Low pressure system in anaesthesia machinePresentation Transcript
Dr. Swadheen kumar Rout1st year P.GDept. of AnaesthesiologyM.K.C.G College & hospital
Anaesthesia machine:- Function is to deliver a precisely-known butvariable gas mixture, including anesthetizingand life-sustaining gases at a controlled &known pressure.TypesIntermittent( gas flows only during inspiration)Ex. – Entonox apparatusContinuous(Gas flows both duringinspiration and expiration)Ex.- Boyle`s machine ,.
Boyle`s anaesthetic machine is a continuous-flow typemachine used for administration of anaesthetic gases. was pioneered by Henry Gaskin Boyle (1875-1941). the original machine from 1917 was carried around in awooden box and used ether and nitrous oxide. It has undergone various modifications with time to increase itssafety & utility.
SYSTEM COMPONENTSELECTRICAL SYSTEM PNEUMATIC SYSTEM- Master switch- Battery back up- Battery recharge- Electric outlet for in built monitor- Circuit breaker- High pressure system- Intermediate pressure system- Low pressure system
Extends from the flow control valves to the common gasoutlet. Consists of : - Flowmeters- Hypoxia prevention safety devices- Unidirectional check valve- Pressure relief valves- Common gas outlet- Vaporiser & their mounting devices
TUBE: made of glass (THORPE tube). Tubes are contained in a chromium platedmetal casing protected by a plasticwindow.Backplate is luminous & detachable. Anti-static coating.INDICATOR: Float / Bobbin made usually ofaluminium. It is free moving device & must not stick totube wall. If it moves erratically, readings may beinaccurate.TYPES Non-rotating H typeRotating typeSlanted groovesBall typeSTOP: Prevents float from plugging the outlet &prevents rising to a point it cannot be seen.
Types of flowmeters1. Variable orifice flowmeters (fixed pressure difference)2. Fixed orifice flowmeters (variable pressure difference)• The area between the outside of the bobbin and the inside of the tapered glasstube represents a orifice / annulus. It can be considered an equivalent to acircular channel of the same cross-sectional areaVariable orifice flowmeters type used mostly today in modern machines.(Synonym- Rotameters)• The glass tube, slightly smaller oncross-section at bottom than at top (tapered tube)•Can be single or double taper.Single taper have gradual increase in diameter from bottomto top, used when there are different tubes for high & low flows.Dual taper have two different taper inside the same tube .one forfine flows (200mL/min to 1L/min) & one for coarse flows.-used when only one tube is used for a gas.
Flowmeters adjust the proportions of medical gases controlled by theanesthesia machine as well as the total gas flows delivered to the patientcircuit . Flowmeters measure the drop in pressure that occurs when a gas passesthrough a resistance and correlates this pressure drop to flow . When the flow control valve is opened the gas enters at the bottom andflows up the tube elevating the indicator. The indicator floats freely at a point where the downwardforce on it (gravity) equals the upward force caused by gasmolecules hitting the bottom of the float. As the bobbin rises with increased flow, the size of the annulus between itand the glass tube increases. In other words, there is a variable orificearound the bobbin which depends on the gas flow.
As the bobbin rises fromA to B, theclearance(annulus)increases (from X toY)
Laminar flow- Flow (Q) through a tube is laminar. In order to drive fluid through a tube, a pressure difference (P = P1-P2)must be present across the ends• There is a linear relationship so that flow is directlyproportional to pressure under conditions of laminarflow (Q@P)• Reducing the diameter (d) in half reduces the flowto 1/16 of its original value if the pressure drop alongthe tube remains the same, i.e. flow is proportional tothe 4th power of the diameter (Q@d4)• Reducing the length by half, doubles the flow(Q@1/L)
Summary: Q @ P Q = flow through tubeQ @ D4 P = pressure across tubeQ @1/L D = diameter of tubeQ @1/n L = length of tuben= viscosity of fluidAll these factors are incorporated in the Hagen-Poiseuille equation:• Viscosity (n) of fluid affects resistance to laminar flow such that the higher theviscosity, the slower the flow (Q@1/n)Substituting radius (r)for diameter
Turbulent flow is often present where there is an orifice, a sharp bend orsome other irregularity which may cause local increase in velocity. Turbulence is also affected by other factors such as viscosity and densityof the fluid and diameter of the tube. The effect of density on onset of turbulent flow can be illustrated by use ofhelium in respiratory disorders Helium reduces the density of the gas inhaled and so reduces the incidenceof turbulent flow, therefore lower resistance to breathing These factors combine to give an index called Reynolds numberV = linear velocity of fluidP = densityD = diameter of tubeU = viscosity• Reynolds number > 2000 means turbulent flow likely• Reynolds number < 2000 means flow likely to be laminar• For a fixed set of conditions, there is a critical velocity at which Reynolds number hasthe value of 2000When the velocity exceeds this critical value, flow is likely to change fromlaminar to turbulent .
As the bobbin risesincrease in the area of the annular orificeflow resistance decreasesflow rate increaseThe rate of flow through the flowmeter tube depends on:- Pressure drop across the constrictionWeight Of Float/Cross-sectional Area- Size of annular orifice- Physical properties of the gas
At low flows:- gas flow around the bobbin approximates to tubular flow (diameterof channel less than length)- gas flow is laminar so viscosity is importantAt high flows:- gas is flowing around the bobbin through an orifice (diameter ofchannel greater than length)- gas flow is turbulent so density is importantTube = Length >DiameterOrifice = Diameter >Length
Flowmeter are calibrated in litres per min. For <1 L/minexpressed in ml or decimal fractions of a litre per minute with azero before the decimal point. Are calibrated at atmospheric pressure(760 torr) and room temperature(200C) basedon physical properties of individual gases. Changes in temperature & pressure affectdensity & viscosity of gas and affect flowmeteraccuracy. As flow changes from laminar to turbulent withinthe flowmeter the flow changes from being directlyproportional to pressure to proportional to thesquare root of pressure and hence the graduationson the flowmeters are not uniform.
There may be one or two rotameters for each gas Single flowmeter layout is the safest but less precise for low flows. If two are present for any gas, the first permits accuratemeasurement of low flows (usually up to 1 L/min) and the other, offlows up to 10-12 L/min Flow indicator tubes for different gases are grouped side by side The various gas flows meet at the common manifold (mixingchamber) at the top . In such a case, the tubes may be arrangedeither in parallel or in series
Parallel arrangement:•Two complete flow indicator assemblies with a flow control valve for eachassembly•The total flow of the gas to the common manifold is the sum of the flows onboth flow indicators•Not presently available because accidental use of low-flow oxygen flowindicator when a high flow is intended is a hazard whenever two oxygenflow control knobs are presentSeries (tandem) arrangement:• One flow control valve for the two flow indicator tubes• Gas from the flow control valve first passes through a tube calibrated up to 1liter per minute, then passes to a second tube that is calibrated for higher flows• Total flow is not the sum of the two tubes but that shown in the higher flowtube.Tandem flow tubes increase accuracy at all flow rates
The O2 flowmeter is positioned on the right side (most distally) ofthe rotameter bank, downstream from the other flowmeters andclosest to the common gas outlet In the event of a leak in one of the other flowmeter tubes, thisposition is the one least likely to result in a hypoxic mixture. In A and B a hypoxic mixture canresult because a substantial portion ofoxygen flow passes through the leak,and all nitrous oxide is directed to thecommon gas outlet.C and D, The safest configurationexists when oxygen is located in thedownstream position
However a leak in the oxygen flowmeter tube can cause ahypoxic mixture, even when oxygen is located in thedownstream position.
Temperature and Pressure Effects Changes in temperature and pressure alter both viscosity anddensity of gases, thereby affecting accuracy of the indicator on theflowmeters. Temperature effects are slight and do not cause significant changes At high altitude, barometric pressure decreases resulting inincreased flow. At low flow rates, flow is laminar and dependent on gas viscosity, aproperty not affected by altitude. At high flow rates flow becomes turbulent, and flow becomes afunction of density, a property that is influenced by altitude. The resulting decreases in density will increase the actual flow rateso the flowmeter will read lower than the actual flow rate. At increased pressure, as in a hyperbaric chamber, the reverse isseen; the delivered flow rate is slightly less than the actual flow rate.
Back Pressure: In machines without an outlet check valve, if pressure at the commongas outlet increases, this is transmitted back to the flowmeters,compressing the gas above the float Pressure above the indicator rises forcing the float down, causing theflowmeter to be read lower than the actual gas flow rateStatic Electricity:Static electricity causes the float to stick to the side of the tube causingreading inaccuracy.These electrostatic charges are negligible as long asthe float rotates freelyHidden Floats:The float may adhere to the stop at the top of the tube even if no gasis flowingThe float may disappear from view if there is no stop present e.g.broken float stop
Protection against hypoxic mixture at the flowmeter level. Prevention of delivery of a hypoxic gas mixture is a majorconsideration in the design of contemporary anesthesia machines.Mandotary minimum oxygen flow: Some machines require aminimum flow (50-250ml/min) of oxygen before other gas will flow. Some machine activate an alarm if O2 flow falls below a certain limit.Minimum oxygen ratio: In modern anesthesia machines, N2O and O2 flow controls arephysically interlinked so that a fresh gas mixture containing at least25% O2 is delivered at the flowmeters when only N2O and O2 areused .Ohmeda = mechanical + pneumatic interlink (Link–25)North American Dräger = pneumatic interlink
A 14-tooth sprocket isattached to the N2O flowcontrol valve, and a 28-toothsprocket is attached to the O2flow control valve. A chainmechanically links thesprockets. For every 2 revolutions of theN2O flow control spindle, anO2 flow control, set to thelowest O2 flow, rotates oncebecause of the 14:28 ratio ofthe gear teeth.
Regardless, of the O2 flow set, if the flow of N2O is increased >75%,the gear on O2 spindle will engage automatically with the O2 flowcontrol knob causing it to rotate and thereby causing O2 flow toincrease to maintain O2 Conc of 25% with a maximum N2O:O2 ratioof 3:1. If attempt is made to increase N2O flow beyond that ratio, O2 flow isautomatically increased & if O2 flow is lowered too much N2O flowreduces proportionately. The final 3:1 flow ratio results because theN2O flow control valve is supplied byapproximately 26 psig, whereas the O2 flowcontrol valve is supplied by 14 psig. Thus, thecombination of the mechanical and pneumaticaspects of the system yields the final oxygenconcentration.
It is a pneumatic O2-N2O interlock system designed to maintain afresh gas oxygen concentration of at least 25% . The ORMC limits N2O flow to prevent delivery of a hypoxic mixture,unlike the Ohmeda Link-25, which actively increases O2 flow . composed of an O2 chamber, a N20 chamber, & a N20 slave control valve; allare interconnected by a mobile horizontal shaft. The pneumatic input into thedevice is from the O2 & N20 flowmeters.These flowmeters have specific resistors located downstream from the flowcontrol valves which create back pressures directed to the O2 & N2O chambers.The value of the O2 flowtube resistor is three to four times that of the N2Oflowtube resistor..The back pressure in the O2 & N2O chambers pushes against rubberdiaphragms attached to the mobile horizontal shaft. Movement of the shaftregulates the N2O slave control valve, which feeds the N20 flow control valve.
The back pressure exerted on the O2 diaphragm, in the upper configuration, is greaterthan that exerted on the N2O diaphragm. This causes the horizontal shaft to move to theleft, opening the N2O slave control valve. N2O is then able to proceed to its flow controlvalve and out through the flowmeter.In the bottom configuration, the N2O slave control valve is closed because ofinadequate O2 back pressure
The ORMC also rings alarms (it has an electronic component)to prevent a hypoxic mixture delivery . Dräger S-ORC (sensitive oxygen ratio controller), newesthypoxic guard system as found on Fabius GS guarantees aminimum FIO2 of 23%. Its fail-safe component shuts off nitrousoxide if the oxygen flow is less than 200 mL/min, or if theoxygen fresh gas valve is closed.
Machines equipped with proportioning systems can still delivera hypoxic mixture under the following conditions Wrong Supply Gas in oxygen pipeline or cylinder. Defective pneumatic or mechanical components. Leaks exist downstream of flow control valves. Inert gas administration( He,CO2) : Proportioning systemsgenerally link only N2O and O2.Use of an oxygen analyzer is mandatory if the operator uses athird inert gas.
Present on some machines (Ohmeda)between the vaporizers and commongas outlet, upstream of where oxygenflush flow joins the fresh gas flow . Positive pressure ventilation & use of O2flush cause back flow of the gas. This back flow can cause “pumpingeffect”, if not prevented, could causeincreased vaporizer outputconcentrations.• Pressure increase can also increase leaks and cause inaccurate flowindicator readings.
The purpose of the outlet check valve, where present, is toprevent reverse gas flow, Newer machines (North American Dräger) are equipped withvaporizers that incorporate a baffle system and speciallydesigned manifold to prevent pumping effect, making an outletcheck valve unnecessary.*Important: Testing the breathingsystem for leaks will not detect a leakin the machine equipped with a checkvalve
Situated on the back bar of the machinedownstream of voporizers or nearcommon gas outlet. Prevents high pressure from transmittedin to machine & from machine to patient. When preset pressure is exceeded valveopens & gas is vented outside. Usually opens when pressure in theback bar exceeds 35 Kpa.
Located in between flowmeter device and common gas outlet. Permanent mounting. Vapourizers and flowmeters are connected to each other andthen bolted with back bar.
Receives all gases & vapors from the machine & delivers themixture to the breathing system. Outlet in most of the machines have 15 mm female slip-jointconnection, with coaxial 22 mm male connection• Machine standards mandates that it be difficult to accidentalydis-engage the delivery hose from the outlet. The pressure delivered at outlet is 5-8 psi.