Anaesthesia Machines

Anaesthesia Machines

IntroductionAnaesthetic gas affects the nervous system, resulting in a numbing of the nerve pathways. There are three types of anaesthesia local, regional and general. This report will only look at general anaesthetic, in which the patient becomes unconscious, unaware of what is happening, has no pain, and is immobile. A general anaesthetic typically employs a gas mixture containing oxygen, nitrous oxide and/or an anaesthetic agent. These gasses are mixed in the anaesthetic machine, and are supplied from wall outlets or cylinders. The pressures are reduced by pressure regulators then pass through an O2 failure alarm. The flow of gas is controlled by flow meters. The gas is mixed and passed over a vaporiser containing the anaesthetic agent. An Anti-hypoxic device ensures that the flowmeters deliver a minimum of 25% O2 to the agent vaporiser. The gas then goes through an over pressure relief valve and back flow valve, then to the common gas outlet (CGO), ready to be given to the patient. There is an emergency O2 flush valve connected to give 100% O2 if required. The gas then enters a patient circuit and ventilator. Expired gas can be stripped of CO2 in a soda-lime filter and recirculated, or scavenged out of the room.

This machine delivers a constant gas flow to the breathing circuit Some patients breath spontaneously; in others however muscle relaxants paralyse the lung muscles, leaving the patient unable to breath by themselves, and need to be ventilated.Gasses

Pipeline source

The hospital gas pipeline is the primary source of gasses for the anaesthesia machine. Medical gases Oxygen Oxygen is produced by fractional distillation of liquid air. Impurities are permitted in medical gases as long as they do not exceed small amounts of known contaminates.

Oxygen is stored as a liquid at -150oC to -175oC in a large flask, as the liquid occupies 1/860 the space a gas would. Safety systems and regulators send oxygen to the hospital pipeline at approximately 50 psi

Nitrous Oxide

Nitrous oxide is manufactured by thermal decomposition of (NH4)2NO3. It is non flammable but supports combustion (same as oxygen). Nitrous Oxide is also stored as a liquid at ambient temperature, in large 745psi H tanks connected to a manifold which regulates the pipeline pressure to approximately 50 psi

Pipeline inlets The gas pipeline inlets are connected with DISS (diameter index safety system) non-interchangeable connections. The check valve, located down stream from the pipeline inlet, prevents reverse flow of gases (from machine to pipeline, or to atmosphere), which allows use of the gas machine when pipeline gas sources are unavailable.

Gasses cylinder source

Capacity, color, markings of cylinders

GasColorUS (internat'l)Service PressurepsiCapacityLPin Position

Oxygengreen (white)1,9006602-5

Nitrous Oxideblue (blue)7451,5903-5

Airyellow(black & white)1,9006251-5

Fig1

Fail-safe, Hypoxic guard, FlowmetersHypoxic guard

The anaesthetic failsafe ensures that whenever oxygen pressure is reduced and until flow ceases, the set oxygen concentration shall not decrease at the common gas outlet. In addition, the loss of oxygen pressure results in alarms, audible and visible, at 30 psi pipeline pressure. This fail-safe system wont prevent hypoxic mixtures. For example, as long as there is pressure in the oxygen line, nothing in the fail safe system prevents you from turning on a gas mixture of 100% nitrous oxide. However Datex-Ohmeda hypoxic guard system, uses a "pressure sensor shut off valve"- at 20 psi oxygen, the flow of all other gases are shut off

Flowmeters

Glass flowmeters

The components of a gas flowmeter are, needle valve, indicator float, knobs, valve stops. Flow increases when the knob is turned counterclockwise (same as vaporizers). At low flows, the annular-shaped orifice around the float is (relatively) tubular so (according to Poiseuille's Law) flow is governed by viscosity. At high flows (indicated on the wider top part of the float tube), the annular opening is more like an orifice, and density governs flows.

Regular mechanical needle valves and glass flowtubes are utilized in the Aestiva;

fig2

Electronic flowmeters

Gas machines with electronic flowmeters have no glass tubes, and the flow rate is indicated with a bar graph on a monitor screen. There is a needle valve (so flow can be generated even without electric power). Flows are captured electronically as follows: flow from the needle valve is conducted to a small chamber of known volume and held there momentarily by a solenoid valve until the transduced pressure within the chamber reaches a preset limit. This gives a known mass of gas. This cycle is repeated sufficiently often for the desired flow rate to occur, and the number of times the solenoid opens is sensed and can be related to flow. Thus, electronic flowmeters allow automated anesthesia record-keepers to chart fresh gas flows. They are also five to ten times as accurate at metering gas flow than glass flowtubes.

fig3Vaporizers

An anaesthetic vaporiser is attached to an anaesthetic machine which delivers a given concentration of a volatile anaesthetic agent.The design of these devices takes account of varying

ambient temperature

fresh gas flow

agent vapour pressureThere are generally two types of vaporisers used on an anaesthesia machine, plenum and Dual-circuit gas-vapour blender which is exclusively used for the agent desflurane.Plenum vaporisersThe plenum vaporiser works by accurately splitting the incoming gas into two streams. One of these streams passes straight through the vaporiser in the bypass channel. The other is diverted into the vaporising chamber. Gas in the vaporising chamber becomes fully saturated with volatile anaesthetic vapour. This gas is then mixed with the gas in the bypass channel before leaving the vaporiser.A typical volatile agent, used is isoflurane. plenum vaporisers are designed to operate within a specific temperature range. They have several features designed to compensate for temperature changes (especially cooling by evaporation). They often have a metal jacket weighing about 5kg, which equilibrates with the temperature in the room and provides a source of heat. In addition, the entrance to the vaporising chamber is controlled by a bimetallic strip, which admits more gas to the chamber as it cools, to compensate for the loss of efficiency of evaporation.Dual-circuit gas-vapour blenderThis vaporiser was created specifically for the agent desflurane. Desflurane boils at 23.5C, which is very close to room temperature. This means that at normal operating temperatures, the saturated vapour pressure of desflurane changes greatly with only small fluctuations in temperature. This means that the features of a normal plenum vaporiser are not sufficient to ensure an accurate concentration of desflurane. Additionally, on a very warm day, all the desflurane would boil, and very high (potentially lethal) concentrations of desflurane might reach the patient. desflurane vaporiser (e.g. the TEC 6 produced by Datex-Ohmeda) is heated to 39C and pressurised to 200kPa (and therefore requires electrical power). It is mounted on the anaesthetic machine in the same way as a plenum vaporiser, but its function is quite different. It evaporates a chamber containing desflurane using heat, and injects small amounts of pure desflurane vapour into the fresh gas flow. A transducer senses the fresh gas flow.A warm-up period is required after switching on. The desflurane vaporiser will fail if mains power is lost. Alarms sound if the vaporiser is nearly empty. An electronic display indicates the level of desflurane in the vaporiser.

Breathing circuit

The circle circuit is the most popular breathing system. It filters carbon dioxide using soda-lime which allows re-breathing of all other exhaled gasesFig4

Circle components Fresh gas inflow source, inspiratory & expiratory unidirectional valves, inspiratory & expiratory corrugated tubing, Y connector, overflow (called popoff, adjustable pressure-limiting valve, or APL valve), reservoir bag, carbon dioxide absorbent canister and soda-lime granules. _1310205554.bin

_1310205623.bin

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