Slides (NOT KEY POINTS) Flashcards
What kind of components do machines have
electrical and pneumatic- operated by air or gas under pressure
Anesthesia workstations and their components are covered by the
American Society for Testing and Materials (ASTM)
Characteristics of Anesthesia Machine
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Source of oxygen
•Means to dispose of CO2
•Device to vaporize liquid anesthetics
•Inhaler assembly/circuit to deliver gases
Components of Modern Anesthesia Machine
Cylinders and/or pipeline source of gas
•Regulators and gauges to control and measure gas in high and low pressure systems
•Valves and flowmeters to control and measure the flow of gas within a delivery assembly or circuit
•Vaporizers to convert liquid anesthetics to gas
•Mixing chambers/CO2 canisters for gases and vapors
Anesthesia Machine Gas Regulators
Regulators function to reduce pressures within the anesthesia machine to a safer level •When gas travels from the high-pressure cylinder to the machine, a pressure regulator decreases pressure to 50 psig •If the pressure in the gas supply module is too high (>75 psig), the pressure relief valve opens and vents the gas to the atmosphere, making a shrill sound
High Pressure Regulators
Function to maintain constant flow with changing supply pressure (50 psi)
•Without a regulator CRNA would have to constantly adjust fresh gas flow
•Separate pressure regulator for each gas
•May be adjustable (transport cylinder) or preset (anesthesia machine)
High Pressure System
Receives gases from cylinders at high, variable pressures and reduces to low, more constant pressure
•Hanger yoke orients and supports the cylinder, provides a gas-tight seal, and ensures unidirectional flow
•Retaining screw allows cylinder valve outlet to be pressed against the washer
•A filter is installed between the cylinder and pressure regulator to prevent particulate matter from entering machine
Regulator Function
Gas flows out of low pressure chamber
•Drop in pressure reduces force generated by diaphragm against the spring
•This allows the valve to open and admit gas from high pressure chamber
•Output pressure adjusted by screw (A)
Cylinder Pressure Gauges
Prior to the check valve a gauge is present which will measure the pressure in the cylinder
•The gas supply pipeline also includes pipeline pressure gauges to show the supply pressure
•Gauges are color coded and are metal needles which stretch with pressure and point to correct pressure (“Bourdon” gauges)
Check Valves
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Allows gas to enter the machine but prevents gas from exiting the machine when there is no cylinder in the yoke
•Allows empty cylinder to be replaced without losing gas
•Are not permanent seals and so should not leave yoke empty
Intermediate Pressure System
Receives gases from the high pressure regulator or pipeline inlet to the anesthesia machine •When master switch is OFF pressure in intermediate pressure system drops to zero •Pneumatic components: oMaster switch oPipeline inlet and pressure indicators oOxygen failure devices oOxygen flush oFlow control valves
Effects of Oxygen Flush
- Pressure transmitted back to other structures in the machine, such as flow indicators and vaporizers (may change output)
- ASTM mandates that vapor output cannot increase by more than 10% with flush
- Hazards include leaks, sticking and obstructing flowmeter output, barotrauma, awareness
Oxygen Supply Failure Safety Devices
Machines possess a pneumatic or electronic alarm device that sounds a warning when the oxygen supply pressure decreases to below 30 psig
Other safety devices include a “fail-safe” valve (oxygen pressure sensor shut-off valve) that will decrease supply pressure of other gases as the oxygen supply pressure decreases
Mandatory Reduction of Gas Flow
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When oxygen supply pressure is reduced below manufacturer specified minimum, delivered oxygen concentration shall not decrease below 19% at the common gas outlet (ASTM standard)
Oxygen Supply Failure Alarm
At least a medium priority alarm enunciated within 5 seconds when oxygen supply falls below 30 psig
Leaks downstream are not detected, as well as crossover pipeline errors
Second Stage (Low Pressure) Regulators
- Some machines have reducing devices just upstream of flow valves that decrease pressure a second time (15-19 psig)
- Purpose of this reducing device is to eliminate fluctuations in pressure supplied to the flowmeter caused by fluctuations in pipeline pressure
Mechanical Flowmeters
Based on principle that flow past a resistance is proportional to pressure
Physical Principles of Flowmeters
Traditional flowmeters are variable orifice (Thorpe type)
•Vertical glass tube is tapered with its smallest diameter at the bottom
•When the flow control valve is opened gas enters at the bottom and flows up the tube
•Gas passes out into circuit and indicator floats freely where gravity equals gas pressure
Flowmeters and Indicators
Factors affecting indicator are thus physical properties of the gas (viscosity and density), weight of the float and annulus of the tube
•Low gas flow means area between indicator and annulus will narrow, decreasing flow
•Flowmeters are calibrated at atmospheric pressure (760 torr) and temperature (20 degrees C), however pressure and temperature affect flow only slightly
Flow Meter Assembly
Flow meters precisely control and measure gas flow to the common gas outlet (leads to patient)
•Flow-control valve regulates the amount of flow that enters a tapered, transparent tube (Thorpe tube)
•A mobile indicator float inside the flow tube indicates the flow rate
Flow Meter Facts
Flowmeters are color coded, and the flow-control knob for oxygen is larger and shaped differently to avoid inadvertent changes
Inaccuracy in Flowmeters
Tube is not vertical
•Back-pressure from a ventilator
•Static electricity causing the float to stick to the tube
•Dirt causing the float to stick to the tube
•Float stuck at the top
Flow Meter Hazards
Oxygen should be on the far right in case of a leak in the line of flowmeters
•Safety mechanisms include mandatory minimum oxygen flow (preset at 200-300 ml/minute) and oxygen/nitrous link (prevents delivery of less than 25% oxygen by “linking” nitrous knob to oxygen knob)
Bourdon Flowmeters
Found on flow-limited regulators
•For delivery of oxygen directly to patient from cylinder
•Adjustable pressure supply, pressure gauge and special orifice
•If orifice becomes partially blocked, will over-read
Common (Fresh) Gas Outlet
- Receives all gases and vapors from anesthesia machine
- Typically CGO has 15 mm female slip-joint fitting, coaxial 22 mm male connector
- Some new machines have two CGO
- CGO should NOT be used to administer supplemental oxygen to patient (may delay use of breathing system during emergency)
Electrical Components
Most components on newer machines, including ventilators, are powered by electricity
•Computer-driven machines should be turned off and restarted every 24 hours (self-checkout)
•Battery power is always available and should be checked
•Only monitors should be plugged into machine’s electrical outlets
Devices Which Require Electrical Power
- Mechanical ventilators
- Electronic monitors
- Digital flowmeters
- Gas/vapor blenders (Tec 6/Desflurane)
- Machines with electronic flowmeters have backup pneumatic flowmeter
