Anesthesia Machine Flashcards
How are vaporizer and FM settings affected by altitude?
AT HIGHER ALTITUDE, VAPORIZER SETTINGS WILL BE HIGHER AND FGF WILL BE LOWER THAN AT SEA LEVEL TO MAINTAIN SAME SETTINGS
Graham’s Law of Diffusion
Rate of diffusion (velocity) = 1/square root (MW)
Density of Gases at Altitude
density of gases decreases
When higher flows set in FM, actual flow of gases will be higher than the set flows, as flow is inversely proportional to the square root of density as per Graham’s law.
Regulatory Body that Oversees AM
American Society for Testing and Materials
o Updated 2005, F1850: Standard Specification for Particular Requirements for Anesthesia Workstations and their Components
2000: Canadian Standards Assoc
Do veterinary AM have to meet any standards?
No
E Tank - oxygen
660L, 1900PSI
14#
E Tank - N2O
1590L, 745psi
14#
G Tank - N2O
13,800L, 745psi
97#
H tank - oxygen
6900L, 2200psi
119#
H tank - N2O
15,800L, 745psi
119#
Anesthesia Machine
- Permits delivery of precise yet variable combination of inhalant anesthetic and oxygen
Common Components
FROGS
FM
Regulator
vapOrizer
Gas supply
Scavenge
What is an anesthesia workstation?
machine + vaporizer(s) + ventilator + breathing system + scavenging + monitors
Advantages of Ax WS?
–integration of monitoring/control functions, and alarms
–data display on single or multiple screens
–reduced external connections that reduce likelihood of misconnections disconnections or kinked connections
–Automatic system checks, built in safeguards in event of machine failure
Disadvantages of Ax WS?
–potential disruption of CMV/ gas delivery
–display failure
–electrical failure
–fires
–liquid spills
–Malfunction in way that anesthesia provider doesn’t recognize
High Pressure Components of Pneumatic System?
Up to 2200psi
Gas cylinders
Hanger yoke/Yolk blocks
High pressure hoses
Cylinder pressure gauge
Cylinder pressure regulator
Intermediate pressure components of a pneumatic system?
40-55psi
Pneumatic part of master switch
Pipeline inlet connections
Pipeline pressure indicators
Piping
Gas Power Outlet
Oxygen pressure failure devices
Oxygen flush
Additional pressure regulators (if present)
Flow Control valves
Low Pressure Components
<30cm H2O - bc pressure in this part transmitted directly to patient lungs
Flowmeters
Hypoxia Prevention Safety Devices
Unidirectional Valves
Pressure Relief Devices
Vaporizer
Conduit btw Vaporizer, Common Gas Outlet
Common Gas Outlet
Hanger Yokes
Fxn: orients, supports cylinder, gas tight seal, ensures unidirectional gas flow
At least one yoke for oxygen, one yoke for N2O +/- second yolk for oxygen
Parts of the Hanger Yoke
Body
Retaining Screw
Nipple
Index Pins
Washer
Filter
Check Valve Assembly
Hanger Yoke Body
principal framework supporting structure
* Swinging gate: distal part hinged, can be swung to side when mounting cylinder
Hanger Yoke Retaining Screw
tightens cylinder valve outlet against washer/nipple of cylinder
* Threaded into distal end of yoke
* Gas-tight seal
Hanger Yoke Index Pins
prevent attachment of incorrect cylinder
* Below nipple, holes into which pins are fitted must be of specific depth
Hanger Yoke Nipple
where gas enters the machine, fits port on cylinder valve
* Impossible to obtain tight seal with cylinder valve if damaged
Hanger Yoke Washer
seal between cylinder and yoke, placed around nipple
* Only one washer per cylinder
* In good condition, not broken or curled
Hanger Yoke Filter
Prevents particulate matter from entering machine
* Between cylinder and pressure regulator or cylinder and flow control valve
Hanger Yoke Check Valve Assembly
unidirectional flow of gas through yoke, esp if no tank on machine
* allows empty cylinder to be replaced with a full one without losing gas
* Prevents transfer of gas from high pressure cylinder to low pressure when both connected to double yoke/turned on
* Plunger: slides away from side when pressure greater
- Cylinder pressure > machine pressure, plunger pushed right –> gas into machine
- Machine pressure > cylinder pressure –> plunger moves left, blocks gas flow
Empty Yoke
use yoke plug/dummy
* In absence of yoke plug gas can flow retrograde through open flow control valve, out through yoke
What should avoid contaminating with grease/oil?
Cylinder valves, yokes = fire hazard
Cylinder Pressure Gauge is what time of tube?
Bourdon Tube
Bourdon Tube
Cylinder Pressure Gauge
coiled party blower hollow metal tube bent into curve, sealed, linked to clock like mechanism with opposite end connected to gas source
* increased gas pressure: inside tube straightens
* decreased gas pressure: tube resumes curved shape
* Movement of sealed end transmitted to an indicator –> moves over calibrated scale (kPa, +/- PSI)
Scale for cylinder, pipeline, machine working pressures?
PSI or kPa
Breathing system pressures?
usually cm H2O
Cylinder regulators
–Allow maintenance of constant flow with changing of supply pressure
–Bring high pressures of gas cylinders down to more reasonable, safe working pressure (40-55psi)
* no regulator: provider constantly altering flow control valve to maintain constant flow through flow meter as pressure and cylinder decreased
* Prevent fluctuations in pressure as tank empties
Required for each gas supplied from cylinder
What regulators are adjustable vs what regulators set by manufacturer?
Pipeline regulators = adjustable
Machine regulators set by manufacturer
ASTM standards for pressure regulators
regulators on anesthesia machines to be set to preferentially use pipeline gases before using gas on the backup cylinder
* Ensure pipeline pressure set 5psi higher than machine’s regulator for reserve oxygen cylinder
Intermediate
o Accepts gas from pressure regulator/central pipeline to flush valve/flow meter on AM
40-55PSI
Multiple routes through intermediate area: flowmeter, flush valve, auxiliary
How does oxygen enter the immediate pressure portion of circuit?
o Oxygen enters via master switch when off, pressure in intermediate system = 0
Downstream of inlets for cylinder, pipeline supplies
O2 flush = independent of switch
Pipeline Inlet Connections
entry pt of gasses from pipeline, Diameter Index Safety System (DISS) connections
Unidirectional (check) valve: prevent reverse gas flow from machine to piping system
Filter required: pore size <100m, can become clogged –> decreases gas flow
Pipeline Pressure Gauge
50-55psi
Indicator: pipeline side of check valve
* Machine side: would not give true indication of pipeline pressure unless cylinder valves closed
If pressure from cylinder (open valve) > pipeline, gas drawn from cylinder
* Always close valve of attached cylinder when using pipeline
Gas Power Outlet
o Gas power outlet: serve as source of driving gas for ax ventilator HALLOWELL
Not found on machines with built-in ventilators
Oxygen Pressure Failure Devices
alarm when oxygen pressure fallen to dangerous level
Mitigate depletion of oxygen supply, deliver 100% anesthetic agent
Oxygen Failure Safety Device
shuts off or proportionally decreases, ultimately interrupts supply of N2O if oxygen supply pressure decreases
Will also interrupt supply of other gases to their flow valves
Depends on pressure, not flow
Oxygen Supply Failure Alarm
<30PSI, medium priority alarm that cannot inactivate
Depends on pressure, not flow
Do not prevent ax gas from flowing if no O2 flow, delivery hypoxic mixture
Oxygen Flush
directs high unmetered flow directly to common gas outlet from pipeline inlet or cylinder pressure regulator
Independent of master switch, vaporizer
O2 Flush FR, pressure
Flow rate: 35-75L/min
~50psi
MOA Oxygen Flush
button, stem connected to ball with ball in contact with a seat
* Button pressed: forced away from seat allowing oxygen to flow to outlet
* Spring opposing ball will close valve when button not pressed
ASTM Standards for Oxygen Flush
front of machine, recessed (avoid accidental activation)
Hazards Assoc with Oxygen Flush
accidental activation, leaking, sticking delivery of oxygen-enriched mixture if using medical air, barotrauma
* Activation during inspiration by ventilator: delivery of high VT, +/- barotrauma
* Not a problem in ventilators that exclude FGF from circuit
* Avoid activating with NRB, low vol circuits (pediatric circle systems)
Flow Adjustment Control
regulate flow of air, oxygen, other gases to flow indicators
Two types: mechanical, electronic
Only one control per gas, must be adjacent to/identifiable with associated FM
Mechanical Flow Adjustment Control
- Used with both mechanical, electronic FMs
- Stem, seat: fine threads, stem only moves short distance when complete turn made
- Control knob: O2 = largest, fluted profile, distinct look/feel from other gas control knobs
Mechanical Flow Control Knob
- O2 = largest, fluted profile, distinct look/feel from other gas control knobs
o Counterclockwise increases flow
o +/- shield or bar to prevent accidental movement
o Should be turned off when not in use
Mechanical Flow Adjustment Control
o Closed valve: pin at end of stem in seat –> occludes orifice, no gas flow
o Open valve (stem outward): opening btw pin, seat –> gas flows through valve; larger space, greater gas flow
Stops:
Off: avoids damage to valve seat (why shouldn’t overturn to close)
Maximum: prevents stem from becoming disengaged with body
Problems with Flow Adjustment Control
control knob sensitive if worn or loose, stem/seat can block flow
Electronic Flow Control
- Knob turned clockwise to increase flow, solenoid valve
- Air-ox or ox-N2O: One control regulates oxygen concentration, another controls total flow
- Flow/pressure transducers, temperature sensors maintain accuracy
Low Pressure
downstream of flow control devices, slight above atmospheric pressure, variable pressures
Flowmeters
indicate rate of flow of gas passing through them
Control rate of gas delivery to low pressure area of ax machine, determine FGF to ax circuit
1 per gas on machine
Electronic or Mechanical
Electronic FMs
representation of mechanical FM on screen, # representing flow
Mechanical FMs
Flow past resistance is proportional to pressure
Thorpe Tube
vertical glass (Pyrex) internally tapered tube with freely mobile indicator
o Narrowest portion at bottom
Single Taper Thorpe Tube
opening gradually increases from bottom to top, different tubes for high and low flows
Double Taper Thorpe Tube
two tapers inside same tube for fine, coarse flows; opening size increases more rapidly >1L/min
MOA Thorpe Tube
Open control valve, gas enters bottom, elevates indicator, flows through annular opening btw indicator and tube, to outlet at top of tube
Indicator floats where downward force caused by gravity = upward force caused by gas pressure at bottom of cylinder
increased gas flow: # gas molecules hitting indicator increases, indicator rises, size of annular opening increases (tapered tube), more gas flows
decreased gas flow: gravity causes indicator to lower
What factors affect rate of flow through Thorpe Tube?
- Pressure drop across constriction
- Size of Annular Opening
- Physical Properties of Gas
- Temp/pressure
Thorpe Tube: pressure drop across constriction
As gas flows around indicator, frictional resistance btw indicator, tube wall –> loss of energy = pressure drop
Pressure drop constant for all positions in tube, = weight of float/cross sectional area
Thorpe Tube: Size of Annular Opening
Annular cross-sectional area varies while pressure drop across indicator remains constant for all positions in tube
Constant pressure flowmeters
Increased flow does not increase pressure drop, causes indicator to rise to higher position in tube –> greater flow area for gas
Thorpe Tubes: low GF, narrow opening
Low gas flow through tube, narrow annular opening
* Longer, narrower constriction
* Laminar flow, HP – flow is function of gas viscosity
Thorpe Tubes: high GF, larger annular opening
- Shorter, wide constriction
- Turbulent flow, Graham’s Law – flow function of gas density
Thorpe Tubes: Temp, Pressure Calibration
Calibrated at atmospheric pressure (760 Torr), room temp 20*C
Temp, pressure changes –> changes viscosity, density of gas, influence accuracy
Calibration: flow at ambient pressure = flow indicated on scale calibrated at sea level * (density of gas at sea level/density of gas at ambient pressure)
Hyperbaric Chamber Flow Rates/FMs
FM delivers less gas than indicated
Lower barometric pressure/higher altitude
actual flow rate greater than that indicated
* Gas less dense
* Less atmospheric pressure, lower flow will raise ball so flow will be more than sea level
Mechanical FM Assembly
tube through which gas flows, indicator, stop at top of tube, scale
* Empties into common manifold (mixing chamber) that delivers measured amt of gases into low pressure system
Indicators: nonrotating float
o Reading: upper rim
o Gas flow keeps in center of tube if tube vertical
Indicators: rotameters (rotating floats)
o Reading: upper rim
o Upper rim diameter larger than body with slanted grooves in rim, causes float to rotate when gases passes
o Visual indicator gas flowing, indicator not stuck in tube
o Prevents fluctuations, reduces where slashed hair, assists passage of small particles, reduces errors caused by friction between tube and indicator
Indicator: Ball
o Reading: middle of ball
o Rib guides hold in center of tube
o Rotation = ball can move freely in tube, reading accurate
Where is the greatest accuracy on a FM?
Middle of the tube
Stop
at top of FM tube, prevents indicator from plugging outlet – damages tube
* Prevents indicator from ascending to point cannot be seen, difficult to ensure off
FM Tube Arrangement
- FMs for different gases = side by side
- Meet at common manifold/mixing chamber
- Can have two FMs for same gas: one for low, one for high flows
If have two FMs for same gas…
o Arranged in series – total flow NOT sum, total flow = flow shown on higher flow tube
o One flow control meter for both tubes
o Gas from flow control valve first passes through tube calibrated up to 1L/min, then passes to second tube calibrated for higher flows
If have multiple FMs…
o Oxygen FM always most downstream/next to manifold outlet so that leak upstream results in loss of other gas, not oxygen
o Placement of air vs N2O variable
Auxiliary FM
- Self-contained FM with own flow control valve, flow indicator, outlet
- Short tube, max flow ~10L/min, barbed fitting on outlet
- Used to supply oxygen to patient without turning on anesthesia machine
Problems with FM
FM scale, tube, indicator = unit, cannot replace individual parts
Tube assembly calibrated for one gas cannot be used for different gas
Degree of inaccuracy that will occur depends on gases for which flow meters were intended
o If gases of similar densities/viscosities, difference in accuracy minimal
Use of wrong FM common with air, N2O bc no standard order
Problems with FM Indicators
Damage caused by sudden projection to top of tube when cylinder opened or pipeline hose connected with flow control valve open,
o Worn by handling
o Dislodgement of stop, rests on top of indicator
FM Leaks
- Leak downstream of indicator, upstream of common manifold = lower than expected concentration of gas in fresh gas
FM ASTM Standards
O2 knob must be uniquely shaped (fluted)
O2 knob must be the right most side of FM bank downstream of all gases (to prevent anoxic mixtures)
O2 always set downstream to avoid anoxic mixture
Hypoxia Prevention Safety Devices
Mandatory Minimum Oxygen Flow
Minimum Oxygen Ratio
Mandatory Minimum Oxygen Flow
- Minimum flow of oxygen (50-250mL/min) before other gases will flow, set by manufacturer
- +/- alarm
- Does not in itself prevent hypoxic gas mixture from being delivered
Minimum Oxygen Ratio
Protection against operator selected delivery of mixture of oxygen, N2O with oxygen concentration < 21% in fresh gas or inspired gas
Two types: mechanical, electric linkage
Minimum Oxygen Ratio: Mechanical Linkage
14 tooth sprocket on and two O flow control valve, 29 tooth sprocket on oxygen control valve
If 25% oxygen concentration reached, pin on oxygen sprocket engages pin on oxygen flow control knob –> Couples turning of oxygen, and two oh valves to maintain minimum 25% oxygen
Allows for independent control of each gas as long as percentage of oxygen above minimum
Minimum Oxygen Ratio: Mechanical Linkage if increase N2O flow below ratio…
O2 flow increased
Minimum Oxygen Ratio: Mechanical Linkage if lower O2 flow too much…
N2O lowered proportionally
Electric Linkage
o Provides minimum ratio of oxygen to N2O flow
o Electronic proportionally valve controls oxygen concentration in fresh gas, computer continuously calculated maximum allowable amount nitrous oxide flow given oxygen flow
Unidirectional Valves
Can have PP transmitted back to machine from BS with controlled or assisted ventilation or with use of oxygen flush valve
Valve = located vaporizers, common gas outlet
* upstream of where oxygen flush joins fresh gas flow
* Valve will lessen pressure increase but not prevent it – gas continually flowing from flow meters
Pressure Relief Devices
Near common gas outlet to protect machine from high pressures
valve opens to atmosphere, vents gas to atmosphere if preset pressure exceeded
May limit ability of anesthesia machine to provide adequate pressure for jet ventilation
Common Gas Outlet
AM –> BC
Receives all gases, vapors from machine - delivers mixture to BS at concentration, flow rate determined by vaporizer setting, flow rates
Common location for disconnection
Fresh gas supply tube, conveys gas to fresh gas inlet and breathing system attaches to common gas outlet
Outlet gas does not usually equal inhaled concentration when using rebreathing circuits – dilution of incoming gas with what is already in the patient circuit
What size is the connection btw the common gas outlet and the breathing system?
15mm (???)
