CO-ORDINATED BY : DR PARULPRESENTED BY : DR GURNEET

ANAESTHETIC MACHINESupply compressed gasesMeasure flow of gasesAdd vapours in known concentrationsDeliver vapours and gases to patient via a breathing systemScavenge wasteMonitor machine and patient

ANAESTHETIC MACHINESPressurized gases are supplied by cylinders or pipelines to the anaesthetic machine which controls the flow of gases before passing them through a vaporizer and delivering the resulting mixture to the patient via the breathing circuit

HistoryOriginal Continuous-flow Anaesthetic Machine designed by Boyle in 1917

Most current anaesthetic machines derived from Boyles Machine

Simpler anaesthetic machines draw over used by Armed Forces (no need for fresh gas supply)

Boyles Anaesthetic Machine

Components of the Anaesthetic MachineGas suppliesPressure gaugesPressure reducing valvesRota metersVaporizersCommon gas outletHigh flow oxygen flushPressure relief valveOxygen supply failure alarmBreathing systemVentilatorScavengingMonitoring devices

Basic Components of Anaesthetic Machine

Anaesthetic machine frameworkBox shaped sections of welded steel or aluminium provide a rigid metal framework mounted on wheels with antistatic tyres and brakes. Antistatic measures improve flowmeter performance and, where flammable vapours are used, reduce the risk of ignition.

Gas Supply - PipelinesCentral supply pointO2, N2O, air, medical vacuum suppliedPipelines throughout hospitalOutlets colour and shape codedFlexible colour coded pipelines run to anaesthetic machinePipelines permanently fixed to machine (Non-Interchangable Screw Thread NIST)Pressure 400kPa (4 bar)

Flexible pipelines have three components:

Schrader probe. To prevent misconnection to the wrong gas service, the probe for each gas supply has a protruding indexing collar with a unique diameter, which only fits the Schrader socket assembly for the same gas. Flexible hosepipe. Originally the hoses were made of black reinforced rubber with short lengths of coloured sheaths at each end. Modern hoses are colour-coded for each gas (oxygen is white; nitrous oxide, French blue; medical air hose, black/white). Non-Interchangeable Screw Thread (NIST) connection to the anaesthetic machine. This ensures a hose connection specific to each gas service. It comprises a nut and probe. The probe has a unique profile for each gas, which fits only the union on the machine for that gas. The nut has the same diameter and thread for all gas services, but can only be attached to the machine when the probe is engaged.

PIPELINE INLETFILTER CHECK VALVE Diameter index safety system (DISS) some machines have an oxygen power outlet that may be used to drive the ventilator or provide an auxillary oxygen flowmeter DISS fittings for oxygen inlet and oxygen power outlet are identical and not be mistakenly interchanged

Gas Supply - CylindersMolybdenum steel or aluminium Cylinders are subjected to mechanical testing before marketing for use. The test is done with one in 100 furnished cylinders.Tensile testFlattening testImpact testBend testHydrolic test or pressure testThere are different sizes of cylinders A, B, C, D,E and others. D and E cylinders of N2O and O2 are in common use.O2, Nitrogen, Air and He are kept in cylinder as compressed gases, but N2O, cyclopropane and CO2 are stored in liquid form.

Oxygen -- Black & white shoulder 2000 PSI (138 Bar)Nitrous oxide Blue -- 750 PSI (52 Bar)Carbon Dioxide Grey -- 720 PSI (50 Bar)Cyclopropane Orange -- 75 PSI (5 Bar)N2O + O2 (entonox) 2000 PSI (138 Bar)The colour coding is also used on the hoses.Safety relief valvePin Index System The pins are below the nipple Each particular cylinder has a fixed pin codeOxygen 2and 5N2O 3 and 5CO2 1 and 6 Air 1 and 5Cyclopropane 3 and 6N2 1 and 4Entonox 7

Bodok seal

Pressure GaugesPressure gauges to measure pipeline and cylinder pressuresO2, N2O and air gauges on front of machineGas colour coded and calibrated

Bourdon Gauge

Pressure RegulatorsFunction:Reduce pressure to protect machineMaintain constant supply to compensate for changing cylinder pressures + demandReduce high and variable pressure found in a cylinder to a lower (40 48 PSI, 272 336 Kpa) and more constant pressure.

.Compression spring acting on diaphragm coupled to inlet valve

ADAMS VALVE : - It reduces the pressure to between 5 cm 10 PSI. It consists of a chamber enclosed to one side by a diaphragm. High pressure gas enters into the chamber from the lower end of the valve, the Diaphragm is distended against the recoil of its own and of a strong spring. The gas passes through the tube on the side of valve.The pressure may be regulated by adjusting the screw at the top of valve.

HIGH PRESSURE RELIEF VALVE Provided downstream of the regulator, opens when pressure exceeds the machines maximum safety limit (95 110 PSI) 700 Kpa.

OXYGEN SUPPLY FAILURE PROTECTION DEVICESOxygen & air can pass directly to its flow control valveOther gases must first pass through safety devices before reaching the flow valveThese devices permit the flow of other gases only if there is sufficient oxygen pressure in the safety deviceSafety devices sense oxygen pressure via a small piloting pressure lineIf piloting pressure line falls below a threshold (20 psig), shut off valves close preventing administration of gases

Most modern machines use a propotioning safety device instead of a threshold shut-off valveCalled oxygen failure protection device (draeger) or a balance regulator (datex ohmeda),proportionately reduce the pressure of nitrous oxide and other gases except for airAll machines also have a oxygen supply low pressure sensor that activates a gas whistle,when inlet gas pressure drops below a threshold value (20-35 psig)

Ritchie Whistle (1960)

RotametersMeasure flow rate of gas passing through themCalibration (accuracy +/- 2.5%)

Components: flow control(needle) valve, tapered tube, bobbin (slits + dot)

Low flow rates: flow laminar + function of gas viscosity (Poiseuilles Law)

High flow rates: flow turbulent + function of gas density

Causes of flow meter malfunctioning includeDirt in the flow meter.Vertical tube mis alignmentStriking or concealment of float at the top of tube.Safety mechanisms in rotametersO2 flow meter is always positioned down stream to all other flow meter.Minimum oxygen flow The oxygen flow valves are usually designed to deliver a minimum flow of 150 ml/min. The method involved the use of a minimum flow resister.

This safety feature helps ensure that some oxygen enters the breathing circuit even if the operator forgets to turn on the oxygen flow.c) constant rotation of the floats d) coating the tubes interior with conducting substancee) Oxygen knobf)OXYGEN NITROUS OXIDE RATIO CONTROLLER Links the two flow valves either mechanically, Pneumatically ,Electronically .

MECHANICALLY LINKAGE : -

There is a 14 tooth sprocket on the N2O flow control valve and a 29 tooth sprocket on the O2 flow control valve. A pin on the O2 sprocket engages a pin on the O2 flow control knob if the flow control valves are adjusted so that 25% conc of O2 is reached. This minimum oxygen ratio device (proportioning system) permits independent control of each gas as long as percentage of O2 is above minimum.

PNEUMATIC LINKAGE : - A movable horizontal shaft connects the diaphragm of the N2O and O2 chambers and the N2O slave control valve. The slave control valve is a ball check valve.ELECTRONICALLY MEASURED : - An electronic proportioning valve controls the O2 conc in the fresh gas. A computer calculates the max allowable N2O flow given the O2 flow. If the N2O flow control valve opened to cause a flow higher than maximum allowable, the proportioning valve reduces the N2O flow to supply a minimum of 25% O2.

AUXILLARY OXYGEN FLOW INDICATOR

Allows delivery of O2 to a patient without turning on the main switch. This can be used to supply O2 directly to patients or can be connected to a resuscitator bag. If there is problem with the anesthesia machine. This can also act as a source of high pressure O2 for transtracheal jet ventilation.UNIDIRECTIONAL (CHECK) VALVE : - Present b/w the vaporizer and common gas outlet. To prevent retrograde flow of gases.

Back BarHorizontal part of machine between rotameters and common gas outletVaporizers mounted here so volatiles can be added to fresh gases

Pressure ranges 0-30 kPa (1 kPa at outlet end)

Non-return pressure relief valve downstream of vaporizers set at 35 kPa prevents barotrauma to rotameters, vaporizers and patient

VaporizersAdd controlled amount inhalational agent, after changing it from liquid to gas, to fresh gas flowAccurate delivery of anaesthetic agent regardless of time, gas flow and temperature Safety mechanisms includetemperature compensationNon-corrosive propertyColour coded dials & bodies

Antispill mechanismColour coded keyed filling devicesSafety interlock device

Common Gas OutletFresh gas mixture supplied to patient or ventilatorHose of breathing circuits attaches to itOften mounted on swivel Supply gas to the breathing circuit. An anti disconnect device is used to prevent accidental detachment of the gas outlet hose that connects the machine to the breathing circuit.

22mm male outlets external diameter15mm female outlets internal diameter

OXYGEN FLUSH VALVE (O2 by pass, emergency O2 receives O2 from the pipeline inlet or cylinder reducing device and directs a high un metered flow directly to the common gas outlet. The flow is between 35 75 litres/min.

In most anesthesia machines the oxygen flush can be activated regardless of whether the master switch is turned ON or OFF.DISADVANTAGES : -Internal leakageSticking in the ON positionBarotrauma.

OXYGEN ANALYZERSG.A should never be administered without it in breathing circuit3 types are polarograpgic,galvanic,paramagneticThey utilize electrochemical sensors that contain cathode and anode electrodes embedded in a electrolyte gel separated from the sample gas by a oxygen permeable membraneShould have alow level alarm Sensor placed in inspiratory or expiratory limb of circle system- not in fresh gas line

Spirometers Used to measure exhaled tidal volume in the breathing circuit on all the anaesthesia machines ,typically near the exhalation valveSome also measure the inspiratory tidal volume just past the inspiratory valveChanges in exhaled tidal volume represent changes in ventilatory settingsAlso may represent circuit laeks,disconnections or ventilatory malfunction

Circuit pressureA pressure gauge or electronic sensor is used to measure breathing circuit pressure somewhere between the expiratory and inspiratory unidirectional valvesRise in airway pressure signal worsening lung compliance, increase in tidal volume, obstruction in breathing circuit & ET tubeDrop may indicate improvement in compliance, leak in circuit

Adjustable pressure limiting valveAPL valves are used in both types of breathing circuit. It allows excess gas to escape when a preset pressure is exceeded, thus minimizing the risk of barotrauma to the patient. Many APL valves do not have calibrations and are adjusted empirically to give a desired peak inspired pressure

Scavenging system

Modern scavenging has four components for collecting, transferring, receiving and disposal of waste gases from the breathing circuit: The collecting system comprises a gas-tight shroud enclosing the APL valve of the breathing circuit (or expiratory port of the ventilator) utilizing 30 mm conical connections. Some systems (Ohmeda AGS) have an over-pressure relief valve which blows at 1 kPa. The transfer system comprises wide bore tubing leading from the collecting systems to the receiving system.

The receiving system comprises a reservoir, air brake, flow indicator and filter. A closed system requires a dumping valve to prevent excessive negative pressure developing (0.5 cm water at 30 litre min1 gas flow) and a pressure relief valve to prevent excessive positive pressure (5 cm H2O at 30 litre min1 gas flow). Disposal systems are active and high flow . The sub-atmospheric pressure required is generated by an exhauster unit, which uses a fan to generate a low pressure, high volume system capable of removing 75 litre min1 at a peak flow of 130 litre min1.

Non inter changeable gas specific connections to pipeline inlet (DISS) with pressure gauges, filter and check valve. Prevent incorrect pipeline attachments, detect failure, depletion, or fluctuation.Safety Features of Anaesthetic Machines

Pin index safety system for cylinders with pressure gauges. One O2 cylinder Prevent incorrect cylinder attachments, provide back up gas supply detect depletion.Low oxygen pressure alarm Detect O2 supply failure at the common gas inlet.Minimum oxygen/N2O ratio controller device (hypoxic guard) Prevent delivery of less than 21% O2.Oxygen failure safety device (shut off or proportioning device Prevent adm of N2O or other gases when the O2 supply fails.

O2 must enter the common down stream to other gases Prevent hypoxia in event of proximal gas leak.O2 conc monitor and alarm Prevent adm of hypoxic gas mixture in event of a low pressure system leak, precisely regulate O2 conc.Automatically enabled essential alarms and monitors Prevents use of the machine without essential monitors.Vaporizer interlock device Prevent simultaneous adm of more than one volatile agent.

Capnography and anesthesia gas measurement guide ventilation, prevent anesthesia over dose, help in reduced an awair ness.Oxygen flush mechanism Rapidly refill or flush the breathing circuit.Breathing circuit pressure monitor and alarm Prevent pulm barotrauma, detect sustained positive, high peak, and negative airway pressures.Exhaled vol monitor Assess ventilation and prevent hypo or hyper ventilation.

Pulse oximetry, BP, ECG monitoring Provide minimal standard monitoring.Mechanical ventilator Control alveolar ventilation more accurately and during muscle paralysis for prolonged periods.Back up battery Provide temporary electrical power (> 30 mt) to monitors and alarms in event of power failure.Scavenging system Prevent contamination of OT with waste anesthetic gases.

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