Medical Gas Systems and Vaporizers Flashcards
Sources of Gases
1) Piped in gases
2) Cylinders = need cylinders as backup for pipeline
Some places don’t have piped in and use cylinders instead
Delivery of Gases
1) Deliver system = flow control knobs in the front
2) Proportional system and safety features = turn N2O knob, the chain link automatically turns oxygen control knob
3) Does not work in reverse
4) Difference in texture of knobs; oxygen is fluted and N2O = pliable feel
5) Color coded
Color Coding of Gases
Green = oxygen Blue = nitrous oxide Yellow = air Black = nitrogen Brown = helium
Cylinder Capacity
E size oxygen: full at 2000-2200 psi with approximately 625 -700L. As O2 is used from the cylinder, the pressure falls in proportion to the amount left in the tank.
Air cylinders: ful at 1900 -2 000 psi with approximately 625L. Like O2, the pressure falls in proportion to the amount left in the tank
N2O cylinder: pressure of 745 psi and have approximately 1590 L and stored a LIQUID. The pressure does NOT indicate amount left in the tank
N2O Cylinder
More available to you since it’s under pressure. Til read 745 psi until its gone. Pressure on the gauge on N2O does not indicate how much is left in the cylinder. Only way you can determine that is weighing. Until all the liquid has evaporated, at which time the pressure will drop in direct proportion to the rate at which gas is withdrawn.
Regulatory Agencies
FDA: purity of medical gases
DOT: department of transportation establishes requirements for manufacturing, filing, transportation, disposition and maintenance of the gas cylinder and containers
Cylinder Marking/Labeling/Tag
Permanent Marking: DOT regulations require specific markings on each cylinder
Labeling: each cylinder must have a label or decal on the side or on the shoulder and can’t cover the permanent marking
Tags: Full, In-use, empty connections by perforations. Denotes the amount of cylinder contents and it not use for identification purposes.
PISS: Pin Index Safety System
Six index positions for pins on the yoke which are linked to the machine. Avoid putting the wrong cylinder to the wrong connector.
Machine alarms don’t know the gas; only recognize pressure and therefore let it flow.
O2 = 2, 5
N2O = 3, 5
DISS: Diameter Index Safety System
Provides non-interchangeable connections for the medical gas lines. Connection consists of body, nipple and nut combination. Only properly mated parts will fit together and allow the threads to engage
Safe Handling of Procedure for Cylinders
1) Cylinder in upright position
2) Never leave empty cylinders on machine
3) Take plastic covering off the port when installing cylinder
4) Never rely only on the cylinder’s color for identification
5) Never oil valves
6) Crack the valve away from you or other personnel
7) Valve should always be FULLY open when a cylinder is in use. Marginal opening may result in failure to deliver adequate gas.
Vaporization
Conversion of liquid to gas
Inhalation agents are liquid that must be converted to vapor to be inhaled
Dependent on: (1) Vapor pressure (2) temperature (3) amount of carrier gas used
Anesthetics = bottles in liquid form. Very potent and must be diluted and converted to gas
Vaporization
Molecules of a volatile agent in a closed container are disturbed between the liquid and gas phases
The gas molecules bombard the surface of the liquid and the walls of the container creating vapor pressure.
As temperature increase, more molecules enter the vapor phase and vapor pressure increases.
Vapor pressure is contingent on temperature and physical characteristics of the liquid = boiling point.
Volatile Agent Vapor Pressure at 20C
Enlurane = 172 mmHg Isoflurane = 240 mmHg Halothane = 244 mmHg Sevolfurane = 160 mmHg Desflurane = 669 mmHg
Desflurane
Very high vapor pressure. Close to atm = goes to gaseous state very readily. Careful filling the vaporizer and don’t spill = if you do clean it up quickly.
Vaporization of a Liquid
Vapor pressure changes with varying temperature
Heat causes the equilibrium to shift that more molecules enter the vapor phase
Lowering temperature causes a shift towards the liquid phase and decrease in vapor pressure
Passing a carrier gas over the liquid shifts the equilibrium towards the vapor phase. The heat of vaporization is supplied from the remaining liquid. This causes a drop in temperature.
Latent Heat of Vaporization
Number of calories required to change 1 gram of liquid into vapor without a temperature change.
The energy for vaporization comes from the liquid itself or from an outside source.
Specific Heat
The number of calories required to increase the temperature of 1 gram of a substance by 1 degree C. It can be used for solids, gas or liquids
The specific heat of an anesthetic agent indicates how much heat must be added in order to maintain a constant rate of vaporization.
Thermal Conductivity
A measure of speed with which heat flow through a substance. The higher the thermal conductivity, the better the substance conducts heat. We don’t want a temperature change in vaporizer. Heat ca affect liquid so much. We want no heat transfer.
Temperature, Altitude and Vapor Pressure
Altitude:
Increase altitude: decrease barometric pressure
Decrease altitude: increase barometric pressure
Boiling Point:
vapor pressure = barometric pressure
Temperature direction relationship
Increase Temperature = increase vaporization
Decrease temperature = decrease vaporization
Vaporizers
Agent specific
Temperature compensated
Variable bypass
Variable Bypass Vaporizer
A portion of the gas flow will pass into the vaporizing chamber where it will become saturated with vapor.
This vapor-laden portion then rejoins the gas flow for dilution to deliverable concentrations.
Movement of strip = determines how much will come out.
Temperature Control Valve
Conductivity = we don’t want vaporizer to have a lot of transfer of heat in or out. Copper conductor of heat and metal specific to allow little transfer of heat possible. Heat comes from the liquid itself to vaporize.
N2O
Stored in cylinder as a liquid. Come out to atm = it’s a gas. That change from liquid to gas does require energy. Energy from the liquid itself. The energy required and liquid temperature drops such that the cylinder cools itself.
Changes in Altitude - Effect on Vaporization
Vaporizer must be re-calibrated to assure accurate % delivered to anesthetic gas.
Increase altitude: decrease barometric pressure
Decrease altitude: increase barometric pressure
Relative to atm pressure = the change in vaporization
Tec 6 Vaporizer
Desflurane: electrically heated, constant temperature/pressure.
It creates a false pressure = it has to be heated. The agent in the vaporizer thinks 1500 mmHg and 669 = it stays as liquid. Increasing pressure to 2 atm = liquid doesn’t go into vapor readily.
Altitude Effect on Vaporizer
Variable bypass: effect is that you are delivering a higher concentration of volatile agent at higher altitudes/lower barometric pressure but maintaining the same partial pressure.
Tec 6: desflurane = would have to deliver higher dialed concentration at higher altitude/lower barometric pressure. Alter your concentration dial to decrease output in lower altitude to avoid delivering overdose of anesthetics.
Copper Kettle
Measured flow, bubble through and dedicated flow meter for this kettle. Gas comes up through central tube inside the vaporizer to the loving cup. Flow of gas is then directed down toward the liquid (bubble through) and high concentrated vapor then exits the vaporizer and is diluted into the fresh gas flow.
Flow Meter
Wider at the top and narrower at the bottom. Why? Based on viscosity and density
Vapor Pressure
Vapor pressure of the agent will effect the liquid delivered. Agent relative to barometric pressure is what matters.
Aladin Cassettes on Workstations
Electronically controlled vaporizers. Machine is coded to recognize what’s in the cassette = atm pressure, vapor pressure of the agent and set to deliver. They also have PISS for safety features.
Mass spectrography: backup for analyzing what gas you are using.
Potential Vaporizer Hazards
Wrong agent in the vaporizer: High - Low - High Low - High - Low Contamination Tipping Overfilling Simultaneous administration of more than 1 vapor Leaks Pumping effect
Screw up on Vaporizers/Misc
Turn down the agent and identify the patient’s response. Drop the concentration and flow.
Leak = the low pressure system check
Pumping Effect: first anesthetic of the day. Everyone crank up the oxygen at once. There is backflow on the system. Gases come up and delivering down on the vaporizer = proportion back up to the common gas outlet.
