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FRCA Gas and Vapours (Gas laws (Assumptions: (Gas particles do not take up…
FRCA Gas and Vapours
Gas laws
Ideal gas equation combines all 3 gas laws and Avogadro's law
PV = nRT
P - pressure
V - volume
n - number of moles
R - gas constant
T - temperature
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Avogadro's law: equal volumes of all gases under same temp and pressure conditions contain the same number of molecules (at STP [0C and 101.3kPa], molecular weight of a gas expressed in grams has a volume of 22.4L)
Graham's law of diffusion: rate of diffusion of a gas is proportional to 1/√density
Therefore high-density gases diffuse slowly
Definitions
Critical Temperature: Temp above which a gas cannot be liquefied no matter how much pressure is exerted
O2: -118C
N2O: +36.5C
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Gas: gaseous substance above its critical temperature
Vapour: gaseous substance below its critical temperature
SVP: Partial pressure exerted by a vapour when a liquid and its vapour are in equilibrium.
SVP varies with temperature only. Independent of ambient pressure.
Latent heat of vaporisation: energy required to turn 1kg of liquid to gas/vapour at a constant temperature.
As energy is taken in for vaporisation (usually from liquid or container), normally the liquid cools and this in turn drops SVP. Units: J/kg
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Boiling point: temperature at which vapour pressure equals ambient pressure above liquid (therefore varies with ambient pressure)
Vaporisers
Need for vaporisers, Types of vaporisers
SVPs of inhaled agents are much higher than that needed to cause anaesthesia. Vaporisers help mix this with a known volume of carrier gas to achieve a certain vol/vol concentration for safe anaesthesia
In-circuit, Low-resistance, usually Drawover.
Drawover occurs when a patient provides negative pressure and draws gas over an in-circuit vaporiser
Oxford miniature (OMV)
Variable bypass, flow-over with metal mesh wicks, low resistance, multiple agents, not temperature compensated, light weight. Chamber only contains 50ml of agent. Cooling limits maximum output to 2-4% with halothane. 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.
Goldman
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.
Boyle's Bottle
Two versions, one for Ether (with bubble-through) and one for Halothane (without)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
Outside-circuit, higher resistance (requires compressed gas), plenum (chamber higher than atmospheric pressure).
Plenum vaporisers: gas split into 2 streams, 1 bypass (variable), other into vaporising chamber. Gas entering chamber gets fully saturated. Streams then rejoin and mix to be delivered to patient.
TEC 2, 3, 4, 5
Temperature compensation needed due to latent heat of vaporisation. SVP will fall as temperature falls and thus inaccurate partial pressure of gas delivered.
Splitting ratio is altered via a valve (usually bimetallic strip) which is nowadays automatic. Modern vaporisers also have a heat sink to allow surrounding heat to be conducted to the vaporiser to maintain the temperature of the liquid.
TEC 6 for Desflurane
SVP 88kPa, but low boiling point (25C) so small temperature changes could cause large changes in SVP.
TEC6 has heater element to heart desflurane to 39C, to increase SVP to 200kPa. Pressure reducing valve then included. Desflurane is then injected into FGF.
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
Schimmelbusch mask
Ether or chloroform dripped onto lint or cloth on mask. Patient inhales vapor. Depth of anaesthesia changed by changing rate of dripping or lifting mask off patient's face
To saturate gas
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Wicks.
Metal or fabric. 1 end dips into liquid anaesthetic agent. Surface tension draws it up and increases surface area for vaporisation
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Storage of gas
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Cylinder Manifolds
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Pipe with several openings connected to cylinders supplying pipeline gases. May be used for O2, N2O, entonox.
The Manifold connects 2 or more groups of high capacity cylinders (J or L). Pressure regulators reduce pressure from 13700kPa to 400kPa.
All cylinders in a group are used at once until pressure falls below certain point where automatic valve switches to next group
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Entonox
50% N2O, 50% O2
At room temperature, SVP of N2O is 50bar.
Therefore theoretical maximum filling of N2O and O2 is 50bar/138bar = 36%.
Entonox manufactured by introducing appropriate amount of liquid N2O into a cylinder, inverting it then bubbling O2 through it. This causes O2 to dissolve in N2O and some to carry up as droplets, This produces 50% mixture of N2O and O2 (Poynting effect.
Pseudo-critical temperature of entonox is -5.5C. Below this, liquefaction of N2O occurs. Lamination occurs when N2O liquid settles and O2-rich gas above it.