Anaesthesia Machine

Dr. Shamsun Nahar ShantaDA StudentDepartment of Anaesthesia Intensive Care and Pain MedicineShaheed Suhrawardy Medical college & Hospital

• NO EQUIPMENT IS MORE INTIMATELY ASSOCIATED WITH THE PRACTICE OF ANAESTHESIOLOGY

THAN THE

ANAESTHESIA MACHINE 

Introduction• The most common type of anaesthetic machine in use in the

developed world is the continuous flow anaesthetic machine, which is designed to provide an accurate & continuous supply of medical gases(such as O2 & NO2)mixed with an accurate concentration of anaesthetic vapour(such as halothane,isoflurane)& deliver this to the patient at a safe pressure & flow.

• Modern machine incorporate a ventilator,suction unit & patient monitoring devices.

Importance of Anesthesia machine• Anesthesia machine use to control patient’s ventilation &

oxygen delivery & to administer inhalational anaesthetics.• Proper functioning of the machine is crucial for patient safety.• Much progress has been made in reducing the number of

adverse outcome arising from anaesthetic gas delivery equipment,through redesign of equipment & machine.

• Misuse of anesthesia gas delivery system is three times more likely than failure of the device to cause equipment related adverse outcome.

Anaesthesia Workstation

• An anesthesia workstation integrates most of the components necessary for administration of anesthesia into one unit

• It is a device which delivers a precisely-known but variable gas mixture, including anaesthetizing and life-sustaining gases.

• Consists of:• The anesthesia machine• Ventilator• Breathing system• Scavenging system • Monitors Added to this may be drug delivery systems, suction

equipment, and a data management system

History• The original concept of Boyle's machine was invented by

the British anaesthetist Henry Boyle in 1917• 1920 – A vaporizing bottle is incorporated• to the machine.• 1926 – A 2nd vaporizing bottle and by-pass• controls are incorporated.• 1930 – A Plunger device is added to the• vaporizing bottle.• 1933 – A dry-bobbin type of flow meter is• introduced.• 1937 – Rota meters displayed dry-bobbin• type of flowmeters• 1952- Pin index safety system• 1958-introduction of Bodok seal

Why should we know about anaesthesia machine ?• For proper & familiar operation of machine.• If the machine get any trouble during anaesthesia of a patient

we cannot wait for the engineer to come. Because it may endanger the patient’s life.

• For rapid detection of any trouble.• For quick correction of the trouble & continuing the

anaesthesia.

Safety features of modern anaesthesia machineTo ensure the delivery of safe gas mixture should include the

following :

1. colour-coded pressure gauges

2. colour-coded flow meters

3. An oxygen flow meter controlled by a coded knob.

4.Oxygen is the last gas to be added to the mixture.

5. Oxygen conc. analyzer

Contd….safety features

6. N2O is cut-off when oxygen pressure is low.

7. O2 : N2O ratio monitor and controller

8. Pin index safety system for cylinders

9. Non-interchangeable screw thread (NIST) for pipelines.

10. Alarm for failure of Oxygen supply.

11. At least one reserve oxygen cylinder should be available on machine that use pipeline supply.

Types of Anesthesia Machine

• Intermittent- Gas flows only during inspiration

Egs: Entonox apparatus, Mackessons apparatus

• Continuous- Gas flows both during inspiration and expiration. Egs :• Boyle Machine• Forregar • Dragger

Basic Schematics

SyStem componentS

Electrical Pneumatic

1.Master Switch2.Power Failure Indicator3.Reserve Power4.Electrical Outlet5.Circuit Breakers6.Data Communication Port

1.High Pressure System2.Intermediate System3.Low Pressure System

Electrical Components

• Master SwitchMaster (main power) switch activates both the pneumatic and electrical functions . On most machines, when the master switch is in the OFF position, the only electrical components that are active are the battery charger and the electrical outlets Standby position - allows the

system to be powered up quickly

Computer-driven machines should be turned OFF and restarted with a full checkout at least every 24 hours.

STANDBY mode is not used for an extended period.

Gases are supplied under tremendous pressure for the convenience of storage and transport.

The anaesthesia machine receives medical gases from a gas supply; controls the flow of desired gases reducing their pressure, to a safe level.

So the pressure inside a source ( cylinder or pipeline ) must be brought to a certain level before it can be used for the purpose of ventilation.

And it needs to be supplied in a constant pressure, otherwise the flow meter would need continuous adjustment.

This is achieved by bringing down the pressure of a gas supply in a graded manner with the help of three pressure reducing zones.

Thus the pneumatic part of the machine can be conveniently divided into three parts- high, intermediate and low pressure systems

• Consists of:Consists of:– Hanger YolkHanger Yolk – Check valve Check valve

– Cylinder Cylinder Pressure Pressure Indicator Indicator (Gauge)(Gauge)

– Pressure Pressure Reducing Reducing Device Device (Regulator)(Regulator)

• Usually not Usually not used, unless used, unless pipeline gas pipeline gas supply is offsupply is off

Hanger Yoke Assembly

The Hanger yoke assembly

1) Orients and supports the cylinder

2) Provides a gas-tight seal

3) Ensures uni-directional gas flow

The workstation standard recommends that there be at least one yoke each for oxygen and nitrous oxide.

If the machine is likely to be used in locations that do not have piped gases, it is advisable to have a double yoke, especially for oxygen.

•Bodok seal

-Cylinders are fitted with yoke with a sealing washer called BODOK SEAL

-It is made up of non combustible material and has a metal periphery which make it long lasting.

-It should be less than 2.4mm thick prior to compression.

-Only one seal should be use between the valve & yoke

It is used to prevent particulate matter from entering the machine.

It is to be placed between the cylinder and the pressure reducing device.

FILTER

Placing cylinder in yoke

Placing a Cylinder in a Yoke1.Cylinder valves and yokes not be contaminated with oil or grease2. Persons placing a cylinder in a yoke should always wash their hands first3. Pin Index Safety System pins are present4.Retract the retaining screw5. The washer is placed over the nipple6.The cylinder is supported by the foot and guided into place manually 7. The port on the cylinder valve is guided over the nipple

and the index pins engaged in the appropriate holes8. The retaining screw is tightened 9. Do not insert the screw in the safety relief device10. Make certain that the cylinder is full and that there is no leak

CHECK VALVE ASSEMBLY

It allows gas from a cylinder to enter the machine but prevents gas from exiting the machine when there is no cylinder in the yoke.

It allows an empty cylinder to be replaced with a full one without having to turn off the `in–use` cylinder.

Prevents transfer of gas from one cylinder to the other with a lower pressure in a double yoke.

It consists of a plunger that slides away from the side of the greater pressure.

It is not designed to act as a permanent seal for empty yoke and may allow small amount of gas to escape.

As soon as a cylinder is exhausted it should be replaced by a full one or a dummy plug.

• In order to minimize losses –

• Yokes should not be left vacant for extended periods

• An empty cylinder should be replaced as soon as possible

• An yoke plug can be used to prevent gas leak or

• An empty cylinder can be left behind after closing the

valve

A Bourdon tube is a hollow metal tube(copper alloy) bent into a curve, then sealed on one side and linked to a clock like mechanism

PRESSURE REDUCING DEVICE (REGULATOR)

The pressure in a cylinder varies. The anesthesia machine is fitted with devices (reducing valves, regulators, reducing regulators, reduction valves, regulator valves) to maintain constant flow with changing supply pressure.

These reduce the high and variable pressure found in a cylinder to a lower (40 to 48 psig, 272 to 336 kPa) and more constant pressure suitable for use in an anesthesia machine.

The machine standard requires reducing devices for each gas supplied to the machine from cylinders.

Physical Principle- A large pressure acting over a small area is balanced by a small pressure over a

larger area

• Pressure regulators have safety relief valves

• If due to any reason there is build up of pressure in pressure regulator then the safety valve blow off at a set pressure of 525 k pa(70psi)

Safety features on pressure regulator

INTERMEDIATE  PRESSURE  SYSTEMBegins at the regulated cylinder supply source at 45 psig includes the pipeline sources at 50 to 55 psig and extends to the flow control valve.

• Consists of:• Pipeline inlet connections

• Pipeline pressure indicators

• Piping• Gas power outlet• Master switch• Oxygen pressure failure devices

• Oxygen flush• Additional reducing devices

• Flow control valves

Check valve

MASTER SWITCH (Pneumatic Component)

The pneumatic portion of the master switch is located in the intermediate pressure system downstream of the inlets for the cylinder and pipeline supplies

The oxygen flush is usually independent of this switch.

The master switch may be a totally electronic switch that when activated controls the various pneumatic components in the anesthesia machine.

When the master switch is turned off ,the pressure in the intermediate system will drop to zero

PIPELINE INLET CONNECTIONS

It is the entry point for gases from the pipelines. The anesthesia workstation standard requires pipeline inlet connections for oxygen and nitrous oxide.

Most machines also have an inlet connector for air. These inlets are fitted with threaded non interchangeable Diameter Index Safety System (DISS) fittings

A unidirectional (check) valve prevents reversed gas flow from the machine into the piping system

Each pipeline inlet is required to have a filter with a pore size of 100 m or less. The filter may become μclogged, resulting in a reduction in gas flow.

PIPELINE INLET CONNECTIONS

PIPELINE PRESSURE INDICATORSIndicators to monitor the pipeline pressure of each gas are required by the anesthesia workstation standard.

They are usually found on a panel on the front of the machine and may be color coded for the gases that they monitor

The workstation standard requires that the indicator be on the pipeline side of the check valve in the pipeline inlet.

If the indicator is on the pipeline side of the check valve, it will monitor pipeline pressure only. If the hose is disconnected or improperly connected, it will read “0” even if a cylinder valve is open

If the indicator were on the machine (downstream) side of the check valve, it would not give a true indication of the pipeline supply pressure unless the cylinder valves were closed. If a cylinder valve is open and the pipeline supply fails, there will be no change in the pressure on the indicator until the cylinder is nearly empty.

Pipeline pressure indicators should always be checked before the machine is used. The pressure should be between 50 and 55 psig (345 and 380 kPa). The indicators should be scanned repeatedly during use.

• Piping is used to connect components inside the machine

• It must be able to withstand four times the intended service pressure

• Leaks between the pipeline inlet or cylinder pressure reducing system and the flow control valve not exceed 25 mL/minute

• If the yoke and pressure reducing system are included, the leakage may not exceed 150 mL/minute.

PIPING

• Some machines have a gas selector switch that prevents air and nitrous oxide from being used together.

GAS SELECTOR SWITCH

GAS POWER OUTLET• One or more gas power (auxiliary gas) outlets may be present on an anesthesia machine. It may serve as the source of driving gas for the anesthesia ventilator or to supply gas for a jet ventilator. Either oxygen or air may be used.

• The ventilator is an integral part of the modern machine and the breathing system and is connected to the ventilator with internal piping. Therefore, the power outlet is not found in many anesthesia machines today.

OXYGEN PRESSURE FAILURE DEVICES

One of the most serious mishaps that occurred with early machines was depletion of the oxygen supply (usually from a cylinder) without the user awareness.

The result was delivery of 100% anesthetic gas.

Numerous inventions have been devised to prevent this

Oxygen Failure Safety Device

Oxygen Supply Failure Alarm

Pressure Sensor Shut-off Valve: Pressure Sensor Shut-off Valve: Datex OhmedaDatex Ohmeda• Operates in a threshold manner: either open or shut

• Oxygen pressure moves the piston and pin upward and the valve opens for N2O

• When pressure of oxygen falls below preset value, force of the valve return spring completely closes the valve

Oxygen Failure Protection Device: Drager

• Based on a proportioning principle rather than a threshold principle

• Pressure of N2O falls in Proportion of decrease of Oxygen. Total cutoff seen at <12psig.

• Seat nozzle assembly connected to a spring loaded conical tapered piston

Oxygen Supply Failure AlarmASTM standard specifies that whenever the oxygen supply  pressure falls below a certain threshold (usually 30 psig), alarm must get activated within 5 seconds. It should not be possible to disable this alarmThey aid in preventing hypoxia caused by problems occurring upstream in the machine circuitry (disconnected oxygen hose, low oxygen pressure in the pipeline, and depletion of oxygen cylinders)

OXYGEN FLUSHThe oxygen flush (oxygen bypass, emergency oxygen bypass) receives oxygen from the pipeline inlet or cylinder pressure regulator and directs a high unmetered flow directly to the common gas outlet.

It is commonly labeled “02+.”On most anesthesia machines, the oxygen flush can be activated regardless of whether the master switch is turned ON or OFF.

A flow between 35 and 75 L/minute must be delivered.The button is commonly recessed or placed in a collar to prevent accidental activation.

It consists of a button and stem connected to a spring loaded It consists of a button and stem connected to a spring loaded ball .The ball is in contact with the seat .When the button is ball .The ball is in contact with the seat .When the button is depressed, the ball is forced away from the seat , allowing the depressed, the ball is forced away from the seat , allowing the oxygen to flow to the outlet. A spring opposing the ball will oxygen to flow to the outlet. A spring opposing the ball will close the valve when the button is not depressed . Delivers close the valve when the button is not depressed . Delivers oxygen at 60 psig.oxygen at 60 psig.

• Reported hazards associated with the oxygen flush include

Accidental activation – causing oxygen-enriched gas mixture, anaesthetic dilution

The flush valve stuck in the ON positionBarotrauma and awareness during anesthesiaInternal leakage

• The anesthesia workstation standard requires that the connection of the flush valve delivery line to the common gas outlet be designed so that activation does not increase or decrease the pressure at the vaporizer outlet by more than 10 kPa or increase the vapor output by more than 20%.

The Flow Meter Assembly• The flow meter assembly controls, measures & indicate the

rate of flow of gas passing through it. The flow meter assembly consists of : 1.Flow control valve

2.Flow meter sub assembly

The flow meter control valve control the rate of flow of a gas through its associated flow meter by manual adjustment of variable orifice, flow control valve also called needle valve or pin valve. The valve mainly consists of control knob, stem & seat.

CONTROL KNOBCONTROL KNOB

• Touch and colour coded

• Joined to stem

• Large enough to be turned easily

• Flow meter sub assembly:

This consists of the tube through which gas flows, the indicator or bobbin or float, a stop at the top of the tube and the scale which indicates the flow. Flow meter tubes are known as Thrope type & are made of borosilicate glass pyrex. Imdicator also called as rota meter or bobbin or float is present within the flow meter tube which moves up & rotate as the gas flows into the tube. The bobbin is made of allominium & has an upper rim which is wider than the body. The upper rim contains slanted flutes which make the bobbin rotate, fluroscent dot 0ver the bobbin make its rotation obsereved easily. Sometimes the float get stuck due to static electricity particularly in dry atmosphere, can be reduced by spraying croxtine.

Low pressure system• The low pressure system is the part of the machine

downstream of the flow meter in which the pressure is slightly above the atmosphere. The component of this system:

1.Vapourisers

2.Back pressure safety device

3.The common gas outlet

Check list for anaesthesia apparatus• Check that that the anaesthesia machine and relevant

equipment are connected to the main electrical supply and switched on.

• Check that all monitoring device are functioning• Oxygen analyzer , pulse oximeter and capnograph are

functioning correctly.• Ensure that each gas pipeline is correctly inserted.• Check that the anaesthesia apparatus is connected to a supply

of oxygen and that an adequate reserve supply of oxygen is available from a spare cylinder

Check list for anaesthesia apparatus• Check that adequate supply of any other gases intended

for use are available and connected• Check the operation of flow meters , ensuring that

bobbin moves freely throughout its range.• Check that vaporizers are fitted correctly to the

anaesthesia machine with locking mechanism fully engaged. Check that the vaporizers are adequately filled and the filling port is tightly closed.

Check list for anaesthesia apparatus• Check the breathing system visually and manually• Check that the ventilator tubing is securely attached• Check that all ancillary equipment

(facemask.LMA,Connector,Laryngoscope etc)• Check that the suction apparatus is functioning correctly

Check list for anaesthesia apparatus• Check that the trolley or bed can be tilted head-down

rapidly• Ensure that an alternative oxygen supply and means of

ventilation are readily available• Document the fact that the anaesthesia machine and

equipment have been checked.

Check list for anaesthesia apparatus• Check the scavenging system• Check the ancillary equipment is present & working• Ensure the monitoring equipment is present, switched

on & calibrated ready for use

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