agents sevoflurane and desflurane, anaesthetic tensions are quickly reached, making these agents particularly suitable for low-flow circle systems after a relatively short induction period.

As a consequence the Humphrey ADE system has been designed to incorporate a new soda-lime canister (Fig 1). The anaesthetist can make his own choice whether he wishes to use the semi-closed or circle modes.

Description of the Humphrey ADE /Circle system

The canister [2] is very quickly and simply attached to the main body of the Humphrey ADE system [1] by two locking nuts (not shown). The two tubes are re-attached at [5]. The system is then used in exactly the same way as in the semi-closed mode, namely with the lever [15] "UP" for spontaneous and manual respiration (Fig la), and "DOWN" for controlled ventilation Fig Ib).

 

 

           Figure 1a                                                                                                                Figure 1b

 

During spontaneous respiration (Fig 1 a) fresh gas flows into the system at the inlet (9), through the canister and one way valve [4] to the patient [5]. The lever allows fresh gas to flow into the reservoir bag [10] which is on the inspiratory limb. On the expiratory side alveolar gas passes from the patient through the one-way valve [3] and into the soda-lime chamber. Expired gas passes down and through the soda-lime to return to the inspiratory limb through the sieve plate [8]. Excess gases vent through the exhaust valve [11].

During controlled ventilation the position of the lever (Fig Ib) causes the exhaust valve [11] and reservoir bag [10] to be automatically excluded, while the ventilator attached at [12] is brought into the system. Fresh gas now flows directly to the patient, while expired gases are re-cycled as described above. The ventilator attached at [12] acts as a piston, pushing expired gas through the canister to the patient. The pressure-limiting valve [13] is always in circuit in all modes.

The canister holds 500gms of soda-lime, while the circuit volume is only 3.5 litres. These characteristics allow for low resistance and a very much quicker response to changes in anaesthetic concentration when compared to the larger absorbers. The canister last for about 8-12 hours, depending on the FGF; changes of soda-lime could be done as a routine every second day at a cost of about £1. A disposable pre-filled canister is also available.

Conclusion:

The Humphrey ADE system allows low FGFs to be used even without carbon dioxide absorption. The addition of a soda-lime canister now allows re-cycling to be easily effected with significant savings with desflurane and sevoflurane, though such savings are best achieved with FGFs 2 1/min or less.

 

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Minimum Fresh Gas Flows for Non-Absorber Breathing Systems

G Meakin

 

Royal Manchester Children's Hospital Manchester, UK

In this lecture I will demonstrate the minimum fresh gas flows required to prevent CO; accumulation in non-absorber breathing systems, and derive formulae relating these to body weight.

The non-absorber breathing systems, or the semi-closed systems, were classified by Mapleson in 1954 into five types A-E [3]. The most important of these are A, the Magill system, and D which exists in two forms, the Barn and me Jackson Rees. The two forms of the Mapleson D system are functionally identical and are frequently referred to jointly as die T-piece systems [4]. During spontaneous ventilation, me absorber breathing systems rely on an adequate fresh gas flow to prevent rebreathing. To prevent rebreathing completely, System A requires a fresh gas flow equal to the patient's minute volume, while systems B-E require fresh gas flows which are 2-3 times as great. The greater efficiency of the Mapleson A compared with the other systems can largely be explained by the presence of tile reservoir bag on the inspiratory limb of the system. This permits the flow of gas to the patient to be interrupted during expiration, thereby keeping the fresh gas and expired gases separate. Further economy in the use of fresh gas is possible by allowing rebreathing of dead space gas stored in the inspiratory limb of the Mapleson A system when the fresh gas flow is equal to the patients alveolar ventilation.

The original suggestion that the T-piece systems require a fresh gas flow greater than twice minute volume to prevent rebreathing during spontaneous ventilation was based on a theoretical model employing a square wave of respiration [3], When more realistic models incorporating changes in the respiratory waveform, respiratory depression due to anaesthesia and allowing for a small amount of rebreathing to occur into the dead space at end-inspiration, it is clear that lower flow rates can be

rebreathing. To prevent rebreathing completely, System A requires a fresh gas flow equal to the patient's minute volume, while systems B-E require fresh gas flows which are 2-3 times as great. The greater efficiency of the Mapleson A compared with the other systems can largely be explained by the presence of the reservoir bag on the inspiratory limb of the system. This permits the flow of gas to the patient to be interrupted during expiration, thereby keeping the fresh gas and expired gases separate. Further economy in the use of fresh gas is possible by allowing rebreathing of dead space gas stored in the inspiratory limb of the Mapleson A system when the fresh gas flow is equal to the patients alveolar ventilation.

The original suggestion that the T-piece systems require a fresh gas flow greater than twice minute volume to prevent rebreathing during spontaneous ventilation was based on a theoretical model employing a square wave of respiration [3]. When more realistic models incorporating changes in the respiratory waveform, respiratory depression due to anaesthesia and allowing for a small amount of rebreathing to occur into the dead space at end-inspiration, it is clear that lower flow rates can be used. Meakin and Coates measured the respiratory effects of reducing the fresh gas flow from two to one tunes the predicted minute volume in 12 children breathing from a Bain system [5]. They found no significant effect on minute volume or end-tidal CO; tension, because the degree of rebreathing was small enough (12.6%) to be accommodated in the dead space at the end of inspiration. The formula Vi = √weight yields flow rates which are 25% greater than the predicted minute volume and is suitable for patients breathing spontaneously from a T-piece [6].

A number of problems can be identified with the Magill system during controlled ventilation. Firstly, the expiratory valve is sited inconveniently close to the head of

the patient; secondly, the valve opens during lung inflation reducing the efficiency of die system by allowing fresh gas and dead space gas to be vented. The first problem can be overcome by adding an expiratory limb to re-site me valve on the anaesthetic machine [7]. The second problem can be solved by enclosing the reservoir bag and the expiratory valve a rigid plastic box. The system that results is called an enclosed Mapleson A system or Enclosed Afferent Reservoir system (EAR) [8-11]; it is available commercially as the Mapleson A mode of the MIE Garden Ventmasta (Vickers Medical Ltd) [12].

ENCLOSED MAPLESON A SYSTEM

 

 

 

In the enclosed Mapleson A system, fresh gas enters a reservoir bag (bellows) enclosed in a transparent plastic container on the inspiratory limb of the system. The expiratory limb passes to an expiratory valve which discharges to the interior of the container. Controlled ventilation is effected by increasing the pressure inside the container by means of a self-inflating bag or a ventilator. The increased pressure in the container squeezes the reservoir bag, while at the same time it holds the expiratory valve tightly shut.

 

hi this way, all the fresh gas and any dead space gas stored in the inspiratory tube is delivered to the patient. The valve opens again at end-expiration allowing alveolar gas to be vented.

Meakin and colleagues determined the efficiency and fresh gas requirement of an enclosed Mapleson A system during controlled ventilation in children [11]. Provided the minute volume to fresh gas ratio was maintained above 1.5, the system was 92% efficient in the use of fresh gas, and normocapnia or mild hypocapnia was produced accurately with a fresh gas flow equal to the predicted alveolar ventilation (0.6 x ^weight), The efficiency of the enclosed Mapleson A system during controlled ventilation was about one-third greater than tliat of the T-piece system studied previously by Nightingale and Lambert, which required a fresh gas flow equal to the predicted minute volume (0.8 ^/weight) [1 1,13]. This finding is consistent with the theory that the greater efficiency of the enclosed Mapleson A system is due to conservation of deadspace gas.

FRESH GAS REQUIREMENTS OF NON-ABSORBER BREATHING SYSTEMS

Enclosed Mapleson A (EAR)

FGF for controlled or spontaneous ventilation (predicted VA) = 0.6 x √weight (kg) T-pieces (Bain. Jackson Rees)

FGF for controlled ventilation (predicted VE) = 0.8 x √weight (kg) FGF for spontaneous ventilation (> predicted Vi;) =√/weight (kg)

REFERENCES

1.    Bruce W. Anaesthetic breathing systems. In: Scurr C. Feldman S. Soni N (eds). Scientific Foundations of Anaesthesia. 4th edn. Oxford. Heinmann Medical Books 1990, 673-687.

2.    Zbinden AM. Feigenwinter P. Hutmacher M. Fresh gas utilization of eight circle systems. British .Journal of Anaesthesia 1991. 67: 492-499.

3.    Mapleson WW. The elimination of rebreathing in various semi-closed anaesthetic systems. British .Journal of Anaesthesia 1954, 26: 323-332.

4.    Rose DK. Byrick RJ, Froese AB. Carbon dioxide elimination during spontaneous ventilation with a modified Mapleson D svstein: studies in a lung model. Canadian Anaesthetist's Society Journal 1978, 25: 353-365.

5.    Meakin G, Coates AL. An evaluation of rebreathing with the Bain system during anaesthesia with spontaneous ventilation. British .Journal of Anaesthesia 1983, 55: 487-496.

6.    Meakin G. Fresh gas requirement of the Bain circuit. British .Journal of Anaesthesia 1991. 67: 663,

7.    Lack JA. Theatre pollution control. Anaesthesia 1976. 31: 259-262.

8.    Fletcher IR, Garden E. Healy TEJ. Poole TR. The MIE Garden ventilator. Anaesthesia 1983,38: 1082-1089.

9.    Miller DM, Miller JC. Enclosed afferent reservoir breathing; systems.

Description and evaluation. British Journal of Anaesthesia 1988. 60: 469-475.

10.   Miller DM. Breathing systems for use in anaesthesia. Evaluation using a physical lung model and classification. British Journal of Anaesthesia 1988, 60: 555-564^

11.   Meakin G, Jennings AD, Beatty PCW, Healy TEJ. Fresh gas requirements of an enclosed afferent reservoir breathing system during controlled ventilation in children. British Journal of Anaesthesia 1992, 68: 43-47.

12.   Ban-ie JR, Meakin G, Campbell IT, Beatty PCW, Healy TEJ. Efficiency of the Carden "Ventmasta" in A and D modes during controlled ventilation of children. British Journal of Anaesthesia 1994: 73: 453-457.

13.   Nightingale DA, Lambert TF. The behaviour of the Jackson Rees circuit with controlled ventilation. In: Proceedings of the Association of Paediatric Anaesthetists Annual Scientific Meeting 1978, 21-24.