Chem + Phys Module 7

Vaporizer

A device for adding clinically useful concentrations of anesthetic vapor to a carrier gas

Constant vapor output in vaporizers are made by the supply of ___

A heat sink (Made up of water or copper) And temp control valves "Temperature compensate" control splitting ratio

temperature

Average movement of molecules

Advantages of low flow anesthesia

Decreased OR pollution (still needs scavenging) Decreased environmental pollution Increased heat and humidity conservation (gas does not have humidification) changes in inspired concentrations occur more gradually Less expensive; Significant savings in N2O, O2, and especially the volatile agents.

Desflurane vaporizer

Desflurane = vapor pressure is close to atm pressure at room temp - 664 vs 760, Vaporizer heats all of desflurane into a gas form d/t elevated pressure inside of that Has gas inside of it and a vapor blender Don't have variable bypass by having a chamber, have the vapor and gas blended together (Similar and and O2 blended in vent to give specific fio2) the only vaporizer you can fill during use Need electricity! The bottle is glass or aluminum so can withstand pressure. Pull cap off and fit bottle into nozzel → pour it in → it heats itself to form gases

Hazards of Vaporizers

Always end up with OD Tipping Incorrect agent Vapor leaks - trace gases given to a pt not intended for Drug with high VP put in low pressure vaporizer → higher concentration Drug with low VP put in high pressure vaporizer → lower concentration

Vapor pressure ____ with temperature

Increases

Fall in temperature of liquid will make it ____ volatile

Less

Heat applied to container with liquid will cause liquid and vapor to ___

More liquid will convert to vapor d/t ↑ in molecular kinetic energy Vapor pressure will increase

How much isoflurane vapor will 100 mL "pick up" as it flows through the vaporizing chamber?

Before the carrier gas enters the vaporizer, its volume is 100% of the total; upon exiting, its volume is augmented by the volatile agent molecules that have been picked up. You have more molecules exciting than emptying. The amount depends on the vapor pressure of the agent involved. 1. For isoflurane, the output concentration (fully saturated) is 243/760 (vapor pressure/atm pressure) = 33% (one third) exiting vaporizer. 100- 33 = 67 (⅔) 2. The oxygen must be 67% (two thirds) of the output. Set up a proportion: 100 mL ÷ 67% = x mL ÷ 33% ; x = 49 mL vapor that's exiting along with the 100 ml that went into the vaporization chamber. Another way to calculate: saturated VP of halothane (238) or isoflurane (243) is around 1/3 of one atm (240/760); the SVP of enflurane (175) or sevoflurane (157) is around 1/4 of an atm (157/760). Thus a vaporizer's output will be one part vaporized agent for every two parts oxygen entering the vaporizer (for H or I) or every three parts oxygen entering the vaporizer (E or S).

____ determines the uptake of vapor

Partial pressure Because of this, it also determines the depth of anesthesia

Draw-over vaporizer

Requires a spontaneously breathing patient. negative pressure (developed by the patient's inspiratory efforts) distal to vaporizer draws carrier gas through it. Instead of pressure being higher before vaporizer and lower afterwards, the patient sucks gas through vaporizer to pick up anesthetic → negative pressure.

Specific heat capacity of water

Specific heat capacity of water is high- 1 kcal / Kg / degree C

Vapor pressure

amt of pressure that a gas in the vapor phase exerts DEPENDENT on the type of liquid and temperature INDEPENDENT of pB measures the molecules in their random paths bombarding the walls of the container

Low flow anesthesia

giving a little more above metabolic demand (<2LPM) No longer in use

Pressuring effect is worsened by

high flow rates, large pressure fluctuations, large pressure differences upon inhalation and exhalation, and low vaporizer settings. With PEEP have higher pressures at peak of inspiration and lower pressures with exhalation Modern machines have one way valves to prevent this.

Formula for Heat capacity =

= specific heat capacity x mass in Gm.

Temp compensated vaporizer via resistance

Change size of opening causes change in resistance (right) Have metallic strips → expand with different temperatures. When colder → expands and pulls away flap → increase size of opening → change resistance → allows FGF to pick up more agent vapor (or incr FGF to pick up more) → maintain concentration

How to solve problems of flow dependence

Copper kettle or Vernitol Uses separate flow meter to control the flow of gas through the vaporizing chamber So the splitting ratio is manually controlled (based on temp, agent, and pB) Compensates for temp

Closed circuit anesthesia

AKA basal flow rate Only giving enough O2 to meet basal metabolic demands and what gets extracted from sampling (50-200ml) No longer in use

Boyle's Bottle (Ohio #8)

Plenum type vaporizer Take liquid and drop it on a mask or in a bag, patient would be breathing spontaneously → sucking air and picking up vapor that way Anesthetic liquid contained in glass bottle and "splitting ratio" (= bypass flow as compared to chamber flow) controlled by a rotary valve. Amount of gas leaving is determined by vapor pressure of the liquid, FGF and temp Liquid gets cool as you take vapor away due to latent heat of vaporization

Care of vaporizers

Services at least once per year Drain and refill vaporizers (esp halothane d/t preservatives) Fill only with appropriate anesthetic up to fill like Do NOT tip > 45 degrees - will allow liquids to go up valve; ↑ dose given

critical temperature

The temperature at which the latent heat of vaporization for a substance becomes zero substance changes spontaneously from liquid to gas without the provision of any external energy. Above the critical temperature, then, the substance cannot exist as a liquid.

Disadvantages of low flow anesthesia

Unfamiliarity. Have to compensate but dialing in more fio2 on FGF and vaporizer. Need to turn vaporizer up way past MAC to get one MAC exhaled Higher rate of granule exhaustion; CO2 absorbent and ETCO2 degree of increase in PaCO2 if exhausted absorbent, incompetent one-way valves, or absorber bypass "on" Requires more monitoring Can change anesthetic concentrations quickly hypercarbia Accumulation of undesired gases and vapors in system (only a problem with closed circuit) Carbon monoxide (from breakdown of Hgb), acetone, methane, hydrogen, ethanol, or toxic metabolites of anesthetic agents. toxin is less likely to be flushed out. Flushing through the vaporizer (older models) at low flows causes a large increase in vapor output. Older vaporizers more susceptible to pumping effect (increased vol %) at lower flows.

Disadvantage of Boyle's bottle

Use is based on clinical judgement of depth of anesthesia b/c output is so variable Easy to OD pt Dependent on FGF, temp, and vapor pressure

Calculate % of FGF needed for vaporizer to pick up agent

Vapor pressure / 760mmhg

Heat taken away from container with liquid (ice bath) will cause liquid and vapor to ____

Vapor will condense to liquid d/t ↓ in molecular kinetic energy Vapor pressure will decrease

Vaporizer controls

Vaporizer control knobs are standardized to turn off with a clockwise turn. Vaporizer inlet and outlet ports are closed in off position to avoid contamination of flowmeters or other apparatus with traces of liquid anesthetic. Always turn to OFF, not just to zero.

Effect of IPPV on vaporizer output

Vaporizer output could vary and be inaccurate d/t intermittent back pressure from the ventilator or bag → gas pushed back into vaporizer → some gas will pick up vapor → higher concentration of VA

Vaporizer position

Vaporizers should be located between the flowmeters and O2 flush valves to avoid flushing gas through the vaporizer (causes increased output of vapor) Turn off vaporizer before flushing to avoid pumping/pressuring effects Oxygen flush valves deliver 25-75LPM → don't want to flush gas at that rate

Latent heat of vaporization

When a substance changes from a liquid to a vapor (or from a solid to a liquid), heat must be supplied even though this change of state takes place at a constant temperature. This heat is known as the latent heat of vaporization when the change is from a liquid to a vapor. "heat/energy needed to convert substance from liquid to vapor, or solid to liquid form. heat/energy will break chemical bonds that hold molecule together → since the energy is used to break the bonds → it doesnt increase the temperature of the liquid or solid. Latent = increased heat energy isnt reflected in a change in temperature.

Full saturation techniques

Wicks of metal or fabric inside chamber: liquid moving up cloth → bigger surface area → saturation Bubble the carrier gas: FGF moves through the liquid → large numbers of tiny bubbles have a large surface area → gas exposed to liquid on surface of the bubble → picks up vapor molecules efficiently.

Pumping effect

Without intermittent back pressure, the concentration delivered during controlled or assisted ventilation can be HIGHER than when vaporizer output flows free May occur in both variable bypass and measured flow vaporizers

Equipment for low flow

accurate flow meters a circle breathing system free from leaks (this includes an endotracheal tube and/or LMA without leakes) an oxygen analyzer a ventilator with a standing bellows or piston an agent analyzer to track inhaled anesthetic concentration a carbon dioxide analyzer (capnography) a modern out of circuit vaporizer

Specific Heat Capacity

amt of heat/energy required to raise the temperature of 1 Gm of a substance by 1degree K. the quantity of heat necessary to produce a given change in temperature in a standard or unit volume of any given substance. Word "specific" refers to-- the unit volume.

When adding gas to vapor/liquid system, temp will ___ ?

decrease b/c more liquid converts to vapor than the reverse The temperature of the remaining liquid also declines, d/t loss of latent heat of vaporization vapor molecules carry away the latent heat of vaporization with them. This results in cooling of the remaining liquid and of the vaporizer walls.

Splitting ratio

depends on relative resistance to flow in bypass and chamber gas flow; which depend on type of flow (laminar, turbulent) and on physical properties of the gases used (density, viscosity).

Classification of Desflurane vapor

electrically heated thermostatically and electromechanically controlled constant temperature pressurized gas-vapor blender.

Specific latent heat

heat required to convert 1 Gm of a substance from one phase to another at a given temperature. Water remains at 100 degrees C when boiling until all is converted to steam, regardless of the amount of heat supplied, or at 0 degrees C until all ice is converted to liquid gas b/c the energy was used to break bonds

A ____ altitude creates a lower boiling point for gas

higher

Drug has same MAC, same vapor pressure, same vaporizer output but at higher elevation will have an ____ in volume percent of drug

increase

Pumping effect is worsened by

increased back pressure, low flows, low volume % dial settings FGF molecules are forced back into the vaporizing chamber → affecting the splitto ratio Modern devices now have one-way valves to prevent this

Vaporizer output (in terms of partial pressure) is

is constant at any dialed setting

Heat capacity

is defined as the amount of heat necessary to raise the temperature of a given OBJECT by 1 degree K. It applies to the object as a whole;

Emergence from low flow anesthesia

is extremely slow with low flows due to rebreathing of exhaled vapor molecules. Most practitioners will use higher flows during emergence, i.e. 4 or more L/min. Turn the drug off. The conc. Will slowly drift down. If you want the patient to emerge fast; turn off agent, turn FGF to high Higher flow rates → no inspired VA → bigger conc gradient between blood and alveolar gas so it comes out more efficiently.

Full saturation in a vaporization chamber requires a

large surface area of liquid that the gas is exposed to

Proper technique to admin low flow anesthesia

low FGF has slow uptake of agent bc diluting conc of oxygen with expired oxygen. So, start with higher flows → get conc up in the brain where we want it → then drop flows. If we are done and want to wake them up. Up the flow rates, concentration more quickly, then can drop flow rates back down

A ____ altitude, creates a higher boiling point for gas

lower

Partial pressure directly related to

penetration of vapor molecules across the alveolar-capillary membrane and blood-brain barrier

Plenum vaporizer

positive pressure upstream of vaporizer provides the driving force for the carrier gas Higher before vaporizer, lower after. The portion of that gas goes in, picks up vapor, comes out, rejoins gas. Used in austere environments and DESFLURANE Need compressed gas

Purpose of vaporizers

produce consistent, predictable output carrier gas leaving the vaporizing chamber is fully saturated regardless of the fresh gas flow chosen. final concentration of anesthetic vapor is controlled by adjustment of the splitting ratio and is independent of flow.

Water is used as a ______ in a vaporizer

reservoir

How does a vaporizer allow us to SAFELY produce anesthesia?

splitting the total flow into a vaporizer portion and a bypass portion. Liquid anesthetics at room temp is many times greater than what we need to produce anesthesia so we mix it with a gas without any vapor "bypass flow" "carrier gas" → "vaporizing chamber flow" → "bypass flow" = appropriate concentration

Boiling point

temperature at which vapor pressure equals atmospheric pressure. → liquid comes to a boi At a higher altitude, lower barometric pressure creates a lower boiling point.

volatile anesthetics acquire their latent heat of vaporization from ____

the environment as they vaporize.

Vapor

the gaseous phase of a substance which is normally liquid at atmospheric pressure and room temperature.

Rate of vapor molecules

vapor molecules enter the liquid, and the rate at which liquid molecules vaporize, are in equilibrium at any given temperature

What do you need to know to do a Vaporizer calculation?

vapor pressure of the agent, percentage of saturated vapor pressure, pB

Latent heat of vaporization in anesthesia

volatile liquid vaporizes → takes its latent heat of vaporization from the skin → cools rapidly → impairs conduction in sensory nerves → analgesia

Gases can be cold or warm when exiting the machine but ____ quickly become ____ temp

Equilibrate ambient

Flow Dependence of gas

High FGF = low dwell time above gas in chamber → flow not fully saturated of carrier gas = lower concentration of gas worst at low flows (<200 mL/min).

Pressurizing effect

LESS vapor output will occur if sustained increases in pressure distal to vaporizer outlet (ie a ventilator where an amount of PEEP is kept on). Continuous positive pressure (PEEP) → have more resistance to gas flow leaving vaporizer → lower concentration delivered Ex. Bottlenecking on highway

Vapor pressure is ____ at lower temperatures

Less

Specific heat capacity of other substances is ____ water

Less than

Seal system @ equilibrium

Liquid → vapor is same

Specific heat for gases is ____

Low because there's less molecules per unit volume that needs to be warmed Small amt of energy needed to warm gas

Most gases will boil at a _____ temp at lower atmospheric pressures

Lower b/c atmospheric pressure that is higher, vapor pressure (and the temperature needed to drive it) must be higher before boiling occurs.

Latent heat of vaporization and heat loss from the patient

Normal metabolic cost of warming and humidifying inspired gases is 10% of basal heat production. Increases to 15% under anesthesia since gases are supplied completely dry (0% humidity) and cool, and the patient's nose is bypassed. This worsens the problem of hypothermia. Humidifier is supplying gases with 100% relative humidity (RH) at body temperature (the saturated vapor pressure of water at 37 degrees C is 47 torr). prevent pt from getting cold rather than warming.