Manchester 1994

Before describing the methods of delivery anaesthetics during low flow / closed circuit conditions, certain general principles have to be discussed. Vapour is the gaseous phase of a substance that is liquid at room temperature and atmospheric pressure. When placed in a closed container, molecules of a potent inhaled anaesthetic in liquid form generate vapour, exerting a vapour pressure. At equilibrium a saturated vapour pressure at a particular temperature (and atmospheric pressure) is pressure. The units of vapour concentration are expressed either as absolute pressure (mm Hg) or in percent (vol %). The latent heat of vaporization is the amount of heat for convening a gram of liquid into vapour.

An anaesthesia vaporizer is a device that facilitates the change of liquid anaesthetic into its vapor phase and adds a controlled amount of this vapor into the flow of gases passing to the patient circuit. The most used contemporary anaesthetic vaporizers (e.g. Tec series from Ohmeda, Vapor from Drager) are concentration calibrated and of the variable bypass design. Older vaporizers (e.g. Copper Kettle or Vernitrol) are measured flow vaporizers, whereby a separate flow of oxygen passes through the vaporizer.

In both types a saturated vapour is created in the vaporizing chamber, and subsequently diluted to the useful clinical concentration. Temperature compensation (to compensate for the heat of vaporization and eventual temperature decrease) is included in the modem variable bypass vaporizers, which function on the principle of splitting ratio. This will be described in detail. Carrier gas, as well as fluctuating back pressure can affect the vaporiser output. The variable bypass vaporizers can be used for all modern inhalational anaesthetics except for desflurane where another principle is used.

Older variable bypass vaporizers did not give a stable output at low flows and were therefore unsuitable for low flow techniques. The output of the modern vaporizer is virtually independent of the fresh gas flow rate, but the maximal output of the vaporizer is identical for high and low gas flow. These conventional vaporizers cannot deliver a sufficient quantity of anaesthetic at low fresh gas flow, particularly at the beginning of low flow or minimal flow techniques.

A novel approach is used in the form of controlled anaesthetic vapour administration (Gambro, Engstrom). In this vaporizer liquid anaesthetic is forced at constant pressure into the electrically heated vaporizing chamber, and a 100% anaesthetic vapour is generated. Delivery of vapour from the chamber is controlled by an electromagnetic valve that pulsates to permit the pressure of the driving gas to deliver known quantities of anaesthetic vapour into the fresh gas supply. The measured fresh gas flow (hot wire anemometer) and the set concentration determine the pulsated quantities of anaesthetic vapour. The vaporizer (max output 8%) is specially designed for minimal flow conditions. This vaporizer is suitable for all modem potent inhalation anaesthetics including sevoflurane) and is also adapted for desflurane.

Desflurane has particular physical properties (saturated vapour pressure of664 mmHg at 20degC and a boiling point of 23.4 degC at 760mmHg) which makes current vaporizers unsuitable.

2 Heating liquid to form vapour under pressure (Tec 6 vaporizer, which is discussed in another lecture; Elsa vaporizer).

4 Cooling desflurane in a variable-bypass concentration-calibrated vaporizer, which is technically not practical.

The direct injection into the system of an inhalation anaesthetic in liquid form has been used for years during closed circuit anaesthesia. The rule of thumb for injecting a fixed amount according to the square root of time (Lowe and Ernst) is well known. Each injection produces a predicted amount of vapour, giving a circuit anaesthetic vapour concentration fluctuating with time (see saw movement curve). Precautions are to be made that liquid anaesthetic cannot enter the patient's airway, but is injected into a small reservoir.

With the availability of modem line measurement of inhalation anaesthetics concentration, manual injection is easily substituted by servo-control. This approach is successfully incorporated in the PhysioFlex apparatus, allowing true closed circuit conditions (methane accumulation), this closed loop control can cause incorrect values.

The anaesthetic forms only a small part of the whole cost of an operation, the largest components of which are the staff salaries and running costs of the operating theatre suite. Within the total costs of the anaesthetic the drugs used form a small, but not insignificant component. It is relatively straightforward to determine the drugs costs within an anaesthetic, but very difficult to do other than perform an estimate of the total costs of the whole anaesthetic. It is likely that this factor is one of the driving forces behind the recent interest in drug costs of anaesthesia.

It is likely that cost has been a consideration in the choice of anaesthetic agents for as long as anaesthesia has existed. It is difficult, however, to find any detailed analysis of those costs before about 1960. In that year, Shackleton (1) described a detailed breakdown from data supplied by the finance department of his hospital, reaching the conclusion that an anaesthetic cost£3.13.9d. (£3.68), of which drugs and dressings made up 13s. 9d. (68p). The cost of the drugs used in 1960 have risen by about 20 times over the intervening 24 years and so the drug cost of an anaesthetic using the same agents should cost about £14.00 today. We do, however, have newer,. better drugs which are more expensive (propofol, enflurane, isoflurane, etc., were not available in 1960) and so the drug cost of an "average" anaesthetic might be expected to be greater than £14.00. Improved manufacturing methods, together with drugs coming off patent do, however, make comparisons difficult (the price of 250 ml of halothane is unchanged in monetary terms - it was £10.00 in 1960 and is approximately £10.00 in 1994).

British practice of anaesthesia has traditionally used semi-open or semi-closed circuits with high flows (6-8 1/min) whereas many other countries have used low flows into a closed circuit. When one considers that more than 80% of anaesthetic gases/vapours emerging from the circuit are wasted using high flows (2) it should be immediately apparent that cost savings are possible from using a low flow system.

Calculation of the costs was illustrated by Dion in' 1992 (3). He provided the following formula (at 21°C):

Cost = V x F x T x MW x C / 2412 x D

Where

V = vaporiser setting (%)

F = fresh gas flow (l/min)

MW = Molecular weight of agent

C = cost of agent (per ml)

D = density of liquid agent (g/ml)

It is clear from this equation that the total cost is directly proportional to the vapour setting, the duration of the anaesthetic and the fresh gas flow. It is possible then to calculate costs of the use of each agent in pence/min of operating dme (see Table 1). At a vaporizer setting of one MAC, desflurane is the most expensive, as might be expected. Desflurane in a low flow into a circle is, however, considerably cheaper than using enflurane at higher flows (see Table 2).

The above is a slight oversimplification. When low fresh gas flow into a circle is used, that only describes maintenance of anaesthesia. High flows are necessary for between 5 and 10 minutes to flush circle, dead space and the patients FRC. Furthermore, the uptake of the agents differs; desflurane reaches equilibrium faster than any of the others. When these points are taken into consideration, it can be seen that desflurane is more expensive than isoflurane for short operations but becomes less expensive with time (4). The consumption of soda lime must also be considered. The use of a circle with low flows requires soda lime to remove CO₂. The active time of a soda lime canister depends upon a number of factors, amongst which are the patients metabolic rate, leaks from the circuit, size and position of the canister, and delays between patients which allows for partial regeneration. Of considerable importance is the fresh gas flow; the lower the flow, the shorter will be the active time of the canister (5).

It is possible to combine many of the variables considered and to determine the economic effects of varying the cost of the agent, the vaporizer setting, the fresh gas flow and the cost of the soda lime together.

A series of 'iso-cost' points or lines can be constructed (6) which confirm the greater cost of using a circle for short operations and permits us to define the exact duration of anaesthesia at which a low flow circle technique becomes cheaper than a higher flow circle or semi-open technique.

When using low flows, it is very important to possess and use adequate monitoring. Most anaesthetists would regard ET CO₂ FIO₂ and agent concentrations to be mandatory in addition to routine anaesthetic monitors. These economic calculations have not taken into account the capital outlay, maintenance and running costs of these machines. With these accurate monitors, a number of groups have taken low flows and circles one stage further and returned to using Goldman vaporizers or injection systems in the circle. These have an even greater advantage (7,8).

It should be clear therefore that the use of low flows has considerable economic advantages over higher flows. It is also much less wasteful. The economic advantages were well illustrated by a study from Northwick Park Hospital in 1991 (9) where it was estimated that simply reducing fresh gas flows to below 41/min should save the hospital approximately £27,000 per year. There can really be little excuse for continuing to use high flows on economic grounds at all!

TABLE 1

COST OF VOLATILE AGENTS

AGENT

COST OF LIQUID AGENT

COST IN

PENCE/MIN *

COST IN PENCE/MIN AT ONE MAC **

Halothane

£10.49 for 250 ml

0.18

0.13

Enflurane

£41.00 for 250 ml

0.82

1.29

Isoflurane

£49.50 for 100 ml

2.52

3.28

Desflurane

£40.00 for 240 ml

0.87

5.25

TABLE 2

COSTS OF THE VOLATILE AGENTS IN PENCE/MIN ASSUMING VAPORIZER SET AT ONE MAC

FGF

8 L/MIN

4 L/MIN

1 L/MIN

250 Ml /MIN

Halothane

62p

31p

Enflurane

£6.19

£3.10

77p

19p

Isoflurane

£15.74

£7.78

£1.97

49p

Desflurane

£25.20

£12.60

£3.15

79p

Given the complexity and problems associated with circle anaesthesia, compared with other breathing systems, the main reason to use the circle is to achieve as low a fresh gas How as possible and so minimise the cost of gases and volatile agents as well as reducing pollution. The minimum flow possible is basal oxygen requirement, in the order of 250 ml/min. However, the conventional technique of using the circle with nitrous oxide and the vaporizer outside the circle (.VOC) requires gas flows considerably above basal requirements during the induction period to exchange nitrous oxide for nitrogen and to achieve an adequate vapour concentration in the circle. Also, during the recovery period, high fresh gas flows are needed to and can be achieved by combining two long established anaesthetic techniques, namely using air/oxygen/volatile anaesthesia along with having the vaporizer in the circle (VIC).

4. The oxygen in the circle mixes with the oxygen/nitrogen in the parient's lungs, producing a mixture with an oxygen concentration of 30 -50%.

5. A vaporizer setting is chosen to produce appropriate inspired and end tidal concentrations of the agent for anaesthesia. This setting needs to be progressively reduced as anaesthesia continues, to maintain the same concentration within the circle.

6. If a leak develops, then gas is lost from the circle and must be replaced by either oxygen or air to maintain the oxygen concentration in the 30-50% range.

7. The vaporizer is switched off well in advance of the end of the procedure to allow recovery to take place. Spontaneously breathing patients can be disconnected from the circle and allowed to breathe oxygen enriched air to speed recovery.

8. As the only control that needs to be used is the vaporizer setting, (in the absence of leaks) the technique is easy to teach and use.

This technique was studied in 305 adults having elective gynaecological and orthopaedic surgery 192 (63%) breathed spontaneously and 113 (37%) were ventilated. In 248 (85%) patients, totally closed anaesthesia was achieved throughout the whole anaesthetic. In 47 (15%) patients, leaks developed requiring oxygen or air to be added to the circle, although most of these leaks were small.

Of these 305 patients, 153 received enflurane and 152 isoflurane. The mean consumption and cost of each agent per hour was measured and compared with those for the conventional circle technique as used in our theatres.

Thus ultimate low flow anaesthesia can more than halve the use and cost of volatile agents as compared with the conventional circle technique. A further saving of about £1 per hour is also made as nitrous oxide is not used with the ultimate low flow technique.

Ultimate low flow circle anaesthesia is therefore a practical technique that is simple to use, makes significant cost savings compared with conventional circle techniques, reduces pollution and by not using nitrous oxide avoids both accidental hypoxia and the deleterious effects of nitrous oxide.


	Manchester 1994
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