Vaporizer

• Vaporizer


  An instrument used to vaporize volatile anesthetic agents (halothane, isoflurane, sevoflurane, desflurane) and deliver it to the patient as a controlled amount of an inhalational agent, after changing it from liquid to vapor, through the fresh gas flow

  
• Vaporizer

  • Must be located between the flowmeter and common gas outlet
  • Normally expressed as a percentage of saturated vapor added to the gas flow like 1%, 2%, 0.75%
  • All anesthetic machines should have an interlocking or exclusion device to prevent concurrent use of more than one vaporizer

  
• Vapor pressure

  The pressure exerted by vapor in equilibrium with solid or liquid of the same substance

  
• As temperature increases


  The vapor pressure increases
• The gas molecules bombard the walls of the container, creating the saturated vapor pressure of that agent
• Vaporization


  Requires energy (the latent heat of vaporization), which results in a loss of heat from the liquid
• As vaporization proceeds


  The temperature of the remaining liquid anesthetic drops and vapor pressure decreases unless heat is readily available to enter the system
• Boiling point


  The temperature at which a liquid's vapor pressure is equal to the atmospheric pressure
• As atmospheric pressure decreases (as in higher altitudes), the boiling point also decreases
• Anesthetic agents with low boiling points are more susceptible to variations in barometric pressure than agents with higher boiling points
• Among the commonly used agents, desflurane has the lowest boiling point (22.8°C at 760 mm Hg)
• Characteristics of the ideal vaporizer

  • Its performance is not affected by changes in FGF, volume of the liquid agent, ambient temperature and pressure
  • Low resistance to flow
  • Light weight with small liquid requirement
  • Economy and safety in use with minimal servicing requirement
  • Corrosion- and solvent-resistant construction
• Draw-over vaporizer


  Gases pass through a very low resistance vaporizer due to the patient's respiratory efforts
• Plenum vaporizer


  Gases are driven through a high resistance, unidirectional and agent specific vaporizer due to gas supply pressure

  
• Conventional (variable-bypass) vaporizers


  • The total background gas flow that enters the unit is split into two streams: a smaller stream passes through the vaporizing chamber and becomes saturated with agent vapor, the remainder of the gas bypasses this chamber
  • A wick may be used in the vaporizing chamber to provide increased surface area for evaporation
  • One adjustment is made to get desired concentration which simultaneously balances the carrier and bypass flows to produce the blend required for the set concentration

  
• Components of a conventional vaporizer


  • The case with the filling level indicator and a port for the filling device
  • Percentage control dial on top of the case
  • The bypass channel & the vaporization chamber with or without wick
  • The splitting ratio is controlled by a temperature-sensitive valve utilizing a bimetallic strip
  • The vaporizers are mounted on the back bar using the interlocking Selecta Tec system

  
• The FGF only enters the vaporizer when it is switched on
• Cooling effect


  During vaporization, cooling occurs due to the loss of latent heat of vaporization. Lowering the temperature of the agent makes it less volatile.
• Compensation for cooling effect


  • The vaporizer is made of a material with high density and high specific heat capacity with a very high thermal conductivity, e.g. copper
  • A temperature sensitive valve (e.g. bimetallic strip or bellows) within the body of the vaporizer automatically adjusts the splitting ratio according to the temperature

  
• Ventilators exert back pressure as they cycle may back into the vaporizing chamber, where more vapor is added
• Compensation for back pressure


  • Long inlet port into the vaporizing chamber
  • Downstream flow restrictors
  • Both the bypass channel and the vaporizing chamber are of equal volumes so gas expansion and compression are equal
• A pressure relief valve downstream of the vaporizer opens at about 35 kPa to prevent damage to flowmeters or vaporizers if the common gas outlet is blocked
• Heated-blender vaporizers


  Designed for desflurane (Suprane) due to its higher volatility compared to other agents
• Heated-blender vaporizers


  • Maintain a constant desflurane vapor pressure and allow accurate delivery
• Electronic vaporizers


  Electronically controlled vaporizers must be utilized for desflurane and are used for all volatile anesthetics in some sophisticated anesthesia machines

  
• Desflurane vaporizer


  • Contains a reservoir of desflurane (desflurane sump) that is electrically heated to 39°C, creating a vapor pressure of 2 atmospheres
  • No fresh gas flows through the desflurane sump, pure desflurane vapor joins the fresh gas mixture before exiting the vaporizer
  • Decreased ambient pressure (high elevation) does not affect the concentration
• Aladin cassette vaporizer


  A novel system that regulates anesthetic vapor concentration with electro-pneumatic proportional flow valve controlled by microprocessors

  
• Aladin cassette vaporizer


  • Provides an accurate amount of anesthetic agent while mitigating the effect of temperature and barometric pressure on evaporation process
  • Automated recognition of inserted gases
  • On-screen data show agent level and usage
  • Automated electronically monitored controlled FGF, temperature, pressure compensation
  • No risk of spillage into bypass channel
  • Can be safely carried in any orientation

  
• Vaporizer filling devices


  • Agent-specific, color coded to fit the safety filling port of the correct vaporizer and anesthetic agent supply bottle
  • Prevent the risk of adding the wrong agent to the wrong vaporizer
  • Decrease the extent of spillage
  • Ensure that the vaporizer cannot overflow

  
• Anesthetic agents


  • Halothane
  • Isoflurane
  • Sevoflurane
  • Desflurane