INTRODUCTION
The Boyle's anaesthetic machine is a continuous flow type of equipment which is used for administration of inhalational anaesthesia and artificial ventilation. It receives gas supply from gas supply unit consisting of cylinders or pipelines, controls this flow of gas to the flow metre, reduces their pressure to the desired safe level, vapourises the volatile anaesthetic agents and finally delivers the gas mixture to a breathing circuit. It was introduced first by HEG Boyle in 1917. After that it was modified at different times which is discussed in ‘Inhalational anaesthesia’ chapter. Now, it has become more modernised and sophisticated by attaching ventilators, O2 failure alarm, hypoxic guard, gas analysing component, different monitors measuring vital parameters of body, electronic sophisticated vapouriser, etc. Now the Boyel's anaesthetic machine have become made extremely sophisticated by incorporating many built in safety features and devices and one or more microprocessors that can integrate, enhance and monitor all the components of the machine. These microprocessors now also provide option for sophisticated ventilator modes, automated record keeping and net working with local or remote computer monitors and as well as with hospital information system. So, it is now called as the anaesthetic work station. Due to this extreme sophistication lots of adverse outcome are now coming infront related to the malfunction of machine. This mainly because of the nonfamiliarity of an anaesthetic machine or work station with the anaesthetist. So this preventable adverse outcome resulting many mishaps can easily be avoided by increasing the familiarity of machine with the anaesthetist through proper education and proper checking the function of machine previously (Fact file-I).
A modern anaestheic machine consists of the following basic components. There are: (i) gas supplying unit such as pipeline or cylinders, (ii) pressure gauges, (iii) reducing valves or pressure regulators, (iv) flow meters, (v) vapouriser, (vi) ventilators, (vii) circle breathing system, (viii) common gas outlet, (ix) monitors, and (x) miscellaneous such as emergency O2 flush, nonreturn pressure relief valve, O2 supply failure alarm, hypoxic guard, suction apparatus, etc. (Fact file-II).
GAS SUPPLY UNIT
Pipeline
Now, in a big hospital or nursing home the supply of gases such as O2, N2O, compressed air, etc, through pipelines to the anaesthetic machine situated at different OT, or ventilators in ICU, ward, etc, from a central source is a common feautre. The advantages of sypplying gases through these pipelines from a central source are that it is easy, convenience, economic and avoid frequent changing of cylinders.
There is also less chance of explosion and increased patient's safety. But the high initial cost is the only disadvantages of it. The pipeline system for delivery of gases consist of (i) a central supplying unit using tank, or concentrator or cylinders for O2, only cylinders for N2O and cylinders or compressor for compressed air, and (ii) distributing pipelines with their outlet located at the point of use. The supplying pipeline is made up of high quality copper alloy which prevents the decomposition of gases and has bacteriostatic property. The usual pressure of gases kept in pipeline is about 400KPa. The size of the pipeline differs according to the demand. The pipes of 42 mm of diameter are usually used for leaving the central supply unit and the smaller diameter tubes of 15 mm are used after repeated branching. They also have specific colour code according to the gas they carry (Fig. 1.1).
The network of central pipelines ultimately terminate in the OT, ICV and/or ward at their outlet which is mounted on the wall or suspended from the ceiling. These outlets of pipeline at the point of use is easily identified by their specific colour code, shape and the name of gas stamped on them. They accept the matched and quick connect / disconnect ‘Schrader’ probe of a flexible colour coded hoses which ultimately connect the outlet of central pipeline to the anaesthetic machine. The anaesthetic machine end of these hoses may be connected with the machine permanently by screw and thread system where the thread is gas specific and not interchangeable, known as noninterchangeable screw threat (NIST) system, or through the ususal yoke assembly with pin index system located on the metal yoke bar of anaesthetic machine where the cylinders are usually attached. The previously described non interchangeable screw and thread system of connection which attaches the hosepipe with the pipeline outlet and the anaesthetic machine is also known as the diameter index safety system (DISS) where a hose of a particular diameter can only be connected to the machine. But the disadvantage in this system is that there is delay in connection with this system. So, the quick connect / disconnect ‘Schrader’ couplers are preferred. Where its other end is connected to the machine through pin index system (Fig. 1.2).
At the central gas suppling room from where the pipelines are distributed there should have a low pressure alarm which will detect the failure of supply of gas through pipeline. A reserve bank of cylinders should be available if primary supply through pipeline fails. An anaesthetist is only responsible for the supply of gas from the terminal outlet of pipeline to the anaesthetic machine, while only engineering department will be responsible for gas pipelines behind the wall. There is also risk of rupture or fire in pipeline carrying O2 gas under high pressure from central source to machine due to worm out or damage of it. So for maintenance and to avoid any mishap the pipelines should be tested from time to time according to the guidelines laid by the international or national committee such as tug test to detect wrong connection, single hose test to detect cross connection, etc. (Fig. 1.3).
The central sources of gases which are distributed by the pipeline may be a storaged tank or cylinder manifold or O2 concentrator for O2, only cylinder manifold for N2O and cylinder manifold or compressor machine for compressed air. In the oxygen storage tank the O2 is stored as liquid between −160° and −170°C temperature at pressure of 5 to 10 atmosphere. Actually, the O2 storage tank acts as huge thermo flask which is made up of double layer steel with vacuum between them. The innersides of these two steel layers is lined by a chemical, named perlite. The vacuum between these two steel layers acts as an insulator and maintain the temperature of inside of the tank. During the use of tank the evaporation of liquid O2 requires heat which is known as the latent heat of evaporation and it is taken from the remaining liquid O2. Thus it helps to maintain such low temperature of the remaining liquid portion of O2 inside the tank. By a coiled copper tubing the cold O2 gas which comes out from tank is warmed for use.
Then a pressure regulator allows the gas to enter the pipelines with pressure at 400 KPa. There is a safety valve on the tank which allows the gas to escape in emergency when excessive pressure builds up inside the tank during no use or under demand use of O2. During excessive use of O2 the control valve which is usually kept closed is opened and it allows the liquid O2 to pass through an uninsulated coils of copper tube. During passage of liquid O2 through this uninsulated copper tube, it evaporates within the tube and can supply more gas. The storaged O2 tank usually rests on a weighing balance. It measures the mass of liquid O2 in tank and thus gives an idea of the total contents of it. A differential pressure gauge is also used which measures the difference of pressure between the liquid O2 at the bottom and the gaseous O2 at the top of the tank and gives an idea of the total contents of a tank. This is because as the liquid O2 evaporates, its mass and pressure at bottom decreases, so by measuring this difference in pressure between the bottom and the top the actual contents of the tank is calculated. At one atmospheric pressure and 15°C temperature 1 litre liquid O2 gives 842 litres of gaseous O2. When the supplying tanks of hospital become empty then liquid O2 is pumped in the tank from an outside O2 tanker by a cryogenic hose assembly. During this process of filling the spillage of cryogenic liquid O2 on the handling person can cause frost bite, cold burns, and hypothermia. Reserve bank of cylinders should always be kept ready when O2 is used from a storage tank particularly in case of sudden accidental failure. The O2 tank should always be housed away from the main hospital building due to fire hazard (Fig. 1.4).
Other than storaged tank, manifold system is also used in a small hospital or nursing home as a central source of supply of O2, N2O, and compressed air. In this system large bulk cylinders (Type E) are used and divided into two groups which alternately supply gases in the pipe line. The number and size of cylinders in each group depends on the expected demand of gas used by the hospital. All the cylinders of each group are connected to a common pipe line through a nonreturn valve and a pressure gauge. This common pipeline from each group then in turn is connected with the distributing pipeline system through a check valve and a pressure regulator. In each group all the cylinders are opend at the same time and allows them to empty simultaneously. When all the cylinders of one group become empty, then the manifold system allows the supply to change over automatically to another group of cylinders. This change over is achieved through a pressure sensitive automatic device that also activates an electric audio signal to alert staff. At the same time the exhausted group of cylinders are turned off automatically. All the exhausted cylinders of previous group are then replaced by fresh full cylinders immediately. The manifold system for the supply of N2O gas may cool to very low temperature due to the latent heat of vapourisation. So, the water vapour of atmosphere may condense or even freeze on the outer surface of the pipeline. This can block also the pipeline and the flow of N2O if it contains some water vapour which is freezed inside the pipe line at this very low temperature. So, a thermostatically controlled heater may be needed at the outlet of N2O manifold system to warm the gas at 47°C which prevents the condensation of water vapour within the gas of pipeline and allow uninterrupted flow of it. Like the O2 tank this manifold system for central supply of O2, N2O and compressed air should also be housed in a well spaced and ventilated room which is constructed by fire proof tiles. This room should be located away from the main hospital building and on the ground floor for the easy access of transport trucks. This room which is used to store tank or manifold system of cylinders should not be used as the general store room for the other empty and full cylinders which are not in use. So, all the empty cylinders should be removed immediately after exhausted. The motor driven compressor for the central supply of air and the O2 concentrator for the central supply of O2 and the central vacuum plant should also be located there. The central vacuum pipeline should be provided with colour code, separate pressure gauge and high and low pressure signal device.
The compressed, oil free medical air which is cleaned by filters is also supplied in hospital through pipelines to run many power driven tools in ICU and OT and for other clinical use at pressure of 400 KPa. They may be supplied from manifold system consisting of large cylinders containing compressed air or more economically by a motor dirven compressor. The anaesthetic machines and the blender of most intensive care ventilators accept the connection from the 400 KPa outlet of compressed air pipeline.
The O2 concentrator is a device which extract O2 from air by differential absorption method and supply it. They become a small one which is designed to supply O2 only for a single anaesthetic machine or a single ventilator. Otherwise it can be large enough to supply adequate O2 through pipeline system. The small O2 concentrators are of light weight, portable and can be used at remote location or for domestic purpose. In this O2 concentrator machine there is a compressor which first filter air from atmosphere and then compressed it. After that this compressed air is exposed to multiple columns of zeolite (hydrated 4aluminium silicate of alkaline earth metal) molecular sieve at a certain pressure which retain N2 and other unwanted components of air except the O2 and argon. Hence the argon cannot be separated from the concentrated O2 produced by this type of machine. Thus maximum concentration of O2 by 95% in volume is achieved. The columns of zeolite molecular sieve in O2 concentrator which absorb N2 and other gases releases them again in atmosphere when it is heated and vacuum is applied. This O2 concentrator can be used in vast majority of cases, but not in circle system. This is because in this system its use leads to gradual accumulation of argon. However this can be avoided only by high gas flow. The main disadvantage of O2 concentrator is its high initial cost which can be recovered easily later by free O2 supply. The other disadvantages of it are risk of fire, contamination of zeolite sieve and sometimes malfunction.
Cylinder
Boyle's anaesthetic machine is equipped with O2 and N2O gas cylinder which are used when there is no provision for pipeline supply of gases or during emergency when the pipeline supply of gases have failed. These gas cylinders are made up of light weight seamless molybdenum steel designed to withstand intense internal pressure when gases are stored in gaseous (O2) or liquid form (N2O) under high pressure. The cylinders which are used in MRI suit are made up of aluminium, However the very large bulk cylinders are made of manganese steel. Very light weight cylinders of O2 also can be made from aluminium alloy with fibre glass covering by epoxy resin matrix. These are used for domestic or transport purposes in ambulance or mountaineering purposes (Fig. 1.5).
The cylinders supplying gases are identified by their specific colour code, labelling stamped on their shoulder and plastic or paper collar. In the past different countries use different colour for their cylinders containing different gases and there was no pin index system. So, with time when this colour is lost and the level is indistinguishable, then any interchange of N2O and O2 cylinder during attachment to the anaesthetic machine can lead to mortality. So an international standard which is given in table was laid out regarding the colour code and pin index (discussed later) by which the cylinders can easily be identified and cannot be interchanged when they are attached to the anaesthetic machine (i.e. it is practically impossible to attach any cylinder to wrong yokes). There are two international standard of colour code according to the school of UK and USA. However, in India UK standard is followed (Table 1.1).
The cylinders are also manufactured in different sizes which are usually named by alphabet from A to L. Among these the size A is the smallest and size L is the largest. The smallest sized A cylinder can hold 1.2 L of water and the largest sized L cylinder can hold 50L of water. Among these the A and H size cylinders are not used for medical purposes. The cylinders which are attached with the anaesthetic machine are usually of D and E size and the cylinders of size J are commonly used for cylinder manifold system for central supply. The O2 and N2O cylinder of size D contains 400 L of O2 and 940 L of N2O respectively. Whereas the O2 and N2O cylinder of size E contain 680 L and 1800 L of O2 and N2O respectively. A full O2 cylinder of any size at atmospheric pressure can deliver O2 which is 130 times of its original capacity.
The O2 is stored in a cylinder as a gas at a pressure of 1900 psi and N2O is stored in a cylinder as a liquid at a pressure of 750 psi. So, the cylinder which contain a gas in the form of liquid such as in N2O and CO2 is partially filled. Hence, this amount of partial filling of cylinder is described as the filling ratio and is defined as the weight of fluid in a cylinder divided by the weight of water required to fill the cylinder. The cylinders containing gases in liquid form are not filled up fully. This is because the partial filling of cylinders with liquid such as in the case of N2O and CO2 reduces the risk of explosion due to the sudden dangerous increase in pressure within the cylinder sudden increase in evaporation of liquid gas during sudden increase in temperature of atmosphere. So, in cold country such as in UK the filling ratio in N2O and CO2 cylinder is kept at 0.75, whereas in hot countries the filling ratio of these two types of cylinders is kept at 0.67.
Only a gas containing cylinder during its emptying at constant temperature shows a linear and proportional decrease in cylinder pressure. But this does not happen in case of cylinder which are filled with gas as liquid such as N2O and CO2. Here, initially the pressure in side the cylinder remains constant, because more gas is produced by 5evaporation from liquid to replace the gas that is used. After that once when all the liquid has been evaporated to gaseous state, then the pressure in the cylinder starts to decrease with the process of emptying. So, the O2 pressure gauge shows continuously the contents of a cylinder which is proportional to the gauge pressure and the N2O pressure gauge does not show the actual content of the cylinder. The N2O pressure gauge shows a constant pressure of N2O gas which is present above the liquid and till the later is completely depleted. Then the pressure in the gauge starts to drop. So, the N2O pressure gauge does not show the actual contents of its cylinder till the whole liquid is completely evaporated to gas. During the emptying of such cylinder containing gas as liquid, the temperature of it also decreases. This is because of the withdrawn of latent heat for vapourisation of liquid within the cylinder from the outside atmosphere, leading to the formation of ice on the outside of the cylinder. As the pressure gauge of N2O cylinder can not tell the total content of it, so a full N2O cylinder can only be identified by comparing the weight of it with that of an empty one, or in other way a full cylinder will give a ringing sound when tap by a metal while a empty will give a dull thud sound.
The cylinders are tested following manufacture at regular intervals, usually 5 years, by:
- visual inspection from outside or inside (endoscopic),
- tensile test where strips of a cylinder are cut longitudinally and stretched till they elongate with yield point not being less than 15 tons/sq inch,
- flattening test where one cylinder is kept in between two compression blocks and pressure is applied to flatten it till the distance of these two blocks becomes 6 times of the thickness of the cylinder wall without crack,
- bend test where a strip of 25 mm width is cut from the cylinder wall and equally divided into 4 strips which are then bend inwards till the inner edge is apart proving cylinder wall will not develop any crack,
- impact test where three longitudinal and three transverse strips are cut from a cylinder wall and struck by mechanical hammer with mean energy needed to produce a crack it should not be less than 5 ft lb for transverse strip and 10 ft lb for longitudinal strip,
- pressure or hydraulic or water jacket test where the cylinder is subjected to high pressure which is more than 50% of their normal working pressure without damage. All these test are usually done for at least one out of every 100 cylinders. The gases and vapours should be free of water vapour when stored in cylinders. Becasue water vapour may freeze and block the exit port of cylinder when the temperature of cylinder valve decreases tremendously on opening. All the cylinder after filled with gas should be stored in a dry, well ventilated, fire proof room and not subjected to extremes of heat. They should not be stored near flammable materials like grease or oil, etc., or near any source of direct heat or fire (Fig. 1.6).
Fig. 1.6: The different parts of a cylinder and different combination of pin index. Gas exit port accommodates the yoke nipple and the hole accommodate the pine of pin index system
A cylinder has four parts such as body, shoulder, neck and valve. The size of the body of a cylinder varies according to the designations which are named as A to L and is painted according to the colour code. The upper part of the cylinder which is called as the shoulder suddenly becomes narrow. This is called as the neck. The shoulder and the neck of a cylinder is also painted according to the colour code which may be the same with the body or not. The neck ends in a tapered screw thread into which the valve of a cylinder is fitted. This thread between the neck and the valve of a cylinder is sealed by a special material which melts if the cylinder is exposed to excessive heat suddenly, and allows the contents of a cylinder to escape avoiding the risk of explosion. There is a plastic disc around the neck whose colour and shape indicates the year when the cylinder was last examined. The marks engraved on the shoulder of a cylinder are: date of last test performed, test pressure, chemical formula of the contents of this cylinder and tare weight (weight of empty cylinder). Every cylinder should also have a paper lebel which is sticked on the body or will hang from the neck. This will show: cylinder size code, specification of contents (which include name, chemical symbol, pharmaceutical form, proportion of gases in a gas mixture), batch number, maximum cylinder pressure in bars, nominal cylinder contents in litre, filling and expiry date, direction of use, hazard and safety instruction, storage and handling precaution, etc. (Fig. 1.7).
At the top every cylinder is fitted with a valve which is known as the cylinder valve. Several types of cylinder valves like flush type, bull nosed, straight type, angle type, etc., are available. But the noninterchangeable flush type of valve with pin index system which is commonly used to attach a cylinder at the yoke bar of anaesthetic machine will be discussed here. It is screwed into the neck of a cylinder via a threaded connection which is sealed by a special material with low melting point.
This cylinder valve is made of brass and plated with chromium or nickel which allow the rapid dissipation of heat, if generated due to compression during filling (Fig. 1.8A).
The chemical formula of gas by which the cylinder is filled up is engraved on its valve. The valve seals the contents of a cylinder and is used to start, regulate and stop the flow of gas from the cylinder by a spindle which is described below. On the top of the cylinder valve there is an on/off stem or spindle with packing nut. When this stem is turned with a spanner, then it allows the gas to flow through its outlet situated on the valve. In modern modification the top of the valve is so designed that the on and off of the cylinder can be done by manual turning of the stem or spindle with the packing nut without the need of a spanner. There is an outlet hole and another two holes below the previous one at one side of the cylinder valve which fits with the yoke assembly of Boyle's machine through a specific noninterchangeable pin index system consisting of a nipple and two pins. A compressible yoke sealing washer named Bodok seal should be placed between the anaesthetic machine and the cylinder valve to make a gas tight joint when the cylinder is connected to the machine at the yoke assembly. Corresponding to the two pins of the pin index system, there is also two holes on the same side of cylinder value below the outlet port. If the yoke nipple is damaged or the pins of yoke assembly and the holes of cylinder valve are not aligned properly (i.e. pin index of a particular cylinder valve does not match with the yoke assembly), then the gas exit port of the cylinder valve will not seal tightly against the Bodok washer and the gas will leak. The Bodok washer is made of carbon impregnated rubber with a metal ring around it. It is 2.4 mm thick and only one seal is allowed inbetween the cylinder valve and the yoke assembly to fit the cylinder without leak. The excessive tightening of the screw of yoke assembly to press the cylinder valve against this seal may also damage it (Fig. 1.8B).
The cylinder valves are usually wrapped by a plastic covering after filling to protect it from anything which can enter the exit port and block it. The valve should be slightly opened and then closed before connecting the cylinder with the anaesthetic machine. This procedure usually cleans dust, oil, and grease from the exit port which would otherwise enter the anaesthetic machine and may damage it. The cylinder valve should be turned on slowly during use when attached to the anaesthetic machine. Because it prevents the sudden rise in pressure and temperature of gas while flowing through in the machine's pipe line. During closure of cylinder, overtightening of valve should also be avoided. Because it may damage the seal between the valve and the neck of a cylinder (Fact file-III).
Bourdon Pressure Gauge (Fig. 1.9)
It is attached with the anaesthetic machine to measure the pressure of gas with in the cylinder such as O2, N2O, and compressed air, or pipe lines after connecting with the anaesthetic machine. However, one which is designed to measure the pressure of cylinder should not be used for pipeline and vice versa. Because it may lead to inaccurate result and can cause damage to the pressure gauge. Inside this type of pressure gauge there is a robust, but flexible coiled tube made of copper alloy. It is closed at its inner end and is connected through a lever to a needle pointer which moves over a dial indicating pressure. The other end of this coiled tube is opened to a gas supply line coming from cylinder or pipeline (Fig. 1.10A).7
After turning on the valve and opening the cylinder the gas under high pressure first starts to flow into this colied copper tube of pressure gauge and causes it to uncoil or straight out. Then this movement of the tube during uncoiling causes the needle pointer to move on dial and indicates pressure of gas inside the cylinder or pipeline. Each pressure gauge is calibrated for a particular gas, colour coded and bears the name and symbol of gas for which types of gas cylinder or pipeline it is used. At the front, every pressure gauge is protected by a cover of heavy fibre glass. So, in case of any breakage of coiled copper tube, the gas escape from behind, rather than the front. The measured pressure in the pressure gauge is depicted in unit such as KPa or lbs/sq.inch or Kg/sq.cm. When the central pipeline for O2 supply is connected to the anaesthetic machine, then the pressure gauge at the connection shows 4 bars or 60 psi pressure (Fig. 1.10B).
Reducing Valve or Pressure Regulator
The gases are usually presented to anaesthetic machine under different high pressure from different types of cylinders or pipelines. So, they are passed through a single or multiple reducing valves which are placed between the cylinder or pipelines and the rest of the components of anaesthetic machine to decrease their high variable pressure to a safe constant operating pressure before reaching the gas to flow metre. Otherwise in the absence of these valves when the pressure of a cylinder decreases with use, then in order to maintain the supply of gas to patient at a constant flow and pressure, continuous adjustment of flow metre is required. These reducing valves also allow a delicate control of gas flow through flowmetre and protect the different sophisticated component of anaesthetic machine against the sudden surges of high pressure of gases (Fig. 1.11).
In this type of pressure regulator or reducing valve there are two chambers such as a high pressure chamber and a low pressure chamber, connected through a gap which is guarded by a valve. The high pressure chamber gets its gas flow through its inlet directly from cylinder. The small valve intervening between the high and low pressure chamber is attached to a diaphragm which is again attached to a spring through which the pressure regulator can be adjusted to get the supply of gas flow at a constant low pressure.
After entering the gas into the high pressure chamber directly from cylinder the force exerted by the gas under high pressure tries to close the gap by the small valve and decrease the gas flow to low pressure chamber from high pressure chamber. On the other hand, the opposite opening force exerted by the spring and diaphragm tries to open the valve. Then a balance is reached between these to forces leading to a constant fixed opening or gap which leads to a constant flow of gas under fixed desired pressure to low pressure chamber from high pressure chamber and ultimately passes out (Fig. 1.12).
If the gas in cylinder contains water vapour, then when the gas with water vapour enter the low pressure chamber from high pressure chamber, then due to loss of heat due to expansion of gas in low pressure chamber, there is chance of ice formation inside the regulator causing malfunctioning of it. There is also the chance of rupture of diaphragm leading to mulfunctioning of it. So, these regulators should be serviced at regular intervals and the rubber diaphragm is checked and renewed. The control of high pressure in pipeline is also achieved by a flow restrictor (a separate type of device which control the flow of gas) and a second stage pressure regulator. If there is only flow restrictor and no pressure regulator for pipeline, then when there is some change in pipeline pressure, the flow metre should be adjusted accordingly. A one way valve is also placed within the cylinder supply line within the anaesthetic machine next to the inlet of yoke. Their function is to prevent the back flow and loss or leakage of gas through an empty yoke (if a cylinder is not connected there) from working cylinder. They also prevent the transfilling of gas when one cylinder is full and working and the other cylinder is empty. Recently this one way valve is incorporated with in the design of a pressure regulating or reducing valve.
This type of reducing valve is also called the preset pressure regulator, because by adjusting the screw before hand during manufacturing we can adjust the diaphragm and subsequently the valve which is situated between the high and low pressure chamber. Thus we will be able to keep a fixed low pressure in low pressure chamber from which the gas will be delivered continuously at low pressure to the flow meter. In India BOC uses the preset regulator or reducing valve which are set to deliver gases at constant low pressure of 60 lbs/sq.inch by adjusting the screw and thus subsequently adjusting the inside diaphragm and valve of regulator. There is another type of preset pressure regulator which is called the Adam's valve. These are used in many anaesthetic machines (Fact file - IV).
Pressure regulators are so adjusted that the anaesthetic machine uses both the gas from pipelines when the pipeline pressure is 50 psi or greater and cylinder valve is also simultaneously open. So, when the gases from pipeline are being used, the cylinder valve should be closed. This is because the machine will always use gas from the source that has higher pressure. But, sometimes if the pipeline pressure drops below that supplied by cylinder and its valve is open, then some gas will be withdrawn from the cylinder. Thus gradually the cylinder will be exhausted without the knowledge of anaesthetist and then it will not help during emergency.
Flow Metre (Fig. 1.13)
It is incorporated into the Boyle's anaesthetic apparatus to measure the flow rate of gases such as O2, N2O and air passing through them.
The flow metre used in anaesthetic machine is also known as the rotameter. The other type of flow metre used in industry are: Waterside, Heidbrink, Connell, Foreggar, etc. The flowmetre consists of a series of specially designed glass tube (Thrope tube) with rotating bobbin inside it to measure the flow of individual gases in the flow metre. The tubes are placed within a chromium plated metal casing and in front there is a transparent plastic window which helps in clear reading and protection of flow metre tube from damage and dust. A detachable radiolucent plate is also provided at the back of the metal casing of flowmeter to facilitate the observation of a working and rotating bobbin within the tube during use in a darkened operation theatre. A flow metre basically has two components such as flow control valve and flow metre tubes with rotating bobbin inside it (Fig. 1.14).
The flow control valves control the flow of gas through the tubes of flow metre by manual adjustment of its knobs. It is situated at the base of the flow metre and its body is made of brass. The stem of the adjusting knob screws into the body of flow control valves and ends as a needle which is placed at the site of inflow of gas to the tube of flow metre. The flow control knobs which are attached to the stems of flow control valve are labelled and colour coded for their respective gases. In some design the O2 control knob is larger and has a longer stem than the other stems and knobs used for other gases. So, this makes it easily recognisable and acts as a safety measure. In some designs a flow control knob guard is attached to protect against the accidental adjustment of flow metres (Fig. 1.15).
The tubes of flow metre are especially made of tapered glass tube with a rotating bobbin inside it. They are set strictly in vertical position on the body of flow control valve. Because inclined position of tube gives incorrect reading by causing friction of bobbin on the wall of the tube and thus producing resistance during the flow of gas. Each tube is individually calibrated at room temperature and one atmospheric pressure for that gas which flows through it giving accuracy of about ±2% in measuring the rate of flow of gases. For flows below 2 L/min the measuring units are 100 ml/min and for flows above it the measuring units are L/min. The rotating bobbin or ball in the flow metre tubes which shows us the rate of gas flow through it are made of light aluminium. They are held floating within the tube by the gas flowing around it through the gap between the tube's wall and bobbin. During floating the effect of gravity on the bobbin is counteracted by the flow of gas. When the bobbin is lifted by the flow of gas, then the upward pressure caused by the gas and the weight of the bobbin is in equilibrium at that height of bobbin showing the rate of flow of gas. The flow metre tubes are tapered in such a fashion that the clearance or gap between the bobbin and the tube wall gradually widens from the bottom to the top. So, at low flow rate the clearance between the bobbin and the wall of the tube is longer and narrower acting as a tube and at these circumstances the flow is laminar, governed by the viscosity of gas. On the other hand, at high flow rate the clearance between the bobbin and the wall of the tube is wider and shorter acting as a orifice. Thus, under these circumstances the flow is turbulent and governed by the density of gas. So, each flow metre is calibrated for its specific gas, according to its density and viscosity (Fig. 1.16).
Fig. 1.15A: The route of flow of gases through the tubes of flow metre. B shows that the gap between the bobbin and tube wall increases as bobbin goes up
The flow metre can give inaccurate result if the bobbin sticks to the wall of the tube due to dirt from contaminated gas supply and / or due to static electricity caused by the continuous friction arising from rotating bobbin during floating.
The problem of dirt can be eliminated by using filter at the gas inlet site of anaesthetic machine and the problem of static electricity can be solved by making the bobbin of antistatic material or applying some antistatic spray over it or coating the tube's interior with a conductive substance which grounds the system and reduces the effect of static electricity. At the upper margin of the bobbin there are many cuts or slits (flutes) at the sides. So, when the gas flows by the side of it, the flutes cause the bobbin to rotate. There is radiolucent dot on the bobbin and it indicates that the bobbin is rotating and is not stuck to the wall of the tube. There are two bobbin stops which are made of spring and is situated at the extreme either end (top and bottom) of the tube. It always ensures the visibility of bobbin during operation at the extremes of flow. According to the shape and size different types of bobbins are also used in the flow metre such as ball, nonrotating H float, skirted and nonskirted. But usually the ball and the skirted varities are commonly used in anaesthetic machine. The reading of the flow metre is taken from the top of the bobbin. But when a ball is used then the reading is generally taken from the midpoint of the ball. When very low flow is required such as in circle breathing system, then an arrangement of two flow metre tube for each gas which are attached in series in rotameter are used for the fine adjustment of flow. But these two tubes are controlled by single flow control valve and knob (Fig. 1.17).
The O2 tube of a flow metre is kept at extreme right of all the gas tubes. Because when it is placed at extreme left of all the tubes, then if any crack develops in a flow metre distal to it, O2 may leak through this distal crack and may deliver hypoxic mixture to the patient. So, to avoid this problem O2 is the last to be added to the gas mixture and is finally delivered to the back bar of anaesthetic machine. During mechanical ventilation pressure rises at the common gas outlet when the bag is compressed by ventilator or manually. This is transmitted back to the gas in the tube of flow metre above the bobbin which results in the drop of it during inspiration and inaccurate reading. This can be prevented by attaching a flow restrictor at the down stream of flow metre (Fact file - V).
Antihypoxic Devices
There are many devices which are incorporated in the modern anaesthetic machine to stop the flow of N2O in the absence of flow of O2 or there should be a minimum 25% concentration of O2 in the gas mixture or the machine will give audible alarm when O2 pressure drops in the pipeline of machine. These antihypoxic devices are consists of hypoxic guard and O2 failure alarm. This hypoxic guard device maintain minimum 25% flow of O2 in the gas mixture or when O2 flow is reduced below 25% of total flow then the N2O flow will be automatically reduced.
Fig. 1.17: How the O2 will leak from cracks at different sites if its tube is placed at extreme left
It works by mechanical, pneumatic or electronic principle. In mechanical method the N2O and O2 flow control valves are linked together by a chain. This chain relays the movement of O2 knob to the N2O knob. So, when the O2 flow control knob is turned to reduce the flow of this gas, then the chain link will also move and reduce the flow of N2O as if always a minimum 25% of O2 mixture can reach to the patients or without the flow of O2, the N2O will not flow alone. The O2 flow control knob can be independently opened further. But it cannot be closed below a setting that will produce less than 25% O2 in the gas mixture if N2O is used. In pneumatic method there is special type of valve known as the ratio mixer valve where O2 is exerting pressure on one side of the diaphragm and 11N2O on the other side. Thus when there is increased flow of N2O, then it will also cause increased flow of O2 maintaining a minimum 25% concentration of it. But when O2 flow is only increased, the flow of N2O will not increase. In electronic device a paramagnetic O2 analyser is used to analyze the mixture of gases which are sampled continuously. Then if due to any reason the O2 concentration falls below 25% in the inspired gas mixture, then the flow of N2O will also be stopped and give an alarm (Fig. 1.18).
In O2 supply failure alarm when the pressure in the pipeline of machine carring only O2 drops below a certain fixed lavel then the O2 is directed through a whistle to produce a sound causing alarm. This alarm is activated only when the pressure of O2 in the machine gas pipeline falls below 200 KPa. This alarm cannot be switched off unless the O2 supply is restored.
Vapourisers
A vapourisers is a device by which a controlled amount of volatile anaesthetic agent is added to the fresh gas flow mixture after vapourising it from liquid. Initially all the types of vapourisers are divided under two broad headings: variable bypass vapourisers and measured flow vapourisers (Fig. 1.19).
In variable bypass vapourisers the fresh gas flow is first splitted into two so that only a small portion of it passes through the vapourisers and when it passes over the liquid volatile anaesthetic agent in the vapourising chamber it becomes saturated with the vapour of this anaesthetic agent. Then, it leaves the vapouriser to mix with the remaining fresh gas flow that has gone through the bypass. The final desired concentration is achieved by varying the splitting ratio between the bypass gas and the gas that enters the vapourising chamber of a vapouriser using an adjustable valve (regulating dial). On the otherhand, vapouriser can be designed so that it heats the anaesthetic liquid to a temperature above its boiling point and make it a gaseous state which is then allowed to leave the vapouriser in calculated amount (measured) controlled by regulating dial to mix with the fresh gas flow in achieving desired concentration of it. These are known as the measured flow vapourisers. The example of this measured flow vapouriser are TEC-6, TEC-6 plus, D-TEC, where desflurane is only used.
The variable bypass vapourisers are again divided into two types such as draw over vapourisers and plenum vapourisers. In draw over vapourisers a portion of fresh gas flow regulated by controlled knob is allowed to simply flow over the liquid volatile anaesthetic agent and to pick up the vapour of this agent. They are of low resistance and the anaesthetic vapour leaving the vapouriser is not saturated with the vapour of liquid anaesthetic agent. The splitting valve which divides the gas flow to the vapourisers and to the by pass channel is of wide bore and works over a wide range of flow rates. This type of vapouriser is also known as the ‘inside the circuit’ vapouriser, because being of low resistance and having no unidirectional valve, it acts as a continuous component of the circuit of the anaesthetic machine. On the otherhand, in plenum (which means high resistance, and unidirectional) vapouriser the carrier gas which enters the vapourising chamber of vapouriser is made to be saturated by the vapour of volatile anaesthetic agent present in the vapouriser and pressurised (cause is described later) so that it is rather forced to mix with the bypass fresh gas flow than it simply blows over the surface of the volatile liquid anaesthetic agent in vapourising chamber taking the unsaturated vapour of it like draw over vapouriser. The vapourising chambers of plenum vapourisers act as pressurised chamber containing saturated vapour of volatile anaesthetic agent at all times from where continuous flow of gas saturated with anaesthetic agent comes out. This is made possible by increasing the capacity of vapourisation of liquid volatile anaesthetic agensts to very high level by adding wicks and baffles inside the vapourising chamber, and restricting the exit of gas from the chamber by control valve than the vapourising capacity.
These plenum vapourisers are also called the ‘outside the circuit’ vapourisers, because they do not acts as the part of anaesthetic circuit due to high resistance and spilitting valve is of narrow bore. They do not act over the wide range of flow rate like draw over vapourisers and so it allow the vapouriser to calibrated very accurately. The examples of the draw over vapourisers are: Boyel's ether bottle vapouriser, Goldman vapouriser, Oxford miniature vapouriser (OMV), EMO vapouriser and TEC-3 vapouriser. The TEC-3 vapouriser is a draw over vapouriser but temperature compensated. 12The examples of plenum vapourisers are: all the temperature compensated (TEC) vapourisers of Datex ohmeda series such as TEC-4, TEC-5, TEC-7 and Drager vapouriser 19 and 2000 series. These different models or types of plenum vapourisers differ among them only in their interior design regarding the arrangement of baffles and wicks to increase the capacity of vaporization made by different companies to remove some technical disadvantages of previous one, but their basic mechanism of action is same. The old model plenum TEC-3 vapouriser is out of market now. At present the commonly used model of plenum vapourisers are TEC-5 and TEC-7 of Datex ohmeda series and Drager vapouriser of 19 and 2000 series. The Aladdin vapouriser which is discussed in more details in chapter is another example of plenum vapouriser with electronic control for vapourisation. The TEC-6 vapouriser which is developed to use only desflurane is the example of measured flow vapouriser (Fig. 1.20).
All the plenum vapourisers are temperature compensated. It means with cooling of volatile anaesthetic agent during vapourisation, the delivered vapour concentration of it after vaporization of anaesthetic agent does not reduce. This is achieved by controlling the splitting ratio of fresh gas flow which enter the vapouriser by a temperature sensitive bimetallic strip which is made of two strips of metal with different coefficients of thermal expansion bonded together. It allows more flow into the vapourising chamber by bending as the temperature decreases and vice versa. This bimetallic strip or temperature sensitive valve is located in the vapourisation chamber in TEC-2 model. Whereas in the TEC-3, 4 and 5 model it is situated outside the vapourisation chamber (Fact file - VI).
All the variable bypass plenum vapourisers are flow compensated and its explanation is described below. On entering the vapouriser the fresh gas flow is splitted into two streams. The main larger stream of gas flows through the bypass channel and the smaller narrow stream of gas flows through the vapourising chamber of vapouriser. After vapourisation these two gas streams again reunite when they leave the vapouriser together. This is controlled by a regulating dial which dictate how much gas will enter the vapourising chamber, then after being completely saturated with anaesthetic agent, will be reunited with the fresh gas flow coming through bypass channel. The vapourising chamber of a plenum vapouriser is such designed that the gas leaving it is always fully saturated with vapour of volatile liquid anaesthetic agent before it joins with the gas of bypass stream whatever may be the amount of fresh gas flow into the vaporising chamber. This is achieved by increasing the surface area of contact between the gas entering the vapourising chamber and the volatile liquid anaesthetic agent by adding wicks soaked by the agent and a series of baffles. Thus whatever may be the rate of fresh gas flow through vaporising chamber the delivered concentration of anaesthetic agent can be controlled by both controlling the flow of gas entering the vapourising chamber and thus controlling the rejoining of gas saturated with anaesthetic agent with the main gas flow by controlling the dial. Thus it will not be influenced by the rate of fresh gas flow through vaporizer like the draw over vapriser such as goldman, ether vaporiser, etc. In modern designs the anaesthetic concentration of volatile anaesthetic agent supplied by vapouriser is independent of gas flow through vaporising chamber between 0.5 to 15 L/min. Hence, they are flow compensated.
But the changing of composition of gas from 70% N2O to 100% O2 may increase the concentration of anaesthetic agent due to the greater solubility of N2O in volatile liquid anaesthetic agents. In comparison to the flow compensated vaporisers the draw over vapourisers are not flow compensated, because under certain dial setting the portion of gas entering the vapourising chamber varies with the rate of fresh gas flow, and the gas coming out from the vapourising chamber is not fully saturated. Thus the rejoining of gas mixed with variable concentration of anaesthetic agent to the main gas flow is not controlled. During vapourisation due to the loss of latent heat of vapourisation the cooling of anaesthetic agent occurs and makes it less volatile, reducing vapourisation. So, in order to compensate this heat loss two measures are taken. One, the vapouriser is made up of such material which has high density, high specific heat and high thermal conductivity such as copper. So it acts as heat reservoir and readily gives heat to the cooled anaesthetic agent, maintaining its temperature and vapourisation nearly constant. Two, a temperature sensitive valve made of bimetallic strip allows more flow into the vapourising chamber by bending as the temperature decreases and vice versa.
All the modern vapourisers are agent specific. So, filling of them with wrong agents should be avoided. For example filling of an halothane specific vapouriser by sevoflurane would lead to anaesthetic concentration in under doses and vice versa. This is because the vapour pressure of halothane at one atmospheric pressure and 20°C is 243 mm of Hg, whereas the same of sevoflurane is 160 mm of Hg. So, if sevoflurane is used in halothane TEC-7 or TEC-5 vapouriser, it will cause near about 40% lesser amount of anaesthetic concentration to be released and vice versa. So, the modern TEC-5 and TEC-7 vapourisers (TEC-4 is also obsolete now)are equipped with agent specific filling ports and colour coded agent specific filling devices to prevent the use of wrong agent in wrong vapouriser. In older vapourisers during IPPV there may be transient reversal of flow of fresh gas and pressure into the vapouriser through the by pass channel and it will lead to the delivery of unpredictable concentration of anaesthetic agent. This is known as the pumping effect which is more pronounced with low gas flow. Hence in modern vapourisers some change in design and placement of a one way check valve limit the occurance of this problem. During transport or due to any reason if vapourisers are tilted excessively then the anaesthetic agent may spill over and flood the by pass channel of vapouriser which may lead to sudden delivery of dangerously high concentration of anaesthetic agent when first used. So, during handling of it when not attached to machine, excessive tilting of vapouriser should be avoided.
Ventilator
All the modern anaesthetic machines are equipped with ventilator for IPPV during anaesthesia which are of bag in bottle type in most of the cases. They have mainly the CMV mode for ventilation, but some have the facilities to provide other few more mode for ventilation modes such as SIMV, CPAP and PEEP. These bag in bottle type of anaesthetic ventilators have mainly two basic components: a driving unit and a control unit. The driving unit consists of a chamber with tidal volume ranging from 0 to 1500 ml and an ascending or descending type of bellow receiving fresh gas flow within it. In paediatric version the tidal volume in chamber ranges from 0 to 400 ml, whereas in adult version the tidal volume in chamber ranges from 100 to 1500 ml. The controll unit of the ventilator contains variety of controlling knobs, display system and alarms. The controlling knobs include the respiratory rate, tidal volume, airway pressure, I/E ratio, power supply, etc., to regulate the IPPV of patient during anaesthesia (Fig. 1.21).
In these types of bag in bottle ventilator compressed air or O2 is used as the driving pressure. On entering the driving chamber of ventilator this driving gas forces the bellows down in case of ascending type of bag in bottle ventilator and delivers the fresh anaesthetic gas mixture to the patient which was accommodated in side the bellow during the previous expiratory phase of respiratory cycle. The volume of driving gas entering the chamber is always equal to the tidal volume and remains completely separated from the fresh gas flow which remarks in side the bellow. Then during expiration the bellow again ascends due to the flow of fresh gas mixed with expired gas within it and the driving gas comes out (Fig. 1.22).
There are another type of bellow known as the descending bellow. Here, during expiration the gas sucked from patient into the bellow by a weight placed at the base of it. So, the probable advantage of this type of descending bellow is the absence of expiratory resistance. This advantage is not available in ascending bellow as it is claimed that the pressure required to fill the bellow both by fresh gas from machine and expired gas from patient adds some expiratory resistance to patient and may prevent complete exhalation.
Therefore, it is claimed that ascending bellow provides a degree of PEEP (2 to 4 cm of H2O) which may otherwise be beneficial.
The another advantage of ascending bellow is that it collapses to an empty position and remains stationary at that empty position if there is any leak in bag or disconnection of circuit. Whereas in descending design of bellow it automatically hangs down to fully expanded position, even if there is any leak or disconnection of circuit and may continue to move almost normally. In such condition the driving gas would also be able to enter the bellow through the leak and dilute the anaesthetic gas mixture within it which is not possible in ascending variety.
The arrangement in descending bellow also allows the driving control unit to be placed above the bellow in the free standing version. So the bellow of it could be placed on the lower shelf of an anaesthetic machine with the controls panel easily at hand. However the descending bellow is now no longer popular (Fact file- VII to IX).
Miscellaneous
Non return pressure relief valve
It is situated on the back bar of anaesthestic machine after the vapouriser or at the common gas outlet. Here it acts as nonreturn valve and helps to prevent the effect of back pressure on the vapourisers or flow metre during positive pressure ventilation by ventilator or manual. It also opens when the back bar pressure exceeds 30 KPa and acts as pressure relief safety valve.
Emergency O2 flush
It is presented in the machine as nonlocking button and when is activated by manual 15pressure then pure O2 at the flow rate of 30 to 70 L/min is supplied to the patient from the common outlet of anaesthetic machine bypassing the flow metre and vapourisers. It also is used to flush the breathing circuit or to rapidly refill the breathing bag. It should not be activated when the minute volume divider ventilator is in use. Injudicious use of this emergency O2 flush may dilute the anesthetic gases and will cause the inadequate depth of anaesthesia and awareness. It may cause barotrauma also when the patient is connected to a completely closed breathing circuit.
Common gas outlet and O2 analyzer
All the anaesthetic machine has only one common outlet supplying fresh anaesthetic gases mixed with O2 to breathing circuit in contrast to multiple inlets which supply different gases to the machine. Modern machines are equipped with devices which measure the flow of gases through this common outlet and gives signal during detachment of this outlet from the breathing circuit.
It is fundamental to monitor the inspired O2 concentration (FiO2) or partial pressure of it in the gas mixture delivered to the patient. Without an O2 analyser which measures the inspired O2 concentration an anaesthetic machine is always incomplete and general anaesthesia should never be administered. It is placed in the inspiratory or expiratory limb if close circuit is used. Otherwise, if the circuit has one limb, then it should be placed at the patient's end of it. But it should not be placed at the fresh gas line. Due to O2 consumption by patient the partial pressure of O2 in expiratory limb is slightly lower than that of the inspiratory limb. An audible alarm can be set for high and low concentration of O2 such as 40% and 28% respectively.
Three types of O2 analyzers which measures FiO2 are used in modern anaesthetic machine. These are: paramagnetic, fuel cell (or galvanic), or Clark electrode (polarographic). The paramagnetic sensor works on the principle that only O2 is attracted by the magnetic field whereas the other gases are repelled. This attraction and repulsion of individual gas depends on their concentration and partial pressure in the sample gas. It is costly than the others and has no consumable parts without requiring frequent replacement. Its response time is very fast than the galvanic and Clark electrode O2 analyzers and can differentiate the partial pressure of O2 between the inspired and expired air as it measures the inspired and expired O2 concentration simultaneously on breath by breath basis. However, this analyzer is affected by water vapour in sample gas. Therefore a water tap is incorporated in the design. The another advantage of this paramagnetic sensor is, it is self calibrating. The galvanic and polarographic sensor is also called the electrochemical sensor, because both of them contain the anode and cathode electrodes embedded in an electrolyte gel which is separated from the gas sample by an O2 permeable membrane. After diffusing through the membrane, O2 reacts with the electrode in the gel and produces a current which is proportional to the concentration and partial pressure of it in the sample gas. Thus they measure the partial pressure of O2 as a percentage. These galvanic and polarographic O2 analyzer have slow response time (20 to 30 seconds), because they are dependent on membrane diffusion of O2. These sensors have limited life span to about 1 year, because of the exhaustion of material of it due to continuous exposure to O2. It needs regular service and calibration is achieved by using 100% O2 and room air (21% O2). It reads only inspiratory or expiratory O2 concentration and water vapour does not affect its performance.
N2O and other inhalation anaesthetic agent concentration analyzer
The measurement of inspired and end tidal concentration of N2O and other inhalational anaesthetic agents are very important, mainly when the circle system is used. This is because the expired inhalational anaesthetic agents are recirculated and are added to the fresh gas flows which is also carrying the volatile anaesthetic agents. So, the ultimate inspired concentration of inhalational anaesthetic agents is different from the setting of vapouriser, especially during low flow. Hence, the modern analyzers can assure the inspired concentration of all the inhalational anaesthetic agents such as N2O, halothane, isoflurane, sevoflurane, desflurane, etc. The principles by which the concentration of inhalational anaesthetic agents are measured are: infrared technique, ultraviolet ray absorption technique, mass spectrometry, Raman spectroscopy, Piezoelectric quartz crystal oscillation technique, etc.
In infrared technique a light of wavelength of 4.6 nm is used for N2O. On the otherhand an infrared light of wavelength of 8 to 9 nm is used for other volatile anaesthetic agents. This is to avoid interference from the methane and alcohol that happens at the lower 3.5 nm wavelength. Some infrared analyzers are not agent specific. These must be programmed by the user for specific agent being administered. Incorrect programming result in incorrect result. In Piezoelectric oscillation technique a lipophilic coated Piezoelectric quartz crystal is used which undergoes continuous changes in its frequency of oscillation when lipid soluble inhalational anaesthetic agent is exposed to it. This changes in oscillation is directly proportional to the concentration of agent. Mass spectrometer is used to analyze the inhalational anaesthetic agents on breath to breath basis. In this technique the principle of action is to change the particles of sample gas by bombardment of them with electron beam and then to separate the components arising from this bombardment by a magnet into different spectrum according to their specific mass: charge ratio.
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The relative concentration of ion in a spectrum of a specific mass: charge ratio is determined by the concentration of a particular agent in gas mixture (Table 1.2).
Measurement of tidal and minute volume
During GA the measurement of tidal volume and from it the measurement of minute volume is very critical which the anaesthetic machine performs by Wright spirometer (respirometer), hot-wire anemometer, ultrasonic flow sensor and pneumotochograph etc. These are used in all the modern sophisticated anaesthetic machine to measure the exhaled tidal volume by attaching them in breathing circuit near the exhalation valve. Some machines measure the inspiratory tidal volume by attaching these just distal to the inspiratory valve. However, in the latest model of Datex-Ohmeda machine these are attached near the Y-connection of patient to measure the actual delivered and exhaled tidal volume.
In Wright respirometer there is a rotating vane which is surrounded by multiple slits and this vane is attached to a pointer on a dial. When the gas passes through it then the slits which surround the vane create a circular flow and rotate the vane with pointer on front dial. The vane does 150 revolutions for each litre of gas passing through it. In clinical use the respirometer reads accurately the tidal volume within the range of 4 to 24 L/min. A minimum flow of 2 L/min is required for the respirometer to function accurately. A paediatric version of spirometer is also now available which can measure the tidal volume in the range of 15 to 200 ml per breath. A sophisticated version of this Wright spirometer uses the reflection of light technique to measure the tidal volume more accurately. Other modification of this Wright spirometer is the use of semiconductive device where the tidal volume is measured from the changes in magnetic field and converting it electronically.
The hot-wire anemometer is used in Drager-Fabius anaesthetic machine to measure the tidal volume. Here, electrically heated fine platinum wires are used. The cooling effect of these wires by increasing gas flow through it causes a change in electrical resistance which is proportion to the gas flow and is determined by the current needed to maintain a constant wire temperature. In ultrasonic flow sensors an upstream and downstream ultrasonic beams are passed at an angle from where the shift of doppler frequency is measure which is proportional to the flow of gas or tidal volume. In pneumotachograph the parallel bundles of tubes of small diameter in a chamber or a mesh screen is used which provide resistance to air flow and drop of pressure. This drop of pressure across the resistance is sensed by a differential pressure transducer and is proportional to the flow rate. Thus calculation of flow rate over time measures the tidal volume. Moreover analysis of this volume, pressure and time relationship will give us the potential valuable information about lungs and airway mechanics.