The anaesthetic machine

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 07/02/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 3.6 (5 votes)

This article have been viewed 12340 times

The anaesthetic machine

The anaesthetic machine receives medical gases (oxygen, nitrous oxide, air) under pressure and accurately controls the flow of each gas individually. A gas mixture of the desired composition at a defined flow rate is created before a known concentration of an inhalational agent vapour is added. Gas and vapour mixtures are continuously delivered to the common gas outlet of the machine, as fresh gas flow (FGF), and to the breathing sytem and patient (Figs 2.1 and 2.2). It consists of:

Pressure gauge

This measures the pressure in the cylinder or pipeline. The pressure gauges for oxygen, nitrous oxide and medical air are mounted in a front-facing panel on the anaesthetic machine (Fig. 2.3).

Some modern anaesthetic machine designs have a digital display of the gas supply pressures (Fig. 2.4).

Problems in practice and safety features

Pressure regulator (reducing valve)

Pressure regulators are used because:

They are positioned between the cylinders and the rest of the anaesthetic machine (Figs 2.7 and 2.8).

Problems in practice and safety features

Second-stage regulators and flow restrictors

The control of pipeline pressure surges can be achieved either by using a second-stage pressure regulator or a flow restrictor (Fig. 2.9) – a constriction, between the pipeline supply and the rest of the anaesthetic machine. A lower pressure (100–200 kPa) is achieved. If there are only flow restrictors and no regulators in the pipeline supply, adjustment of the flowmeter controls is usually necessary whenever there is change in pipeline pressure.

Flow restrictors may also be used downstream of vaporizers to prevent back pressure effect (see later).

Flowmeters

Flowmeters measure the flow rate of a gas passing through them. They are individually calibrated for each gas. Calibration occurs at room temperature and atmospheric pressure (sea level). They have an accuracy of about ±2.5%. For flows above 1 L/min, the units are L/min, and for flows below that, the units are 100 mL/min (Fig. 2.11).

image

Fig. 2.11 A flowmeter panel.

Mechanism of action

1. When the needle valve is opened, gas is free to enter the tapered tube.

2. The bobbin is held floating within the tube by the gas flow passing around it. The higher the flow rate, the higher the bobbin rises within the tube.

3. The effect of gravity on the bobbin is counteracted by the gas flow. A constant pressure difference across the bobbin exists as it floats.

4. The clearance between the bobbin and the tube wall widens as the gas flow increases (Fig. 2.12).

5. At low flow rates, the clearance is longer and narrower, thus acting as a tube. Under these circumstances, the flow is laminar and a function of gas viscosity (Poiseuille’s law).

6. At high flow rates, the clearance is shorter and wider, thus acting as an orifice. Here, the flow is turbulent and a function of gas density.

7. The top of the bobbin has slits (flutes) cut into its side. As gas flows past it, the slits cause the bobbin to rotate. A dot on the bobbin indicates to the operator that the bobbin is rotating and not stuck.

8. The reading of the flowmeter is taken from the top of the bobbin (Fig. 2.13). When a ball is used, the reading is generally taken from the midpoint of the ball.

9. When very low flows are required, e.g. in the circle breathing system, an arrangement of two flowmeters in series is used. One flowmeter reads a maximum of 1 L/min allowing fine adjustment of the flow. One flow control per gas is needed for both flowmeters (Fig. 2.14).

10. There is a stop on the oxygen flow control valve to ensure a minimum oxygen flow of 200–300 mL/min past the needle valve. This ensures that the oxygen flow cannot be discontinued completely.

Problems in practice and safety features

1. The flow control knobs are colour-coded for their respective gases. The oxygen control knob is situated to the left (in the UK) and, in some designs, is larger with larger ridges and has a longer stem than the other control knobs, making it easily recognizable (Fig. 2.15). In the USA and Canada, the oxygen control knob is situated to the right.

2. The European Standard for anaesthetic machines (EN 740) requires them to have the means to prevent the delivery of a gas mixture with an oxygen concentration below 25%. Current designs make it impossible for nitrous oxide to be delivered without the addition of a fixed percentage of oxygen. This is achieved by using interactive oxygen and nitrous oxide controls. This helps to prevent the possibility of delivering a hypoxic mixture to the patient. In the mechanical system, two gears are connected together by a precision stainless steel link chain. One gear with 14 teeth is fixed on the nitrous oxide flow control valve spindle. The other gear has 29 teeth and can rotate the oxygen flow control valve spindle, rather like a nut rotating on a bolt. For every 2.07 revolutions of the nitrous oxide flow control knob, the oxygen knob and spindle set to the lowest oxygen flow will rotate once. Because the gear on the oxygen flow control is mounted like a nut on a bolt, oxygen flow can be adjusted independently of nitrous oxide flow.

3. A crack in a flowmeter may result in a hypoxic mixture (Fig. 2.16). To avoid this, oxygen is the last gas to be added to the mixture delivered to the back bar.

4. Flow measurements can become inaccurate if the bobbin sticks to the inside wall of the flowmeter. The commonest causes are:

5. Flowmeters are designed to be read in a vertical position, so any change in the position of the machine can affect the accuracy.

6. Pressure rises at the common gas outlet are transmitted back to the gas above the bobbin. This results in a drop in the level of the bobbin with an inaccurate reading. This can happen with minute volume divider ventilators as back pressure is exerted as they cycle with inaccuracies of up to 10%. A flow restrictor is fitted downstream of the flowmeters to prevent this occurring.

7. Accidents have resulted from failure to see the bobbin clearly at the extreme ends of the tube. This can be prevented by illuminating the flowmeter bank and installing a wire stop at the top to prevent the bobbin reaching the top of the tube.

8. If facilities for the use of carbon dioxide are fitted to the machine, the flowmeter is designed to allow a maximum of 500 mL/min to be added to the FGF. This ensures that dangerous levels of hypercarbia are avoided.

9. Highly accurate computer controlled gas mixers are available.

Vaporizers

A vaporizer is designed to add a controlled amount of an inhalational agent, after changing it from liquid to vapour, to the FGF. This is normally expressed as a percentage of saturated vapour added to the gas flow.

Vaporizers can be classified according to location: