Equipment

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Chapter 12 Equipment

Anaesthetic Equipment

Gases

Cylinders

Gas cylinders are manufactured from chromium molybdenum steel as a seamless tube.

Colours. Conform to International Standards Organization (Table 12.1).

Table 12.1 Gas cylinder colours

	Body	Shoulder
Oxygen	Black	White
Nitrous oxide	French blue	French blue
Air	Grey	Black/white
Carbon dioxide	Grey	Grey
Helium	Brown	Brown
Cyclopropane	Orange	Orange
Entonox	French blue	French blue/white
Nitric oxide	Pink	Pink

Marks on cylinders. Test pressure, dates of test, chemical formula of gas and tare weight (i.e. weight when empty).

Gas pressures

• N₂O – 51.6 bar (liquid). No reduction in cylinder pressure until 75% empty

• CO₂ – 44 bar (liquid)

• Cyclopropane – 4 bar (liquid)

• O₂ – 137 bar

• Entonox – 137 bar.

Filling ratio = weight of gas in cylinder/weight of water cylinder would hold. For CO₂ and N₂O, filling ratio = 0.75 in temperate climates and 0.67 in tropics.

Vacuum

Vacuum required to give 0.53 kPa pressure below atmospheric pressure, producing 40 L/min suction of air.

Anaesthetic machine safety features

• Copper reinforced gas hoses to prevent kinking, with specific colours for each gas. Non-interchangeable Schraeder valves at wall with different threads to connect to the anaesthetic machine

• One-way valves at yokes to prevent leaks

• Pin index system for cylinders

• Pressure-reducing valves (from 137 to 4 bar)

• Sintered bronze filters proximal to rotameters to prevent ingress of dust

• Oxygen flow knob on rotameter more proud than others with serrated surface

• Oxygen enters fresh gas flow from rotameter last

• Rotameters have a stop to prevent bobbin disappearing from site at high gas flows

• Gold/tin coating on surface of rotameter to prevent static electricity causing bobbin to stick

• Flow restrictors sited proximal to rotameters to protect them from sudden surges in pressure and distal to rotameters to protect them from back pressure

• Interlocking vaporizers

• Oxygen failure alarm

• Emergency O₂ flush without locking valve (>35 L/min)

• Air entrainment if oxygen delivery fails

• Blow-off valve at 43 kPa to protect back bar

• Blow-off valve in patient circuit at 5 kPa.

Checking Anaesthetic Equipment 3

Association of Anaesthetists of Great Britain and Ireland 2004

Full checklist is the responsibility of the anaesthetist and should be performed prior to each operating session.

1. Check that the anaesthetic machine is connected to the electricity supply (if appropriate) and switched on

Note: Some anaesthetic workstations may enter an integral self-test programme when switched on; those functions tested by such a programme need not be retested.

• Take note of any information or labelling on the anaesthetic machine referring to the current status of the machine. Particular attention should be paid to recent servicing. Servicing labels should be fixed in the service logbook.

2. Check that all monitoring devices, in particular the oxygen analyser, pulse oximeter and capnograph, are functioning and have appropriate alarm limits

• Check that gas sampling lines are properly attached and free of obstructions.

• Check that an appropriate frequency of recording non-invasive blood pressure is selected.

3. Check with a ‘tug test’ that each pipeline is correctly inserted into the appropriate gas supply terminal

Note: Carbon dioxide cylinders should not be present on the anaesthetic machine unless requested by the anaesthetist. A blanking plug should be fitted to any empty cylinder yoke.

• Check that the anaesthetic machine is connected to a supply of oxygen and that an adequate supply of oxygen is available from a reserve oxygen cylinder.

• Check that adequate supplies of other gases (nitrous oxide, air) are available and connected as appropriate.

• Check that all pipeline pressure gauges in use on the anaesthetic machine indicate 400–500 kPa.

4. Check the operation of flowmeters (where fitted)

• Check that each flow valve operates smoothly and that the bobbin moves freely throughout its range.

• Check the anti-hypoxia device is working correctly.

• Check the operation of the emergency oxygen bypass control.

5. Check the vaporizer(s)

• Check that each vaporizer is adequately, but not over, filled.

• Check that each vaporizer is correctly seated on the back bar and not tilted.

• Check the vaporizer for leaks (with vaporizer on and off) by temporarily occluding the common gas outlet.

• Turn the vaporizer(s) off when checks are completed.

• Repeat the leak test immediately after changing any vaporizer.

6. Check the breathing system to be employed

• Inspect the system for correct configuration. All connections should be secured by ‘push and twist’.

• Perform a pressure leak test on the breathing system by occluding the patient-end and compressing the reservoir bag. Bain-type co-axial systems should have the inner tube compressed for the leak test.

Vaporizers

Because desflurane has such a high saturated vapour pressure (88 kPa), standard vaporizers are unsuitable for its storage and delivery. Use of a conventional vaporizer would require very high fresh gas flows to achieve 1 MAC equivalent of desflurane. The low boiling point of desflurane (24°C) also makes a conventional vaporizer unsuitable. The Tech 6 desflurane vaporizer (Fig. 12.1) uses a servo-controlled electronic system which heats the vaporizer chamber to a constant 39°C (higher than the boiling point) at a pressure of 1500 mmHg. The desflurane is delivered into the fresh gas flow (FGF) at equal pressures through a pressure-regulating valve which increases desflurane delivery as the FGF increases. Unlike conventional ventilators, use of the Tech 6 vaporizer at high altitude requires manual adjustment to increase desflurane concentrations.

Figure 12.1 Desflurane vaporizer.

(Reproduced with permission from New Generation Vaporizers, Pharmacia.)

Ventilators

Nuffield 200 series ventilator

This is a time-cycled pressure generator. It has variable expiratory and inspiratory timers and a variable inspiratory flow rate control (Fig. 12.2).

Figure 12.2 Nuffield Penlon 200 ventilator – inspiratory mode.

(Reproduced with permission from Davey et al 1992.)

Paediatric Newton valve. Capable of delivering tidal volume between 10 and 300 mL at flow rates of 0.5–18 L/min. At small tidal volumes, pressure-controlled ventilation is preferable to volume-controlled ventilation because the final volume delivered is dependent upon circuit leaks, circuit compliance and fresh gas flow rates.

Bibliography

Andrews J.J., Johnston R.V. The new Tech 6 desflurane vaporizer. Anesth Analg. 1993;76:1338-1341.

Association of Anaesthetists of Great Britain and Ireland. Checking anaesthetic equipment, vol 3. AAGBI, London, 2004.

Cartwright D.P., Freeman M.F. Vaporisers. Anaesthesia. 1999;54:519-520.

Davey A., Moyle J.T., Ward C.S., editors. Ward’s anaesthetic equipment, ed 3, London: WB Saunders, 1992.

Gardner M.C., Adams A.P. Anaesthetic vaporizers: design and function. Curr Anaesth Crit Care. 1996;7:315-321.

Howell R.S.C. Medical gases (1) Manufacture and uses. Kaufman L., editor. Anaesthesia review, vol 7. Churchill Livingstone, Edinburgh, 1989;87-104.

Howell R.S.C. Medical gases (2) Distribution. Kaufman L., editor. Anaesthesia review, vol 8. Churchill Livingstone, Edinburgh, 1990;195-210.

Breathing Circuits

Mapleson’s classification of breathing systems

For all adult circuits, a 110 cm hose holds a volume of 550 mL. T-piece reservoir should equal the tidal volume (more reservoir volume causes increased resistance and re-breathing).

Figure 12.3 Mapleson’s classification of breathing systems.

Table 12.2 Breathing circuit flow rates

Mapleson classification	Spontaneous ventilation	IPPV
A	70 mL/kg per min	2.5 × MV
B	2.5 × MV	2.5 × MV
C	2.5 × MV	2.5 × MV
D	2.5 × MV	70 mL/kg per min
E – Adult	2.5 × MV	2.5 × MV
E < 20 kg	3 (1000 + 100 mL/kg)	1000 + 100 mL/kg (minimum flow = 3 L)
or	3 (5 × frequency × kg)	5 × frequency × kg
Lack (coaxial A)	50 mL/kg per min
Bain (coaxial D)		70 mL/kg per min