Chris Thompson 2008 VAPOURISERS Physical Concepts
Saturated Vapour Pressure (SVP) - partial pressure of the vapour phase of a substance when at equilibrium with its liquid phase (e.g. in a closed container). Increases rapidly as boiling point approaches. Boiling Point - temperature at which SVP equals ambient pressure. Latent Heat of Vapourisation - heat energy required to convert a given volume of liquid to a vapour while keeping both at the same temperature (kJ/kg or J/g). Approx 40 cal/g for volatile agents. Specific Heat - energy required to raise the temperature of a substance by 1°K. For water this is 1cal/g/°K. Thermal Conductivity - speed with which heat flows through a substance; Cu > Al > brass > steel >> glass.
Plenum - a chamber at higher than atmospheric pressure, for distribution purposes. In-circuit vapourisers have resistance in the bypass limb, pressurising the vapour chamber relative to the outlet. Variable bypass vapouriser - one in which the total gas flow is divided in two streams by a variable resistance proportioning valve. Usually a small percentage enters a vapourising chamber, picking up molecules of volatile agent, while the majority travels through a bypass line. Volume vapourised - typically 200 ml vapour per ml of liquid anaesthetic ml liquid used/hour = 3 x % x FGF Pumping effect - pulsatile back-pressure increases output, because it 'ventilates' the plenum chamber via the inlet line, contaminating the bypass gas with vapor. Large pressure swings, large vapour chamber volume and short line from bypass into vapourising chamber all make this worse. Modern vapourisers have overcome this with small vapourising chambers and long inlet lines. Pressurising effect - (constant) backpressure reduces output. For example, high resistance in the line after the vapouriser will compress all gas before it. This increases the ratio of carrier molecules relative to vapour molecules in the vapour chamber, because the number of vapour molecules is fixed, whereas at higher pressures there will be more carrier gas molecules. The absolute percentage of agent per unit volume leaving the vapouriser is correct, but when the gas mixture expands post-obstruction, there are fewer molecules of agent per volume of carrier than there should be. This is a minor effect. Classification:
Copper Kettle Halothane, Isoflurane; flow required to vapour chamber in ml = FGF * 20 * percent required. For example, with 6 l/min FGF, set flow into copper kettle to 120ml, picks up 60ml vapour (partial pressure is about 1/3 of atmospheric), 60ml vapour in 6060 ml total FGF = (approx) 1%. Hence for 1% halothane the splitting ratio is approximately 120/6000 ml or 2%. Enflurane; flow required to vapour chamber in ml = FGF *30 * percent required Goldman
Three position lever. AC Delco on glass; originally a carburettor float bowl reservoir.
Boyles Bottle
Aladdin Cassette
Can deliver partial pressure or true% depending on software settings. Tec 6 Desflurane
Output in partial pressure terms diminishes with altitude, unlike all other vapourisers. The solution was quite ingenious. A resistor in the bypass line increases upstream pressure, and that pressure increases in proportion to FGF. The same resistance, when contrasted with the resistance of the manual knob at the top, results in the splitting ratio. The vapour chamber is heated to 39°C (vapour pressure 1500mmHg). Vapour leaves the chamber through a separate electronically-controlled flow control valve, and is much lower after this valve. In fact this valve maintains the post-vapour chamber vapour line pressure at a value exactly equal to the pre-resistance bypass gas pressure. Should FGF increase, both the bypass line pressue and the vapour line pressure will increase to the same extent. It is then a simple matter for the manually operated mechanical splitting valve on the top of the vapouriser to set the ratio between the two and hence the output concentration. Because the Thus the vapou the manually-operated variable resistance control knob on the top of the vapouriser. Because both the pressure Key Fillers
Sequencing of non-interlocked vapourisers - from lowest to highest vapour pressure / potency; in decreasing order of MAC: |
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Variable bypass, incomplete vapourisation, flow-over without wicks, in-circuit, non-agent-specific (but intended for Halothane), no temperature compensation, no interlocks. Economical if taken from place to place.
Two versions, one for Ether (with bubble-through) and one for Halothane (without)
Variable bypass (electronically controlled vapour flow regulation valve, resistor in the byapss line, flow-sensor in both vapour and bypass line, CPU external to cassette calculates valve position), Flow-over with wicks, in-circuit, temperature compensated (temper ature sensor in chamber) 
Like all calibrated variable bypass TEC vapourisers, the Desflurane TEC 6 has a manually operated mechanical variable-resistance proportioning valve on the top of the unit, and a resistance in the bypass line. The clever bit was how to make it work reliably, knowing that Desflurane can boil so close to room temperature. Output between 1% and 18% from 200 to 10000 ml/min FGF.
