ANAESTHESIA MACHINE

DR. NEELAKSHI KALITANKP SIMS& LMH Nagpur India

HISTORY

• The earliest anaesthesia machines provided anaesthesia via the insufflation method.

• C.A Elsberg’s continuous flow machine was described in 1911 and underwent modifications .

• The popular Shipway model was used to provide anaesthesia to King George V of England,designed by Francis E. Shipway.

• Of major importance of design was compression of gases in metal cylinders.

• Oxygen & nitrous oxide cylinders were available as early as 1885.

• This allowed development of compact machines.

• Boyle anaesthetic machine is a continuous – flow type of machine used for administration of inhalational anaesthetic agents.

• It was introduced by Henry Edmund Gaskin Boyle in 1917.• It has undergone modifications • 1920-1926 – vaporiser bottles added• 1930- plunger device in vaporizes bottle• 1933- dry bobbin type of flowmeter instead of water sight

-feed type• 1937- rotameters replaced dry bobbin type of flowmeter• Various safety devices have been introduced and

modernized.

Boyle’s machine

vaporizerbellow

Corrugated tube

Soda lime

Flow meter

ventilator

APL valve

Scavenging system

Comprises of 3 different pressure systems High pressure system : from cylinder to

pressure reducing valves. Intermediate pressure system : from pressure

reducing valves to flowmeters. Low pressure system : from flow meters to the

common gas outlet on machine.

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High Pressure System Intermediate Pressure System Low Pressure System

1. Flow Indicators2. Unidirectional Check valve3. Pressure Relief Device4. Vaporizer Mounting Devices5. Common Fresh Gas Outlet

1. Pipeline Inlet Connections2. Pipeline Pressure Indicators (Gauge)3. Piping4. Gas Power Outlet5. Master Switch6. Oxygen Pressure Failure Devices7. Oxygen Flush Valve8. Second Stage Reducing Device9. Flow Control Valves

1. Hanger Yoke Assembly2. Cylinder Pressure Indicator

(Bourdon Guage)3. Pressure Reducing Device

(Regulator)

The Anesthesia MachineHigh Intermediate Low Pressure Circuit

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High Pressure System• Receives gases from the high pressure E

cylinders attached to the back of the anesthesia machine (1900 psig for O2, 745 psig for N2O )

• Consists of:– Hanger Yoke (reserve gas cylinder holder)– Check valve (prevent reverse flow of gas)– Cylinder Pressure Indicator (Gauge)– Pressure Reducing Device (Regulator)

• Usually not used, unless pipeline gas supply is off

MEDICAL GAS CYLINDER COMPONENTS:

BODY: constructed of steel with alloy containing molybdenum(0.15-0.25%) & or chromium(0.8-1.1%)

- Minimizes weight and wall thickness - In recent years steel carbon fiber cylinders are

constructed, lighter,holds more gas. - Aluminium cylinders for MRI environment. - Marking of 3AA – steel, 3ALM – aluminium.

VALVE : Cylinders are filled and discharged through a valve (spindle valve) attached to the neck.

- made of bronze or brass - used to start,regulate and stop gas flow - chemical formula of gas engraved on the valve Valve consists of following parts Body : basic structure.Has an outlet on one side with pin holes to fit

into the yoke.On opposite side conical depression present where screw of yoke fits.

Port: point of exit for the gas. Stem: or spindle,fitted on top of valve,rotated during valve opening

or closing. Handle or Handwheel: used to open or close a cylinder

valve.counterclockwise opens and clockwise closes it. - a handle,spanner is used to open a small cylinder. - Large cylinder valve has a permanently attached handle wheel.

- Valve is fitted to the top of a cylinder - Made gas tight with a seal washer made of soft alloy - No oil or grease should be applied to any part of the

high pressure system. Types of cylinder valves: a)Diaphragm valve:includes a valve stem with

adjustable screw which raises or lowers a metal diaphragm.

Advantages:- opened fully with only one half to three-quarter turn.

- Seat does not turn,less likely to leak. - No stem leakage due to diaphragm.

b)Packed type or flush type: has a valve stem and resilient non flammable plastic seal.

- directly acting valve, by turning the stem outlet can be opened or closed.

- commonly used in modern anaesthesia machines

Other types of cylinder valves: a) Bull nosed b) Straight type c) Angled type - Bodok seal is placed between the

valve and yoke of anaesthesia machine.

Bull nosed valve

PRESSURE RELIEF DEVICE

• Spring loaded device• Each cylinder valve has a safety relief device • Prevents bursting of cylinder• Gas escapes through it to atmosphere, recloses after

normal pressure is achieved• Set pressure at which it will escape is marked on the

valve• In air,helium, oxygen,nitrogen,carbon dioxide

cylinders upto 500 psig pressure relief device is present.

RUPTURE DISC :

• Non-reclosing device with a disc against an orifice• Disc ruptures if predetermined pressure is reached• Protects against excess pressure as a result of high

temperature or overfilling FUSIBLE PLUG :• Thermally operated• Protects from excessive pressure caused by high

temperature.• Made of brass or bronze with alloy, yield temp: 70-74 deg C (158 – 165deg F) , 98-104 deg C (208-219 deg F)

NON INTERCHANGEABLE SAFETY SYSTEM

• PIN INDEX SAFETY SYSTEM: 1952 - Safety mechanism so that one cylinder cannot be fitted at the other’s

position - it consists of holes on the cylinder valve and 2 pins on the yoke

positioned to fit into these holes. - the port will seal only with proper alignment of pins and holes - pins are 4mm in diameter,6mm long, pin 7 is slightly thicker - the 7 hole positions are on the circumference of a circle of 9/16 inch

radius centered on the port - 6 pin positions are located at an interval of 12 degree, with pin 7 located between 3 and 4.

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Gas Index pinsOxygen 2,5

Nitrous oxide 3,5

Cyclopropane 3,6

O2-CO2(CO2<7.5%) 2,6

O2-CO2(CO2>7.5%) 1,6

O2-He(He>80.5%) 4,6

O2-He(He<80.5%) 2,4

Air 1,5

Nitrogen 1,4N2O-O 7

PIN INDEX SAFETY SYSTEM

• VALVE OUTLET CONNECTIONS FOR LARGE CYLINDERS - large cylinders have threaded outlet (bull nose)

connections. - when the threads of of this outlet mesh with those of the

nut,the nut may be tightened. - the outlets and connections are indexed by diameter,

thread size, right and left handed threading external and internal threading and nipple seat design.

CYLINDER SIZES:• Size A is smallest, smaller than it size AA is for use in

army.• Size E – most commonly used in anaesthesia

machine, patient transport, resuscitation• Size D – for limited supplies of gases• Aluminium cylinders have same or shorter length

than steel cylinders and a larger outer diameter

Cylinder Characteristics Size E

Oxygen Nitrous Oxide

Carbon Dioxide

Air Helium Nitrogen

Color White Blue Gray Black/White

Brown Black

State Gas Liquid and gas

Liquid and gas

Gas Gas Gas

Contents (L) 660 1590 1590 625 500 610

Empty Weight (kg)

5.90 5.90 5.90 5.90

Full Weight (kg) 6.76 8.80 8.90

Pressure Full (psig)

1900 745 838 1900 1600 1900

CONTENTS AND PRESSURE

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• In a cylinder with non liquefied gas pressure declines steadily as contents are withdrawn, pressure can be used to measure cylinder content.

• The weight of cylinder can also be used to measure the contents.• In a cylinder with liquefied gas ,pressure depends on vapor pressure of the

liquid , not an indication of amount of gas in the cylinder as long as the contents are partly in liquid form.

• The pressure remains nearly constant until all liquid has evaporated after which the pressure declines until the cylinder is exhausted.

• Weight can be used to determine cylinder contents regardless of the state of gas in the cylinder.

TESTING OF CYLINDERS

• Should be tested every 5 years or with special permit every 10 years.

• Test date ( month&year) must be permanently stamped on the cylinder.

Tensile test: done in 1 in 100 finished cylinders• Strips cut longitudinally from cylinder are stretched

until they elongate.• The ‘yield point’ should not be < 15 tons/ square inch

FLATTENING TEST:- One cylinder is kept between 2 compression blocks,- pressure applied to flatten it till distance of blocks become 6 times the

thickness of walls- Well approved if there is no crack IMPACT TEST :- 3 longitudinal and 3 transverse strips taken from a finished cylinder and struck

by a hammer- Mean energy needed to produce a crack should not be less than 5ft lb for

transverse strips and 10ft lb for longitudinal strips. BEND TEST :- 1 is chosen from 100 finished cylinders- 25mm width ring is cut and divided into 4 equal strips.- Each strip is bent inwards until inner edges are apart not greater than diameter

of strip- Approved cylinder should develop no cracks

HYDRAULIC TEST OR PRESSURE TEST: The test is usually done by the water jacket method.

• Here the proof pressure applied inside is 236.2 kgf/cm2. • Change in cylinder volume on applying and withdrawl of

proof pressure is measured by changes in water level in a gauge projecting from upper end of water jacket.

• The permanent stretch should not exceed 10% of the total stretch under proof pressure.

• This test also determines whether cylinder is leak proof.

FILLING OF CYLINDERS

• Pressure in a filled cylinder at 70 degree F ,not to exceed service pressure marked on cylinder

• For some nonliquified ,nonflammable gases like O2 ,He,

CO2 - O2 mixtures & He- O2 mixture 10% extra allowed• For cylinders with liquified gas ,maximum amount of gas

allowed is defined by a filling density/ filling ratio filling density = wt of gas in a cylinder/ wt of water the cylinder would hold at 60 deg F• Filling density is not the same as the volume of full

cylinder occupied by the liquid phase

COLOUR CODING

• Helps identify the gases• Non fading,durable,water insoluble paint used to

cover cylinders• Colour coding used on valve protection caps, hoses,

connectors,knobs & gauzes on medical equipments

• Oxygen • White• Nitrous oxide • Light blue

• Medical air • Black and white

• Suction • Yellow• Nitrogen • Black• Carbon dioxide • Grey

• Helium • Brown

International colour coding

Cyclopropane Orange

MARKINGS

• DOT & TC regulations – specific markings on each cylinder

• Permanently stamped on the shoulder of a cylinder

DOT specification DOT – 3A 2015 SL. NO - 8642 Manufacturer - XYZ Owner’s symbol - JCN • Arranged in a horizontal line

around the shoulder

LABELING

• Each cylinder must bear a label on the side or shoulder of cylinder• A diamond shaped figure denoting the hazard class of contained gas with a single word like DANGER, WARNING or

CAUTION.• Label contains name & address of cylinder manufacturer /distributor,

content, vol in litres at 70 deg F, wt when empty & full• Expiratory date of contents• Should be possible to read date with cylinder connected to machine. TAGS• 3 sections labelled FULL , IN USE & EMPTY connected by perforations

RULES FOR SAFE USE OF CYLINDERS

• Handled by trained personnel• Should be kept away from combustible substances• Should not be contaminated with oil or grease• Should not be subjected to temperature>54 deg C or -7 deg C, away

from flames , sparks• Connections to piping ,pressure regulators etc should be leak free• Discharge port of pressure relief device must not be obstructed• No interchange of hoses,gauzes for a different gas• Valve should be kept closed except when cylinder is used• No part of cylinder or valve should be tampered with• Markings, tags should not be defaced• Cylinder should not be used as a roller or support

• Cylinders should not be dropped,dragged,slid or rolled, cart should be used to transport

• Should be properly secured to prevent being knocked over or falling

• Should not be allowed to strike each other• Unserveicable cylinders should be disposed of by

trained personnel

STORAGE OF CYLINDERS

• Definite area should be designated,not stored in an operation theatre• Area should be cool,clean,of fire resistent material.• Flooring should be conductive,adequate ventilation• If in open area,protection against weather,no direct sun rays• Storage in a secure area,prevent nitrous oxide theft• Combustible material kept away• Smoking or open flames prohibited• Should not be exposed to dampness,corrosive fumes chemicals• Should be upright or horizontally in racks• Should be protected from mechanical shock• Proper grouping when different gases are stored in same area

HAZARDS OF CYLINDER USE

• Incorrect cylinder• Incorrect contents , valve, labelling , colour• Inoperable or damaged valve• Asphyxia• Fires• Explosion• Projectile damage• Contaminated cylinder contents• Nitrous oxide theft• Overfilled cylinders• Thermal injury• Blocked flowmeter

COMPONENTS OF HIGH PRESSURE SYSTEM

• Hanger Yoke:orients and supports cylinder,provides a gas-tight seal, ensures unidirectional gas flow

parts: Body: principal framework of yoke• Threaded into frame of machine, supports cylinder• On swinging gate type of yoke,distal part is hinged• Hinged part can be swung to side Retaining Screw: • Is threaded into distal end of yoke Hanger Yoke

• Fits into conical depression of cylinder valve• Tightening presses cylinder valve outlet against washer and nipple to

achieve gas tight seal.

Nipple: gas enters machine through nipple, if damaged no tight seal

Index Pins: are below the nipple Washer: or Bodok seal is placed around nipple to

produce a seal between cylinder valve and yoke• Bodok seal is made of noncombustible material, has

a metal periphery to make it long lasting• Should be < 2.4 mm thick• Only one seal allowed between the valve and yoke Filter: (100µm maximum) installed between cylinder

and pressure regulator or flow control valve, prevents entry of particulate matter into machine

Check valve assembly: allows gas from cylinder to enter machine but prevents gas from exiting machine when yoke has no cylinder

• Allows replacement of cylinders without losing gas• Prevents transfer of gas from a cylinder with high pressure

to one with low pressure if connected to a double yoke and turned on simultaneously

• A yoke should never be left vacant• After tightening a cylinder to yoke should look for

leaks( hissing sound)• After cylinder is attached valve should be closed• Safety device in yoke: Wood’s alloy-fusible plug of bismuth ,lead,cadmium,

&tin.melts at temp 150-170 deg F,prevents explosion. Copper frangible disc ruptures at very high pressure.

CYLINDER PRESSURE INDICATOR( GAUZE)

• Displays cylinder pressure for each gas• Indicator may be near a cylinder or on a panel infront of

machine• Scale must be 33% greater than maximum filling pressure• Indicators are of Bourdon tube type• Calibration is in kilopascals(kPa) or pounds per square inch

(psi)• Span angle is from 180 deg and 280 deg from lowest to

highest, • With lowest between 6 o’clock and 9 ‘o clock on clock face• If it ruptures gas is vented from back side• Present in oxygen cylinders,not required in nitrous oxide and cyclopropane cylinders

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Mechanism of pressure gauge

• Hollow metal tube of copper alloy, bent into a curve , sealed and linked to a clock –like mechanism.

• Other end is connected to gas source.

• Increase in pressure causes tube to straighten.As pressure decreases , the tube resumes its curved shape.

• These motions are transmitted to an indicator which moves on a calibrated scale through clock-like mechanism.

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Pressure Reducing Device• Pressure in cylinder varies, to maintain constant flow

with changing supply pressure regulators are provided.

• Reduces the high and variable pressures found in a cylinder to a lower and more constant pressure found in the anesthesia machine (4O-45 psig).

• Physical principle: a large pressure acting over a small area is balanced by a smaller pressure acting over a larger area.

• Pc – inlet pressure•R – inside of regulator,with

gas at reduced pressure• opening between C& R

occluded by area A1• Pr – outlet pressure• A2 – area of flexible

diaphragm where Pr acts• S – stopcock

• S closed, forces are in balance,seat seals opening from cylinder,no gas flows

• Stopcock S is open• Gas flows from R,pressure Pr drops• Forces not balanced as Pc x A1 > Pr x A2• Flexible diaphragm becomes flatter• Balance tips to right• Seat does not occlude cylinder

opening,gas flows from cylinder to R• When stopcock is closed gas will

continue to flow briefly into R until Pr increases to balance forces.

• Regulator will yield constant reduced pressure only if Pc is constant.

• If Pc is reduced when cylinder pressure falls,Pr must decrease to balance forces

• Flow indicator will need constant adjustment with this type of pressure regulator

• S1 – main spring • Exerts downward force on diaphragm• Force pushing diaphragm up – Pr x A2• Pushing downward – (Pc x A1) +Fs1• Fs1 – force exerted by spring• if Pc,Pr,A1,A2 remain unchanged,imbalance

of forces occur since force of spring is added to force of Pc acting on A1

• To compensate A1 decreased,A2 increased or both

• Force exerted by Pr acting on diaphragm is opposed by 2 forces, constant force from spring& variable force from Pc acting on the seat

• If Fs1 is larger than Pc,large variations in Pc will cause only slight variations in Pr.

• Pr depends on Fs1.• Fs1 varied with adjustable screw,Pr can be

varied.

• One more addition to regulator is necessary

• S2 – a sealing shut-off spring is added

• This acts to force the seat against the opening from the cylinder

• Prevents gas from flowing from C to R when adjusting spring is completely relaxed and stopcock open.

• Fs2 is smaller than Fs1.• Finally, a change in cylinder

pressure will produce a change in outlet pressure.

Reducing devices are preset so that the machine uses only gas from the pipeline (wall gas), when the pipeline inlet pressure is 50 psig. This prevents gas use from the cylinder even if the cylinder is left open (i.e. saves the cylinder for backup if the wall gas pipeline fails)

Cylinders should be kept closed routinely. Otherwise, if the wall gas fails, the machine will automatically switch to the cylinder supply without the anesthetist being aware that the wall supply has failed (until the cylinder is empty too).

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Intermediate Pressure System

• Receives gasses from the regulator or the hospital pipeline at pressures of 40-55 psig

• 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

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Pipeline Inlet Connections• It is the entry point of gases from

pipelines.Mandatory for N2O and O2 , also for air and suction too

• Inlets are non-interchangeable due to specific threading as per the Diameter Index Safety System (DISS) fittings.

• Each inlet must contain a check valve to prevent reverse flow from machine to piping(similar to the cylinder yoke)

• Filter present at inlet,pore size 100µm or less.

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Pipeline pressure indicators• Needed to monitor the pipeline pressure of each gas. • Usually found on a panel on the front of the machine

,may be colour coded.• Should register pressure between 50-55 psig. • Indicator should be on the pipeline side of the check

valve in the pipeline inlet.• If a cylinder valve is open and pipeline supply fails

there will be no change in indicator pressure till cylinder is nearly empty.

• If gas pressure coming from a cylinder via a pressure regulator exceeds pipeline pressure and a cylinder valve is open, gas will be drawn from the cylinder.

• Cylinder valve should be closed when pipeline supply is in use.

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Pipings• Used to connect components inside the machine.

• Must be able to withstand 4 times higher pressure without rupturing.

• Leaks between pipeline inlet \ cylinder pressure reducing device and flow control valves should not exceed 25ml per minute.

• If pressure reducing system is included , maximum allowable leakage should not exceed 150 ml per minute.

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Gas Power Outlet

• Supply oxygen or air to ventilators.

• Check valve present.

• Spring loaded valve present.

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Oxygen Failure Safety Devices

• Machine standard requires that an anesthesia machine be designed so that whenever the oxygen supply pressure is reduced below normal, the oxygen concentration at the common gas outlet does not fall below 19%

• A Fail-Safe valve is present in the gas line supplying each of the flowmeters except O2. This valve is controlled by the O2

supply pressure and shuts off or proportionately decreases the supply pressure of all other gasses as the O2 supply pressure decreases

• Historically there are 2 kinds of fail-safe valves– Pressure sensor shut-off valve (Ohmeda)– Oxygen failure protection device (Drager)

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Oxygen Supply Failure Alarm• The machine standard specifies that whenever the oxygen

supply pressure falls below a manufacturer-specified threshold (usually 30 psig) a medium priority alarm shall blow within 5 seconds.

• Electronic alarms: A pressure operated electric switch operates this alarm\– Ohmeda: 28 psig– Drager: 30-37 psig

• Pneumatic alarms (aka Bowman’s Whistle): Uses a pressurized canister that is filled with oxygen when the anesthesia machine is turned on. When the oxygen pressure falls below a certain value, the alarm directs a stream of oxygen through a whistle

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Limitations of Fail-Safe Devices/Alarms

• These devices depend on pressure and not flow so have limitations.

• Fail-safe valves do not prevent administration of a hypoxic mixture , do not prevent anaesthetic gas from flowing if there is no flow of O2 .

• These devices prevent hypoxia from problems occurring upstream in the machine circuitry (disconnected oxygen hose, low oxygen pressure in the pipeline and depletion of the oxygen cylinder).

• Do not prevent hypoxia from accidents such as pipeline crossovers or a cylinder containing the wrong gas.

• Equipment problems that occur downstream (like leaks , partial closure of oxygen flow control valve) are not prevented by these devices.

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Second-Stage Pressure Regulator

• Located just upstream of the flow control valves

• Receives gas from the pipeline inlet or the pressure regulator and reduces it further to 26 psig for N2O and 14 psig for O2

• Purpose is to eliminate fluctuations in pressure supplied to the flow indicators caused by fluctuations in pipeline pressure

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Oxygen Flush Valve (O2+)• Receives O2 from pipeline inlet or cylinder

pressure regulator and directs high, unmetered flow to the common gas outlet (downstream of the vaporizer)

• Machine standard requires that the flow be between 35 and 75 L/min

• Single –purpose, self closing device.Designed to minimize unintentional activation.Labelled “O2 +”.

• Hazards– May cause barotrauma– Dilution of inhaled anesthetic– Accidental activation– Internal leakage,– Flush may stick

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• Flush valves for gases other than oxygen are not permitted.

• Oxygen flush can be activated regardless of whether machine is turned ON or OFF.

• Pressure delivered is 60 psig.

62

Flow adjustment controls

• Controls rate of flow of gas through it’s associated flow indicator by manual adjustment of a variable orifice.

• Current standard requires that there be only one flow control valve for each gas. It must be adjusted or identifiable with it’s flow indicator.

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Components

• Body - screws into the anaesthesia machine

• Stem and Seat- have fine threads,when valve is closed the pin at the end of the stem fits into the seat,occluding the orifice.

- when stem is turned outward an opening between pin and stem is created allowing gas to flow.

• Control knob – joined to the stem.

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Control Knob

• Touch coded.• Color coded.• Joined to stem.• It should be large enough so that it can be

turned easily.

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• ROTATORY STYLE KNOB:• Flow control knob for O2 must have fluted

profile. • be as large as or larger than that for any other

gas.• Knobs are turned counter clockwise to increase

the flow and clockwise to decrease the flow.• all other flow control knobs must be round.• If other types of flow control knobs are present

the O2 control must look and feel different from the other controls.

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USE :• Flow control knobs should operate smoothly and be

easy to adjust.

• Knob should be turned clockwise only until the flow of gas ceases, because further tightening may result in damage to pin/ seat.

• When not in use gas source should be closed or disconnected.

• Flow control valves should be checked to see that they are closed before use of machine is resumed.

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Problems with flow control valves

Inadvertent alterations.

Loose or worn knob may respond to a light touch or accidental brushing.

Leakage through open flow control valves.

Inability to turn control knob.

Failure to allow adequate gas flow.

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Low Pressure System

• Extends from the flow control valves to the common gas outlet

• Consists of:– Flow meters– Hypoxia prevention safety devices– Unidirectional valve– Pressure relief devices – Common gas outlet– Vaporizers and their mounting devices

FLOWMETER ASSEMBLY TUBE: made of glass,Thorpe tube.• Glass tubes intended for ball indicator have rib-

guides which are thickened bars that run the length of tube.

• Rib guides hold the ball indicator in the center of tube

• Can have a single or double taper Single taper tube have gradual increase in

diameter from the bottom to top - used where there are different tubes for low &

high flow Dual taper tubes have 2 different tapers on

the inside of same tube.One for fine flows and one for coarse flows

- used when only one tube is used for a gas

INDICATOR: float or bobbin,made of aluminium• Free - moving device within the tube• if it moves erratically readings may be inaccurate• In non-rotating float type reading is taken at the upper rim• Bobbin is made antistatic to prevent sticking to wall of

flowmeter.• Rotating indicators (rotameters) have upper rim ,diameter of

which is larger than body.Slanted grooves or flutes are cut into the rim.There is often a coloured dot on one side of indicator.

• When gases pass between the rim and tube wall flutes cause the indicator to rotate.Reading taken at the upper rim

• Ball indicator reading is taken at the ball’s mid point • Types: non rotating H type,ball,rotating.

STOP: present at the top of flowmeter tube prevents indicator from plugging the outlet.

• Also prevents indicator from rising to a point where it cannot be seen.

HAZARD: stop might break off and fall on indicator – registers less flow than is actually occuring.

SCALE:is marked on or immediately adjacent to tube• Flowmeters are calibrated in liters per minute.• For < I L/ min expressed in milliliters or decimal

fractions of a liter per minute with a zero before the decimal point.

Oxygen:• Graduated in 100ml/min,divisions from 100 to 2000ml/min and

in 1000ml/min divisions from 2000 ml – 5000ml/min. Carbon-dioxide:• Graduated in 100ml/min divisions from 100-2000ml/min Cyclopropane :• Graduated in 50 ml/min divisions from 50-750ml/min Nitrous Oxide:• Graduated in 1L divisions from 1- 10 L/min• The 4 tubes are contained in chromium plated metal casing

protected by a plastic window.• Detachable luminous backplate provided• Modern machines have flowmeter lights

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Flowmeter • When the flow control valve is

opened the gas enters at the bottom and flows up the tube elevating the indicator

• The indicator floats freely at a point where the downward force on it (gravity) equals the upward force caused by gas molecules hitting the bottom of the float

• Because the tube is tapered the annular opening around the indicator increases with height and more gas flows around the float

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Flowmeter Physics• The rate of flow through the flowmeter tube

depends on 3 things– Pressure drop across the constriction: As gas flows around

the indicator it encounters frictional resistance between the indicator and tube wall.there is loss of energy reflected in a pressure drop. This pressure drop is given by:

weight of float/cross sectional area– Size of annular opening: The annular area varies while the

pressure drop across the indicator remains constant for all positions in the tube.

– Physical characteristics of the gas:• Low Flow: Small annular space, therefore flow is laminar,

therefore flow is a function of gas viscosity .• High Flow: Large annular space, therefore flow is turbulent,

therefore the flow is a function of gas density .

• Flowmeters are calibrated at atmospheric pressure (760 torr) and room temp( 20 deg C).

• Changes in temp & pressure will affect density and viscosity of a gas and affect flowmeter accuracy.

• In a hyperbaric chamber flowmeter will deliver less gas than indicated .

• With decreased barometric pressure (increased altitude), the actual flow rate will be greater than that indicated.

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Arrangement of the Flow-Indicator Tubes

• In the presence of a flowmeter leak (either at the “O” ring or the glass of the flow tube) a hypoxic mixture is less likely to occur if the O2 flowmeter is downstream of all other flowmeter

• In A and B a hypoxic mixture can result because a substantial portion of oxygen flow passes through the leak, and all nitrous oxide is directed to the common gas outlet

AUXILIARY OXYGEN FLOWMETER• Self contained flowmeter with its

own flow control valve,flow indicator,& outlet

• Short tube with maximum flow of 10L/min

• Usually on the left side of the machine

• Can be used to supply O2 to patient without turning ON the machine

• Older machines – works on pipeline supply,in newer ,works on cylinder & pipeline supply both.

PROBLEMS WITH FLOWMETERS

• Inaccuracy- if mixing of components occur• Indicator problems – damage due to sudden

projection to top of the tube. - worn or distorted• Leaks – if flow control valve is left open, there is no

cylinder or yoke plug in the yoke• Using the wrong flowmeter – when flowmeter

sequence is altered

HYPOXIA PREVENTION SAFETY DEVICES

MANDATORY MINIMUM OXYGEN FLOW:• Some machines require a minimum ( 50-250ml/min) flow of O2 before other

gas will flow• Some machines activate an alarm if O2 flow goes beyond a certain minimum.

MINIMUM OXYGEN RATIO:• Device to protect against operator selected delivery of a mixture of O2 & N2O

having O2 conc below 21% O2 .

MECHANICAL LINKAGE:• Exists between O2& N2O flow control valve

• There is a 14- tooth sprocket on N2O flow control valve & 29-tooth

sprocket on O2 flow control valve

• Flow control valves are adjusted so that when 25% O2 conc is reached ,a pin on O2 sprocket engages a pin on O2 flow control knob.This causes O2& N2O flow control valves to turn together to maintain minimum O2 of 25%.

• If attempt is made to increase the N2O flow beyond that ratio, the O2 flow is automatically increased.

• If O2 flow is lowered too much the N2O flow is decreased proportionally

• An electronic system can be used to provide a minimum ratio of O2 to N2O flow

ALARMS:• Available to alert the operator that O2-N2O flow ratio has

fallen below a preset value

UNIDIRECTIONAL (CHECK ) VALVE

• During controlled ventilation a positive pressure from breathing circuit can be transmitted back to the machine

• Using O2 flush valve may also cause this• Unidirectional check valves are present to minimize

these effects• Valve is located between vaporizers and common gas

outlet ,upstream of where O2 flush flow joins fresh gas flow

PRESSURE RELIEF VALVE

• May be attached downstream of vaporizers on the back bar itself or near common gas outlet

• Prevents high pressure being transmitted to the machine

• Whenever preset pressure is exceeded valve opens to atmosphere and gas is vented outside

• Usually opens when pressure in the back bar exceeds 35kPa

• This valve limits the machine to provide jet ventilation

• Also known as the pop-off valve.

COMMON ( FRESH ) GAS OUTLET

• Receives all the gases and vapors from the machine and delivers the mixture to breathing system

• Some outlets have a 15-mm female slip-joint fitting with a coaxial 22-mm male connector

• Machine standard mandates that it be difficult to accidentally disengage the delivery hose from the outlet

• The pressure delivered at the outlet is 5 -8 psi

BACK BAR

• Part of frame of the Boyle’s machine which supports the rotameter,vaporizers and other accessories

• There are 2 metal rods in back bar• Flowmeters and vaporizers are connected

with each other and then bolted with the back bar.

SAFETY FEATURES OF THE BOYLE ANAESTHESIA MACHINE

• Antistatic low friction black rubber wheels to the machine trolley.

• Cylinders : - colour coding - safety pin index system - safety relief valve - filling of cylinder• Bourdon’s pressure gauze• Pressure reducing valve

• Flow meters : - colour,touch & placement coding - rotating bobbin - antistatic spray in flowmeters - downstream placement of oxygen flow meter. - master and slave safety mechanism for gas delivery between N2O & O2• Radio-fluorescent plastic sheet behind flow meters• Placement of vaporizers• In older machines - copper plunger in Boyle’s bottle - Trilene interlock with closed circuit• Vaporizer inter-lock device• Oxygen Flush Valve• Sodalime and its indicator in closed circuit• Black tubings of breathing system

DAILY CHECKLIST FOR ANAESTHESIA MACHINES

INSPECT MACHINE• Vaporizers: Turn off. Refill (if necessary). Tighten filler cap.• CO2 absorber: Check fill level and usage. Replace after

10-12 hours of use.• One-Way Valves (Circle system): Check function by

breathing through circuit. CONFIRM OXYGEN SUPPLY IS ON• Check cylinder pressure and/or piped gas lines. FLOWMETER• Check bobbin/ball moves freely and spins.

PRESSURE CHECK MACHINE/CIRCUIT (each morning and any time bags or tubes are changed)• Close pop-off valve.• Occlude breathing tube by placing thumb over ET tube

connection port.• Pressurize circuit to 30cm H2O by using 3L/min O2 flow to

fill bag.• Turn off O2 flow and watch the pressure gauge for pressure

loss.• Turn on vaporizer (no O2 flow) and watch for drop in

pressure.• At any point, if pressure loss is detected, increase O2 flow

slowly until pressure holds steady.• *****FLOW ON O2 METER = RATE OF LEAK*****

• Confirm that leakage does not exceed 300ml/min.• If leak is >300ml/min the leak needs to be addressed.• Open pop-off valve and confirm release of pressure. CHECK SCAVENGING SYSTEM• Confirm connected to pop-off valve.• Turn on extractor (active systems) and confirm rate

of extraction.• CHECK VENTILATOR FUNCTION (IF FITTED)

AMERICAN SOCIETY OF ANESTHESIOLOGISTS GUIDELINES FOR DETERMINING ANESTHESIA MACHINE OBSOLESCENCE

Absolute Criteria • An anesthesia machine shall be considered to be obsolete

if any of the following criteria apply. • I. Lack of essential safety features • A. Minimum oxygen ratio device (O2&N2O proportioning

system) on a machine that can deliver nitrous oxide• B. Oxygen failure safety (“fail-safe”) device • C. Oxygen supply pressure failure alarm D. Vaporizer interlock device

• E. Pin Index Safety System • F. Non-interchangeable, gas-specific (e.g., Diameter Index

Safety System (DISS)) connectors on the gas pipeline inlets II. Presence of unacceptable features • A. Measured flow (flowmeter-controlled) vaporizers (e.g.,

Copper Kettle, Verni-trol) • B. More than one flow control knob for a single gas

delivered to the common gas outlet of the machine • C. Vaporizer with rotary concentration dial such that the

anesthetic vapor concentration increases when the dial is turned clockwise

• D. Connection(s) in scavenging system of the same (i.e., 15-mm or 22-mm) diameter as a breathing system connection

• III. Adequate maintenance no longer possible

• Relative Criteria • Consideration should be given to replacing an anesthesia machine if any of

the following apply: 1. Lack of certain safety features A. Means to isolate the APL (adjustable pressure-limiting) valve during

mechanical ventilationB. Oxygen flow control knob that is fluted and larger than the other flow

control knobs C. Oxygen flush control protected from accidental activationD. Main On/Off switch for electrical power to integral monitors and alarms E. Anti-disconnection device at the fresh gas outletF. Airway pressure alarm (for detecting sustained positive pressure, negative

pressure and high peak pressure) II. Problems with maintenance III. Potential for human error IV. Inability to meet practice needs

THANK YOU!