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Vapour - gaseous phase of a substance below its critical temperature. Saturated Vapour Pressure SVP - partial pressure of the vapour phase of a substance when at equilibrium with its liquid phase e.
Increases rapidly as boiling point approaches. Boiling Point - temperature at which SVP equals ambient pressure. Desflurane boils at Volumes percent - percentage concentration of gas in a mixture; e.
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.
Fraction Diverted - the proportion of Fresh Gas entering the vapourising chamber. Volume vapourised - approximately ml vapour per ml of liquid anaesthetic. Pumping effect - pulsatile back-pressure increases output, especially obvious with older vapourisers, at low flows, and with large pressure swings.
Increasing back-pressure will compress gas in the plenum chamber. When the back-pressure drops, vapour-laden gas in the chamber will re-expand and try to get out. It can get out via either the inlet or the outlet line. Via the inlet line, fully-saturated gas can mix with and contaminate the bypass gas. If there is a short connection between the vapour chamber and the outlet line, pumping back and forth between the two will also increase output.
Modern vapourisers have overcome this with smaller vapourising chambers and long inlet and outlet 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. To get that 60ml of vapour we would need to divert ml of the ml into the vapourising chamber. The operator adjusts the fraction of the total fresh gas that is diverted into the vapourising chamber.
Typically only a small percentage of the total fresh gas flow enters the chamber. Gas entering the vapourising chamber is always fully saturated by the time it leaves. Vapour molecules added to the gas molecules entering the chamber increase the volume of gas leaving. The extent of this increase in volume and the amount of vapour picked up per ml of gas entering depends on volatility of the agent in proportion to atmospheric pressure. Every 2 ml of incoming gas picks up 1 ml of vapour.
Hence the amount of vapour added, as a percentage of the incoming fresh gas flow, is about a half of the percentage diverted into the vapouriser. The volume leaving the vapouriser is increased by the small amount of vapour added. The fraction diverted is sometimes expressed as the 'splitting ratio', ie the ratio of the amount bypassed to the amount that goes through the vapouriser.
Sevo and Enflurane are less volatile SVP approx.. They require a greater fraction diverted 3 parts in to pick up 1 part of vapour. I've made an excel spreadsheet that calculates fraction diverted by MAC and dial setting and shows how output varies at altitude. A summary appears in the table below:. Table 1 - Fraction diverted and effects of altitude.
Atmospheric pressure is now just twice that of halothane, so the ratio of halothane to carrier at the output will be The final output would be 12 0ml vapour in ml i. For classical plenum vapourisers, the percentage output increases roughly in proportion to the fall in barometric pressure , but a smaller partial pressure increase. Depth of anaesthesia depends on partial pressure, and the changes in partial pressure are relatively small, so normal vapouriser settings work as expected at altitude.
Most volatile agent monitors measure partial pressure and display this as a Sea Level equivalent percentage.
In this sense the numbers displayed indicate 'anaesthetic effect' and require no mental 'correction' for altitude. If you are working at altitude you should confirm how the monitor operates as they are not all the same.
The TEC 6 Desflurane vapouriser behaves differently. The percentage delivered is essentially held constant, so partial pressure FALLS in proportion to the fall in atmospheric pressure. The dial setting should be turned up to compensate. The Aladdin Cassette system can be programmed to deliver either constant partial pressure output or constant pressure output. For a very detailed mathematical approach from which the above calculations were derived , download Steve Shafer's " How Vapourisers Work ".
Kam's point form notes on volatile uptake provide a more general overview. Intended for specific agents, no filler key, approximate output. Copper Kettle. Copper, measured flow, bubble through, out of circuit, not temperature compensated, non agent specific, manually metered flow vapouriser with temperature gauge.
Oxygen is bubbled through the vapour chamber from a dedicated accurate low flow rotameter and the fully saturated vapour then enters the circuit. Correct inflow can be determined from agent-specific tables that relate temperature, desired percentage output and FGF. For Sevoflurane and Enflurane, with lower vapour pressures:. Intrinsically dangerous because output must be manually changed whenever FGF is changed; failure to do so can lead to over or under-dosage. Useful as a teaching tool; the Aladdin cassette system works basically on the same principle.
Kam's notes on volatile uptake. Variable bypass, incomplete vapourisation, flow-over without wicks, low resistance in-circuit, non-agent-specific but intended for Halothane , no temperature compensation, no interlocks. Glass bowl marked AC Delco; originally an automotive fuel filter chamber. Economical if taken from place to place unused agent can be put back in the bottle, no wicks.
Used in-circuit. Output decreased due to cooling after induction. Patient hyperventilation if light caused increased output.
Boyles Bottle. Variable bypass, incomplete vapourisation, bubble through or flow-over without wicks, low resistance in-circuit, non-agent-specific but intended for Halothane or Ether , no temperature compensation, no interlocks.
Variable bypass, incomplete vapourisation, flow-over without wicks, low resistance, agent-specific for Ether, temperature compensated by bellows, temperature stabilised by water jacket, transportable but heavy 10kg. For more information see this review of drawover anaesthetic apparatus. Variable bypass, flow-over with metal mesh wicks, low resistance, multiple agents, not temperature compensated, light weight.
Chamber only contains 50ml of agent. Two units in series required for a Sevo induction. Intended for field use. Can be used in series with a Laerdahl type self-inflating bag in the field or in series with an EMO for halothane inductions. The first 'modern' precision agent specific vapouriser, launched in by Cyprane in Yorkshire as an update to the earlier Mk 1.
Agent-specific for Halothane, variable bypass, flow over with wicks, low resistance, temperature compensated with bimetallic strip in vapour path, non-tippable, no interlocks, subject to pumping and pressurising effects, non-keyed filler. Bimetallic strip tended to stick due to residual thymol in Halothane. Images and history from Sheffield Museum of Anaesthesia.
Variable bypass electronically controlled vapour flow regulation valve in the output line, resistor in the bypass line, flow-sensor in both vapour and bypass line, CPU external to cassette opens vapour flow valve to deliver desired FG percentage , flow-over with wicks, in-circuit, temperature compensated temperature sensor in chamber , transportable, light-weight.
Used in GE machines. A fan blows warm air over the cassette if its temperature falls below 18 degrees, as may happen with gaseous inductions Sevo or Des. The cassette includes magnets to ID the agent to the machine on insertion, an electrical connection to an internal temperature sensor, and self-sealing ports for gas in and out, but no other electronics. It is tippable and robust. More info. With Desflurane, the pressure inside the canister may exceed the inflow line pressure, at which point the inflow control valve and one-way must close firmly, and the unit functions as a pressurised injector.
Partial failure of one or both of these valves may be associated with high levels of desflurane in the circuit. 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. The solution was quite ingenious. A resistance in the bypass line increases upstream pressure.
As a result, 'upstream' pressure increases in proportion to FGF. This pressure is monitored by a differential pressure transducer that controls an electronic flow control valve so that the 'output' pressure of the vapourising chamber is always equal to the pressure in the fresh gas inlet line.
If FGF increases, both the bypass line pressure and the vapour line pressure will therefore increase to the same extent. The manually operated mechanical splitting valve on the top of the vapouriser only has to set the resistance ratio between fresh gas and vapour to set the output concentration just like it does in any other vapouriser.
NB: output in partial pressure terms diminishes with altitude. This is because the 'thinner' fresh gas flows more freely past the resistance, reducing the measured 'upstream' pressure for any given flow, which in turn reduces the vapouriser outlet pressure and hence number of molecules.
When more than one vapouriser is attached to the backbar, the interlock system should allow only one to be on at a time.
Modern Anaesthesia Vapourisers
Vapour - gaseous phase of a substance below its critical temperature. Saturated Vapour Pressure SVP - partial pressure of the vapour phase of a substance when at equilibrium with its liquid phase e. Increases rapidly as boiling point approaches. Boiling Point - temperature at which SVP equals ambient pressure.
How anaesthesia vaporisers work explained simply.
Michael P. Vapor pressure Molecules escape from a volatile liquid to the vapor phase, creating a "saturated vapor pressure" at equilibrium. Vapor pressure VP increases with temperature. VP is independent of atmospheric pressure, it depends only on the physical characteristics of the liquid, and its temperature. So, even although evaporation proceeds at a rate governed by liquid temperature and is independent of altitude barometric pressure , individual vaporizer types may or may not function the same at altitude.