ANAESTHESIA VAPOURIZERS BYMOHAMED ANWER RIFKY

1. VAPORIZERSMOHAMED ANWER RIFKY Anaesthetic vapour has to be diluted with fresh gas in one of the two ways: 1-By splitting the fresh gas flow so that only a portion passes through the vapourising chamber and the rest bypasses it (Variable bypass vapourisers). 2-By injecting the vapour directly to the total fresh gas flow without any split (Measured flow vapourisers). A-Variable bypass vapourisers: Can be classified into: 1-Plenum vapourisers: For halothane, enflurane, isoflurane, sevofluraneexceptdesflurane. Examples: (i) Tec 5 and 7 vaporizers® (GE) (ii) Vapor 2000® series (Drager) (iii) Sigma Delta® (Penlon). 2-Plenum vapourisers with electronic control: For halothane, enflurane, isoflurane, sevoflurane and desflurane. Example: Aladin cassette vaporizer® (GE). B-Measured flow vapourisers: Can be classified into: FLOW DEPENDENCE Marquet injection vaporizer Penlon Sigma Alpha vaporizer for desflurane. • A-Desflurane Vapouriser: Only for desflurane. • Examples: (i) Tec 6® (GE) (ii) D vapor® (Drager) (Penlon). (iii) Sigma Sigma Alpha® • B-Direct injection of volatile anaestheticvapouriser: For halothane, enflurane, isoflurane, sevoflurane and desflurane. • Examples: (i) The Drager DIVA®(ii) The Maquet 950® series. A vaporizer >> adding clinically concentrations of anaestheticvapour to a stream of carrier gas. The saturated vapour pressure of volatile anaesthetic agents at room temperature >>greater than that required to produce anaesthesia>>so a Vaporizer splittesthe flow of gas to the vaporizer into two streams. Vaporizers Gas can be made to flow through a vaporizer in one of two ways.1-Apositive pressure can be developed upstream of the vaporizer,>> plenum vaporizer.2-negative pressure may be developed in the gas stream distal to the vaporizer (draw-over vaporizer), or a negative pressure may be generated (mechanical means). THE BOYLE'S BOTTLE The bypass is controlled by a rotary valve>> the splitting ratio-Degree of saturation of the gas leaving the chamber is very dependent on the flow-Overcome this problem the vaporizer is also fitted with a plunger and cowl >>the surface>>bubbled through the liquid agent-If the Boyle's bottle is used for other anaesthetics for which high concentrations>>dangerous then the plunger is raised to its full extent-Vaporization draws latent heat>> the saturated vapour pressure of the agent therefore fall>>readjustment of the settings is made-Because the output of the Boyle's bottle is so variable>>vaporizer cannot be calibrated The problem of flow dependence may be overcome >>all the gas emerging from the vaporizing chamber is fully saturated>>the final percentage of anaestheticvapour delivered from the vaporizer is controlled by adjustment of the splitting ratio and is independent of the . FLOW DEPENDENCE flow. A halothane vaporizer at a room temperature of 20°C. The saturated vapour pressure >>32 kPa>>atmospheric pressure is 100 kPa the concentration of vapour in the vaporizing chamber is 32%, . If the control valve is set to allow 10% of the incoming gas to flow through the chamber and 90% through the bypass channel, >> halothane is 3.2%.(the temperature is constant and the surface area of liquid halothane within the chamber is large enough to maintain the saturated vapour pressure). If a fault occurs,in the control valve>>high proportion of gas may pass through the vaporizing chamber >>overdose. To achieve full saturation a large surface area must be available for vaporization .1-metal or fabric wicks, one end of which dips into the anaesthetic liquid while the other end projects up into the chamber>>surface tension between the liquid anaesthetic and the capillary channels in the wicks draws up the anaesthetic and provides a large surface area of anaesthetic2-Bubble the gas through the liquid anaesthetic by means of a sintered disc of glass or metal. This produces large numbers of minute bubbles which have a large total surface area ( copper kettle). The splitting ratio produced in a particular vaporizer depends on the relative resistances to flow of the gas paths through the vaporizing chamber and through the bypass channel Flow dependence is usually worst at low flows, e.g. below one litre per minute>>manufacturer's chart must be consulted . A vaporizer which is calibrated with oxygen flowing through it will generally give a slightly different output with nitrous oxide-oxygen mixtures. The copper kettle series of vaporizers overcame these problems by using flowmeter to control the flow of gas through the vaporizing chamber.(figure in the upper part of the slide).

2. TEMPERATURE CONTROL The glass of the Boyle's bottle >>a poor conductor >>more advanced types of vaporizers are made of metals . For example, consider a halothane vaporizer of 5 kg weight made of copper and containing 200 ml of halothane. Heat capacity of halothane = Volume x Density x Specific heat capaicity= 200 ml x 1.87 g ml-1 x 0.8 J g-1 K-1= 299 J K_1 Heat capacity of copper= 5000 g x 0.39 J g-1 K-1= 1950 J K-1 both = 2249 J K-1- Heat of copper is not as high as that of halothane, Water has an even higher specific heat capacity >>heat reservoir = 200 ml x 1 g ml-1 x 4.18 J g-1 K-1 The relationship of temperature and saturated vapourpressure for halothane is illustrated in the Fig. (20°C the saturated vapour pressure is 32 kPa.In earlier models of the Drager Vapor halothane vaporizer >>A thermometer was provided to monitor the temperature n the TEMPERATURE COMPENSATION Chamber (vaporizing) and a scale on the control knob allowed the anaesthetist to read accurately the percentage of halothane at a particular vaporizer temperature>> accurate concentrations. The copper kettle vaporizer also had a system of manual adjustment, (two gas flows, through the bypass and through the vaporizing chamber- The temperature in the vaporizer was noted and the flow rates through the vaporizer were adjusted accordingly(adjustment now takes place automatically)- Most vaporizers contain a temperature controlled valve which adjusts the splitting ratio. Temperature control valve incorporates1- A bimetallic strip (two metals which have different coefficients of thermal expansion and which are joined together). As the temperature changes, the shape of the strip alters >>bends or straightens. (Fluotec and PAC). 2-Use small flexible bellows containing some fluid that has a high coefficient of expansion. As the temperature changes, the bellows expand or contract and thus open or close a valve. (EMO, Abingdon and Ohio vaporizers). 3- In Drager Vapor 19 vaporizers, expansion of a metal rod acts similarly to adjust>>temperature- The fall of vapour pressure with temperature can be avoided if liquid anaesthetic is added directly to the gas stream, (Siemens vaporizer). The anaesthetic agent is delivered through a fine nozzle. The rate of delivery of the anaesthetic depends upon the pressure difference P1 to P2 across the nozzle>>adjusted by the throttle valve. If flow through the vaporizer is increased, the pressure across the valve is increased >>maintain the same concentration>>(remains accurate despite changes in flow)-Throttle valve is calibrated to indicate the percentage of the anaesthetic. (Siemens vaporizer). FIGURE ON THE NEXT SLIDE. EFFECT OF PRESSURE ON THE VAPORIZER The calibration of some vaporizers is inaccurate with (IPPV) giving increased concentrations of the anaesthetic, particularly if a low flow is used in association with IPPVwith a closed circuit. Intermittent back pressure from the ventilator can occur at the outlet of the vaporizer as the ventilator cycles. If the volume of the vaporizing chamber in the vaporizer is larger than the bypass channel, gas may expand out of the inlet and outlet of the vaporizing chamber when the back pressure from the ventilator falls. Thus, gas containing anaesthetic agent will flow into the bypass channel and increase the output concentration of the vaporizer. overcomed in a number of ways. Firstly, a pressurizing valve can be included downstream of the vaporizer to ensure that the pressure within the vaporizer is constant and greater than the pressure at the ventilator. Secondly, the vaporizer may be designed so that the volume of the bypass channel and vaporizing chamber are equal. If this is the case, expansion takes place equally from both routes and there is noretrograde flow. Thirdly, a long inlet tube to the vaporizing chamber can be provided so that retrograde flow from the vaporizing chamber never reaches the bypass channel. A further problem ( liquid anaesthetic **decanted into the breathing system)!!. ** Fow out from a containe USE AT HYPERBARIC PRESSURE(The figure) reviews the situation with a vaporizer set at 1%halothane,with ambient pressure is 100 kPa (1 bar), the partial pressure of halothane at the outlet is 1% of this>> halothane in the vaporizer is 32 kPa, or 32% of the atmospheric pressure.But at a pressure chamber at 200 kPa (2 bar)>>saturated vapour pressure is unaffected by ambient pressure(still 32 kPa) but this now constitutes 16% of the 200 kPa atmospheric pressure. The splitting ratio >>halving of that >>The vaporizer is thus delivering 0.5% halothane at 200 kPa which results in a partial pressure of halothane of 1 kPa, the same as before (depth of anaesthesia is dependent on the partial pressure of anaesthesiavapour rather than on its percentage) >>the vaporizer can normally be used with the usual settings at different atmospheric pressures. ( contaminated with vapour from the first>>interlock not used P2 P1

3. VAPORIZER POSITION AND CONTROLS On the anaesthetic machine the vaporizer should be placed between the flowmeter block and the emergency oxygen flush control (high flow of oxygen).The cut-out actuated by a failure of the oxygen supply >>downstream from the vaporizer. The vaporizer control knob should be standardized ( turn to 'off' in a clockwise direction) . When the vaporizer is in the Off' position both the inlet and outlet ports to the vaporizing chamber should be (contamination with anaesth.traces). USE OF VAPORIZERS IN TANDEM If . two vaporizers are situated in tandem on the anaesthetic machine simultaneously>>without such a safety factor a simple Boyle's bottle may beplaced in the second position !!. DRAW-OVER VAPORIZERS The internal flow resistance of the draw-over vaporizer must be very low to avoid additional resistance- Vaporizers must be calibrated (retain their accuracy over a wide range of tidal ventilation)>>vaporizers which were used to give methoxyflurane or trichloroethylene (obstetrics),but hyperventilationin early models of these vaporizers did not remain accurate during use. At present draw-over vaporizers are less popular for analgesia but the EMO, the OMV (Oxford Miniature Vaporizer) and the PAC (Cyprane) are used in portable anaesthetic apparatus>>not requiring cylinder gas supplies, they are ideal for use outside hospital.The EMO has a bellows thermal compensation device and has a water reservoir to aid thermal stability. It is designed for use with ether it is safer for use in circumstances when oxygen is not available.Is less suitable for use with a continuous flow (plenum system)>>lower percentage of the anaesthetic than indicated. The OMV is conveniently compact with a small water reservoir but no thermo-compensating valve. It is designed for use with halothane, trichloroethylene or enflurane, preferably with a system of oxygen supplementation. It can be used with a continuous or an intermittent flow, or with a draw-over technique. In a development of the OMV, the Triservice vaporizer( the heat retaining reservoir contains antifreeze in place of water)>>not damaged at low temperatures and continues to function in field use under winter conditions. The PAC series of vaporizers have bimetallic strip temperature compensation, and their performance is not unduly affected by shaking, tilting or transient overturning .Vaporizers are individually calibrated for the anaesthetic agent required and can be used in a plenum or draw-over system. Draw-over vaporizers have also been used within an anaesthetic circle system using carbon dioxide absorption, but careful monitoring of the anaesthetic concentration (overdosage) , particularly IPPV -contamination of the vaporizer with water vapour may affect its performance. checked with a -In the case of halothane and some other anaesthetics it is recommended that the vaporizer is drained and refilled at regular intervals, usually weekly(thymol) -accidental filling of vaporizers with the wrong liquid>>filler tubes and caps. COPPER KETTLE MANUALLY MANUALLY ANUA The VAPOR VAPORIZER COPPER KETTLE