Introductory Lecture Series: The Anesthesia Machine

1
Introductory Lecture Series The Anesthesia
Machine

• Kevin Ewing PGY-4
• July 7, 2004

2
Objectives

• Anesthesia Machine
• Vaporizers
• Circuits
• Ventilators
• Scavenging Systems
• System Checkout

3
The Anesthesia Machine
High
Intermediate
Low Pressure Circuit
4
High Pressure System

• Receives gasses from the high pressure E
  cylinders attached to the back of the anesthesia
  machine (2200 psig for O2, 745 psig for N2O)
• Consists of
• Hanger Yolk (reserve gas cylinder holder)
• Check valve (prevent reverse flow of gas)
• Cylinder Pressure Indicator (Gauge)
• Pressure Reducing Device (Regulator)
• Usually not used, unless pipeline gas supply is
  off

5
E Size Compressed Gas Cylinders
6
Hanger Yolk

• Hanger Yolk orients and supports the cylinder,
  providing a gas-tight seal and ensuring a
  unidirectional gas flow into the machine
• Index pins Pin Index Safety System (PISS) is gas
  specific?prevents accidental rearrangement of
  cylinders (e.g.. switching O2 and N2O)

7
Pressure Reducing Device

• Reduces the high and variable pressures found in
  a cylinder to a lower and more constant pressure
  found in the anesthesia machine (45 psig)
• Reducing devices are preset so that the machine
  uses only gas from the pipeline (wall gas), when
  the pipeline inlet pressure is 50 psig. This
  prevents gas use from the cylinder even if the
  cylinder is left open (i.e. saves the cylinder
  for backup if the wall gas pipeline fails)
• Cylinders should be kept closed routinely.
  Otherwise, if the wall gas fails, the machine
  will automatically switch to the cylinder supply
  without the anesthetist being aware that the wall
  supply has failed (until the cylinder is empty
  too).

8
Intermediate Pressure System

• Receives gasses from the regulator or the
  hospital pipeline at pressures of 40-55 psig
• Consists of
• Pipeline inlet connections
• Pipeline pressure indicators
• Piping
• Gas power outlet
• Master switch
• Oxygen pressure failure devices
• Oxygen flush
• Additional reducing devices
• Flow control valves

9
Pipeline Inlet Connections

• Mandatory N2O and O2, usually have air and
  suction too
• Inlets are non-interchangeable due to specific
  threading as per the Diameter Index Safety System
  (DISS)
• Each inlet must contain a check valve to prevent
  reverse flow (similar to the cylinder yolk)

10
Oxygen Pressure Failure Devices

• Machine standard requires that an anesthesia
  machine be designed so that whenever the oxygen
  supply pressure is reduced below normal, the
  oxygen concentration at the common gas outlet
  does not fall below 19
• A Fail-Safe valve is present in the gas line
  supplying each of the flowmeters except O2. This
  valve is controlled by the O2 supply pressure and
  shuts off or proportionately decreases the supply
  pressure of all other gasses as the O2 supply
  pressure decreases
• Historically there are 2 kinds of fail-safe
  valves
• Pressure sensor shut-off valve (Ohmeda)
• Oxygen failure protection device (Drager)

11
Pressure Sensor Shut-Off Valve

• The Ohmeda pressure sensor shut-off valve is a
  threshold valve which is either open or closed.
• Oxygen supply pressure opens the valve as long as
  it is above a pre-set minimum value (e.g.. 20
  psig).
• If the oxygen supply pressure falls below the
  threshold value the valve closes and the gas in
  that limb (e.g.. N2O), does not advance to its
  flow-control valve.

12
Oxygen Failure Protection Device (OFPD)

• Based on a proportioning principle rather than a
  shut-off principle
• The pressure of all gases controlled by the OFPD
  will decrease proportionately with the oxygen
  pressure

13
Oxygen Supply Failure Alarm

• The machine standard specifies that whenever the
  oxygen supply pressure falls below a
  manufacturer-specified threshold (usually 30
  psig) a medium priority alarm shall blow within 5
  seconds.
• Electronic alarms A pressure operated electric
  switch operates this alarm\
• Ohmeda 28 psig
• Drager 30-37 psig
• Pneumatic alarms (aka Bowmans Whistle) Uses a
  pressurized canister that is filled with oxygen
  when the anesthesia machine is turned on. When
  the oxygen pressure falls below a certain value,
  the alarm directs a stream of oxygen through a
  whistle

14
Limitations of Fail-Safe Devices/Alarms

• Fail-safe valves do not prevent administration of
  a hypoxic mixture because they depend on pressure
  and not flow.
• These devices prevent hypoxia from some problems
  occurring upstream in the machine circuitry
  (disconnected oxygen hose, low oxygen pressure in
  the pipeline and depletion of the oxygen
  cylinder)
• These devices do not prevent hypoxia from
  accidents such as pipeline crossovers or a
  cylinder containing the wrong gas
• Equipment problems that occur downstream (for
  example leaks or partial closure of the oxygen
  flow control valve) are not prevented by these
  devices.

15
Oxygen Flush Valve (O2)

• Receives O2 from pipeline inlet or cylinder
  reducing device and directs high, unmetered flow
  directly to the common gas outlet (downstream of
  the vaporizer)
• Machine standard requires that the flow be
  between 35 and 75 L/min (AS/3 is 35 L/min)
• The ability to provide jet ventilation via the O2
  flush valve is presence of a check valve between
  the vaporizer and the O2 flush valve (otherwise
  some flow would be wasted retrograde)
• Hazards
• May cause barotrauma
• Dilution of inhaled anesthetic

16
Second-Stage Reducing Device

• Located just upstream of the flow control valves
• Receives gas from the pipeline inlet or the
  cylinder reducing device and reduces it further
  to 26 psig for N2O and 14 psig for O2
• Purpose is to eliminate fluctuations in pressure
  supplied to the flow indicators caused by
  fluctuations in pipeline pressure

17
Low Pressure System

• Extends from the flow control valves to the
  common gas outlet
• Consists of
• Flow meters
• Vaporizer mounting device
• Check valve
• Common gas outlet

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Flowmeter assembly

• When the flow control valve is opened the gas
  enters at the bottom and flows up the tube
  elevating the indicator
• The indicator floats freely at a point where the
  downward force on it (gravity) equals the upward
  force caused by gas molecules hitting the bottom
  of the float
• Because the tube is tapered the annular opening
  around the indicator increases with height and
  more gas flows around the float

19
Flowmeter Physics

• The rate of flow through the flowmeter tube
  depends on 3 things
• Pressure drop across the constriction The loss
  of energy as gas passes the float is reflected in
  a pressure drop across the float. This pressure
  drop is given by
• weight of float/cross sectional area
• Size of annular opening The larger the annular
  opening the greater the flow of gas
• Physical characteristics of the gas
• Low Flow Small annular space, therefore flow is
  laminar, therefore flow is a function of gas
  viscosity (Poiseuilles Law)
• High Flow Large annular space, therefore flow is
  turbulent, therefore the flow is a function of
  gas density (Grahams Law)

20
AS/3 Flowmeter

• Electric flow measurement devices determine the
  gas flow according to the pressure difference
  caused by a flow restrictor
• Each flow measurement unit has a
• Pressure sensor sense pressure before and after
  flow restrictor
• Flow restrictor causes a pressure drop which is
  measured by the pressure sensor the pressure
  difference is then used to calculate the flow
  through the flow measurement unit
• There are 4 flow measurement devices in the AS/3
  ADU.
• Flow control section
• O2 flow measurement
• N2O/air flow measurement
• Vaporizer section
• Bypass flow measurement
• Aladin cassette flow

21
Arrangement of the Flow-Indicator Tubes

• In the presence of a flowmeter leak (either at
  the O ring or the glass of the flow tube) a
  hypoxic mixture is less likely to occur if the O2
  flowmeter is downstream of all other flowmeters
• In A and B a hypoxic mixture can result because a
  substantial portion of oxygen flow passes through
  the leak, and all nitrous oxide is directed to
  the common gas outlet
• Note that a leak in the oxygen flowmeter tube
  can cause a hypoxic mixture, even when oxygen is
  located in the downstream position

22
Proportioning Systems

• Ohmeda
• Mechanical integration of the N2O and O2
  flow-control valves
• Automatically intercedes to maintain a minimum
  25 concentration of oxygen with a maximum N2OO2
  ratio of 31
• The Link-25 Proportion Limiting Control System
  does this by automatically increasing the O2 flow
  to prevent a hypoxic mixture

23
Proportioning Systems

• Drager
• The Oxygen Ratio Monitor Controller maintains a
  minimum oxygen concentration of at least 25 /-
  3
• Back pressure from resistors located in the N20
  and O2 flowmeters provides pneumatic input to the
  N2O slave control valve
• The value of the O2 resistor is 3-4 times that of
  the N2O resistor, therefore if O2 flow falls
  below 25, the N2O slave control valve reduces
  the flow of N2O

24
AS/3 Proportional Valve

• Datex-Ohmeda AS/3 Anesthetic Delivery Unit (ADU)
  now uses a linear, electrically driven solenoid
  valve controlled by the CPU.
• The CPU receives input from the O2 and N2O flow
  measurement devices and sends and electric
  current to the solenoid valve
• The valve limits the N2O flow to a maximum of 75
  of the total flow (assuring a minimum of 25
  oxygen)

N2O
25
Limitations of Proportioning Systems

• Machines equipped with proportioning systems can
  still deliver a hypoxic mixture under the
  following conditions
• Wrong supply gas
• Defective pneumatics or mechanics (e.g.. The
  Link-25 depends on a properly functioning second
  stage regulator)
• Leak downstream (e.g.. Broken oxygen flow tube)
• Inert gas administration Proportioning systems
  generally link only N2O and O2
• In general, an oxygen analyzer is the only
  machine safety device that can detect these
  problems (gas sampling done at the Y-piece of the
  patient circuit)

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Low Pressure Limb Check Valve

• Built into Ohmeda (including the AS/3) machines,
  but not Drager machines, between the vaporizer
  and the common gas outlet to prevent positive
  pressure from the patient circuit from being
  transmitted back into the vaporizer and affecting
  the amount of volatile issued from the vaporizer
• Necessary to permit jet ventilation from the
  common gas outlet using the O2 flush valve
• Significant when checking for leaks (see later)

27
Vaporizers

• A vaporizer is an instrument designed to change a
  liquid anesthetic agent into its vapor and add a
  controlled amount of this vapor to the fresh gas
  flow

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Vaporizer Physics

• Partial Pressure The vapor of a liquid exerts
  pressure inside a closed container. That part of
  the total pressure exerted by gasses in a
  container which is due to a specific gas is that
  gass partial pressure
• Vapor Pressure The highest partial pressure that
  can be exerted by a gas at a given temperature is
  its vapor pressure
• Vapor pressure is dependent only on 1) the
  identity of the gas and 2) temperature. It is not
  dependent on pressure
• Boiling Point That temperature at which the
  vapor pressure of the substance equals the
  atmospheric pressure
• Latent Heat of Vaporization Number of calories
  required to change 1 g of liquid to a vapor
  without a temperature change
• The energy for this must either come from the
  liquid itself or an outside source. If it comes
  from the liquid itself then the temperature of
  the liquid decreases, therefore its vapor
  pressure decreases, therefore the amount of
  vaporization decreases.
• Specific Heat Number of calories required to
  increase the temperature of 1 g of a substance by
  1 degree Celsius.
• This tells us how much heat we need to add to a
  liquid to offset the heat lost by vaporization
• Allows us to choose vaporizer materials with high
  specific heats which therefore have minimal
  temperature changes with vaporization

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Classification of Vaporizers
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Generic Bypass Vaporizer

• Flow from the flowmeters enters the inlet of the
  vaporizer
• The function of the concentration control valve
  is to regulate the amount of flow through the
  bypass and vaporizing chambers
• Splitting Ratio flow though vaporizing
  chamber/flow through bypass chamber
• Examples include the Tec 3, Tec 4, Tec 5 and the
  Drager 19.1

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Concentration of Inhaled Anesthetic

• Depending on the temperature and vapor pressure
  of the inhaled anesthetic, the flow through the
  vaporizing chamber entrains a specific flow of
  inhaled anesthetic vapor
• The final concentration of inhaled anesthetic at
  the vaporizer outlet is given by
• entrained anesthetic flow
• entrained anesthetic flow bypass flow
  vaporizing chamber flow
• This is a volume concentration (for example,
  the MAC of isoflurane is 1.5 volume )
• Volume is related to partial pressure in the
  following way
• (volume gas A/total volume) (partial pressure
  gas A/total pressure)
• Patient uptake and depth of anesthesia depend on
  partial pressure, not volume concentration

32
Factors That Influence Vaporizer Output

• Flow Rate The output of the vaporizer is
  generally less than the dial setting at very low
  (lt 200 ml/min) or very high (gt 15 L/min) flows
• Temperature Automatic temperature compensating
  mechanisms in bypass chambers maintain a constant
  vaporizer output with varying temperatures
• Back Pressure Intermittent back pressure (e.g.
  positive pressure ventilation causes a higher
  vaporizer output than the dial setting)
• Atmospheric Pressure Changes in atmospheric
  pressure affect variable bypass vaporizer output
  as measured by volume concentration, but not
  (or very little) as measured by partial pressure
  (lowering atmospheric pressure increases volume
  concentration and vice versa)
• Carrier Gas Vaporizers are calibrated for 100
  oxygen. Carrier gases other than this result in
  decreased vaporizer output.

33
Desflurane

• Volatility
• The vapor pressure of desflurane is 3-4 times
  that of the other inhaled anesthetics that we use
  and its boiling point is 22.8 degrees Celsius
• Normal flow through a traditional vaporizer would
  vaporize 10-20 times as much desflurane as the
  other volatiles (and thats assuming it doesnt
  boil)
• Potency and Heat of Vaporization
• The MAC of desflurane is 4-9 times that of the
  other volatiles, therefore the absolute amount of
  desflurane vaporized over a given time period is
  considerably higher than the other volatiles?this
  would lead to excessive cooling of the desflurane
  vaporizer
• Desflurane would be very intolerant of this
  cooling because it has a very steep
  temperature/vapor pressure curve

34
Tec-6 Vaporizer

• Electronically heated and pressurized to achieve
  controlled vaporization of desflurane
• 2 independent circuits (fresh gas and vaporizer)
• Vaporizer output is controlled by adjusting the
  concentration control valve (R2)
• Pressure in the two limbs is equalized by the
  pressure regulating valve

35
Tec-6 and Altitude

• The Tec-6 works at absolute pressures, therefore
  ambient pressure makes no difference to its
  performance per se. It will accurately deliver
  the desired vol of desflurane, but
• When this gas is brought to ambient pressure at
  high altitudes, this volume percent will
  represent an absolute decrease, in the partial
  pressure of the anesthetic
• To compensate for this the following compensation
  must be made
• Required dial setting Normal dial setting (vol
  ) x (760 mm Hg)/(ambient pressure mmHg)

36
Datex-Ohmeda Aladin Cassette Vaporizer
The ADU identifies the agent type by sensing the
position of identification magnets on the Aladin
cassette?the removable aladin cassette is the
only agent-specific part of the Aladin cassette
vaporizer
37
The Aladin Cassette Vaporizer

• Agent Control Fresh gas enters the Aladin
  cassette from the upstream flow controls. It is
  divided into two streams by a fixed restrictor in
  the bypass unit
• Bypass Flow measured by the bypass flow
  measurement unit
• Cassette Flow measured by the cassette flow
  measurement unit
• Agent concentration is altered by adjusting the
  cassette outflow. This done by the
  electronically-controlled flow-control valve
  controlled by the CPU
• The CPU receives input from 6 sources
• The concentration control dial (that the
  anesthetist sets)
• The flow measurement units of the bypass chamber
• The flow measurement unit of the vaporizing
  chamber
• A temperature sensor located in the vaporizing
  chamber
• A pressure sensor located in the vaporizing
  chamber
• The composition of the carrier gases

38
The Aladin Cassette Vaporizer and Desflurane

• In the AS/3 the same vaporizer is used for
  desflurane as is used for all the other
  volatilesthis poses a problem given desfluranes
  volatility
• At higher temperatures the cassette becomes
  pressurized (desflurane boils at 22.8
  degrees)?this closes the inlet check valve
  effectively shunting all of the carrier gas
  through the bypass chamber, skipping the
  vaporizer chamber entirely. Under these
  conditions the electronically controlled
  flow-control valve simply meters in the
  appropriate flow of pure desflurane vapor
• To avoid the problem of cooling due to
  desfluranes high MAC, the vaporizer is equipped
  with a fan which warms the vaporizer back up
  towards room temperature

39
The Circuit Circle System

• So-called because the components are arranged in
  a circular manner
• Arrangement is variable, but to prevent
  re-breathing of CO2, the following rules must be
  followed
• Unidirectional valves between the patient and the
  reservoir bag
• Fresh-gas-flow cannot enter the circuit between
  the expiratory valve and the patient
• Adjustable pressure-limiting valve (APL) cannot
  be located between the patient and the
  inspiratory valve

40
The AS/3 Circle System
41
Circle System

• Advantages
• Relative stability of inspired concentration
• Conservation of respiratory moisture and heat
• Prevention of operating room pollution
• PaCO2 depends only on ventilation, not fresh gas
  flow
• Low fresh gas flows can be used
• Disadvantages
• Complex design potential for malfunction
• High resistance (multiple one-way valves)
  higher work of breathing

42
Efficiency of a circuit

• Efficiency of a circuit refers to conserving dead
  space gas and venting alveolar gas
• i.e. we want to get rid of alveolar gas which is
  high in CO2 and low in useful gases and keep dead
  space gas which hasn't been used up or polluted
  with CO2
• In the circle system the dead space extends from
  the patient port of the Y-piece to the partition
• The most efficient set-up of the circle system is
  to put the APL valve and one-way inspiratory/
  expiratory valves very close to the Y-piece
• This is because the first gas out of the body
  during exhalation is dead space gas, and the
  latter gasses are alveolar gas. Overflow (i.e.
  venting of excess gas through the APL valve)
  occurs during the latter part of expiration
  during spontaneous ventilation, therefore putting
  the APL close to the patient would vent the
  alveolar gases and conserve the dead space gases
  before they had a chance to mix in the expiratory
  limb of the circuit
• This isnt done in the AS/3 system because it
  would be too bulky

43
The Adjustable Pressure Limiting (APL) Valve

• User adjustable valve that releases gases to the
  scavenging system and is intended to provide
  control of the pressure in the breathing system
• Increased pressure in the breathing system (from
  patient) pushes the diaphragm off its seat
  venting the excess gas into the scavenging system
• The control knob controls the position of the
  diaphragm
• Bag-mask Ventilation Valve is usually left
  partially open. During inspiration the bag is
  squeezed pushing gas into the inspiratory limb
  until the pressure relief is reached, opening the
  APL valve. At this point the additional volume
  the patient receives is determined by the
  relative resistances to flow exerted by the
  patient and the APL valve
• Mechanical Ventilation The APL valve is excluded
  from the circuit when the selector switch is
  changed from manual to automatic ventilation

44
Anesthetic Ventilators

• Power Source Either compressed gas (pneumatic),
  electricity, or both (AS/3). In the event of a
  power failure older pneumatic ventilators will
  continue to function, but the AS/3 relies on its
  limited battery supply
• Drive Mechanism Most anesthesia machines are
  pneumatically driven. A compressed gas (O2 or
  air) compresses a bag or bellows, which in turn
  delivers a volume of gas to the patient
• Note that the AS/3 will automatically switch to
  the gas not in use if the primary driving gas
  fails
• Bellows classification Direction of bellows
  movement during the expiratory phase determines
  the bellows classification
• Ascending Bellows Ascend during the expiratory
  phase. Will not fill in the event of a
  disconnection (safer)
• Descending Bellows Descend during the expiratory
  phase. Will continue up and down movement during
  a disconnection by entraining room air

45
Generic Ascending Bellows Ventilator

• Bellows physically separates the driving gas
  circuit from the patient gas circuit
• During the inspiratory phase the driving gas
  enters the bellow chamber resulting in
• Compression of bellows delivering the anesthetic
  gases within the bellows to the patient
• Closure of the overflow valve, preventing
  anesthetic gas from escaping into the scavenging
  system
• During the expiratory phase the driving gas exits
  the bellows chamber.
• Exhaled gas fills the bellows
• Excess gas opens the overflow valve (PEEP of 2-3
  cmH2O) allowing scavenging of excess gases to
  occur

46
AS/3 Ascending Bellows Ventilator
47
Scavenging Systems

• Scavenging Interface Protects the breathing
  circuit or ventilator from excessive positive or
  negative pressure. There are 2 kinds of
  scavenging interfaces
• Open Contains no valves and is open to the
  atmosphere allowing both positive and negative
  pressure relief
• Closed Communicates with the atmosphere through
  valves
• Gas Disposal Assembly This assembly ultimately
  eliminates the waste gas. There are 2 kinds of
  gas disposal assemblies
• Passive Uses the pressure of the waste gas
  itself to produce flow through the system
• Active Uses a central vacuum to induce flow in
  the system, moving the waste gas along. A
  negative pressure relief valve is mandatory (in
  addition to positive pressure relief)

48
AS/3 Scavenging System

• In the AS/3, the scavenging assembly shown in the
  previous slide is internal to the machine
• Located on the back of the ADU are 5 connectors,
  3 for N2O(1), air(2) and O2(3) pipeline supply,
  one for suction(5) and one for EVAC(4).
• The EVAC connector is the outlet for the
  scavenging system, and is connected to the LHSC
  central scavenging system via pink hosing

49
Checking Anesthesia Machines

• Anesthesia Apparatus Checkout Recommendations,
  FDA. 1993.
• 8 Categories of check
• Emergency ventilation equipment
• High-Pressure system
• Low-Pressure system
• Scavenging system
• Breathing system
• Manual and automatic ventilation system
• Monitors
• Final Position

50
Low Pressure Circuit Leak Test

• Checks the integrity of the anesthesia machine
  from the flow control valves to the common outlet
  (e.g. leaks at flow tubes, O-rings, vaporizer)
• Two types of leak test (depending on presence or
  absence of check valve)
• Oxygen Flush Positive-Pressure Leak Test Only
  used in machines without check valves basically
  just pressurize the low pressure circuit using
  the O2 flush valve and look for leak
• Negative Pressure Leak Test Used in machines
  with or without check valves (i.e. Ohmeda).
  Attach suction bulb to common gas outlet, squeeze
  repeatedly until fully collapsed and ensure that
  it remains collapsed for 10 seconds. Will detect
  leaks as small as 30 ml/min.

51
Circle System Test

• Evaluates the integrity of the circle breathing
  system, which spans from the common gas outlet to
  the Y-piece.
• Tests for leaks, valve integrity and obstruction
• Two parts
• Leak Test Close the APL valve, occlude the
  Y-piece and pressurize the circuit to 30 cmH2O
  using the O2 flush valve. In the absence of a
  leak the pressure should remain at 30 cmH2O for
  10 seconds
• Flow Test (tests for valve integrity and absence
  of obstruction) Breathe through the expiratory
  limb (ensuring that you can exhale but not inhale
  against the unidirectional valve) and the
  inspiratory limb (ensuring you can inhale but not
  exhale) looking for proper unidirectional valve
  motion.

52
AS/3 Automated System Check

• N2O delivery and hypoxic mixture control check
  checks for adequate flow of N2O and for hypoxic
  mixture prevention
• Agent delivery check checks the function
  (including leakage) of the anesthetic agent
  delivery system
• AUTO ventilation check
• Checks the amount of internal and external leak
  (low pressure, bellows, patient circuit). Pass
  leakage less than 150 ml/min
• Calculates the compressible volume of the patient
  circuit. Pass compressible volume less than 10
  ml/cmH2O
• MAN ventilation check checks the amount of
  leakage in the bag hose and in the manual bag.
  Pass leakage less than 100 cmH2O
• Checklist Suction, CO2 absorbent, gas cylinders,
  insp/exp valve, O2 flush, gas monitor

53
The Virtual Anesthesia Machine

• http//vam.anest.ufl.edu/