Anesthesia Ventilators and Scavenging of Waste Gases

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Anesthesia Ventilators and Scavenging of Waste
Gases

• Juan Gonzalez, CRNA, MS
• Clinical Assistant Professor
• Anesthesiology Nursing Program
• School of Nursing
• Florida International University

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Anesthesia Ventilators

• Background
• Patient under General Anesthesia may require
  mechanical ventilation
• A ventilator is used to control the breathing
  pattern for the patient
• Ventilators allow the Anesthesia Provider to
  control respirations hands free

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Ventilators

• Power source is either compressed gas,
  electricity or both (contemporary require both).
• Drive mechanism - modern vents classified as
  double-circuit, pneumatically driven.
• Double-circuit means that a pneumatic force
  compresses a bellows, which empties its contents
  into the patient (aka bellows-in-a box).
• Driving gas is oxygen, air, or a venturi mix of
  O2 and air (Dräger).

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Ventilators

• Cycling mechanism - time cycled, control mode.
• Modern ventilators use solid state electronics
  for timing. Driving gas flow ceases when the set
  tidal volume is delivered to the breathing
  circuit or when a certain pressure is reached.
• Set TV and delivered TV quantities may differ due
  to compliance, losses or leaks!

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Anesthesia Ventilators

• An electronic ventilator
• Re-circulates exhaled patient gas through the
  absorber (where CO2 is removed) and fresh gas is
  added with enough pressure to move the gas into
  the patients lungs
• Helps protect the patient from high airway
  pressures
• Supplies rate, volume, oxygen, and pressure
  monitoring
• Vents excess gas out from the patient breathing
  circuit

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Anesthesia Ventilators

• Types of Ventilators
• Remember that the type is described by how the
  bellows move during EXPIRATION
• Descending Bellows (HANGING BELLOWS)
• You wont find many of these any more.
• The greatest danger is unrecognized disconnection
  of the patient FROM CIRCUIT.
• Bellows may stay distended (apparently full)
  but empty and just giving a false impression of
  being full since gravity is what keeps them
  pulled down.

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Anesthesia Ventilators
This is an example of a weighted hanging
bellows descending bellows ventilator. Not too
easy to find anymore!
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Anesthesia Ventilators

• Ascending Bellows
• Most common type found today
• Safer in that disconnects are more readily
  notable (bellows will not look re-inflated)
• Found on all machines at MSMC and MHI
• Comes with both adult and pediatric bellows which
  are interchangeable

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Anesthesia Ventilators
Typical Example of an ascending bellows
ventilator commonly used today
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Something to Remember!!

• Ascend
• Descend
• Expiration

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Anesthesia Ventilators

• Inhalation
• Bellows fill with fresh gas and re-circulated
  exhaled gas from the patient breathing circuit
• The ventilator control module meters gas from the
  pressured gas supply, called drive gas, to
  pressurize the bellows housing
• The drive gas pressure pushes the bellows down
  and forces the gas mixture into the patients
  lungs (ascending bellows) during inspiration

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Anesthesia Ventilators

• This illustrates the drive gas being forced into
  the bellows housing causing the bellows to
  contract

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Anesthesia Ventilators

• Exhalation
• At the end of inhalation, the control module
  vents drive gas from the bellows housing out the
  exhaust port
• Gas is allowed to flow from the patients lungs
  through the absorber and into the bellows.

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Anesthesia Ventilators
This illustrates how the gas exits the bellows
housing and is vented out of the machine
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Inspiration Expiration
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Anesthesia Ventilators
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Bellows-in-box Ventilator

• A) Begin Inspiration
• Driving gas being delivered into space b/w
  bellows housing
• Exhaust valve (driving gas to atmosphere) closed
• Spill Valve (vents out excess gas to scavenger)
  closed

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Bellows-in-Box

• B) Mid Inspiration
• Driving gas pressure keeps filling space (press
  increases) bellows compressed
• Gas inside bellows pushed into pt
• Exhaust relief valves stay closed
• If too much pressure of driving gas,
  safety-relief valve opens (vent out)

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Bellows-in-Box

• C) End Inspiration
• Bellows fully compressed
• Exhaust relief valves closed

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Bellows-in-Box

• D) Begin Expiration
• Breathing system gases (exhaled fresh) flow
  into bellows (expansion)
• Expanding bellows displace driving gas from
  interior of housing
• Exhaust valve opens (driving gas out to atm)
• Spill valve stays closed

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Bellows-in-Box

• E) Mid Expiration
• Bellows nearly fully expanded
• Driving gas still flowing out to atm
• Spill valve stays closed

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Bellows-in-Box

• F) End expiration
• Breathing system gases continue to flow into
  bellows
• Bellows fully expanded create positive pressure
  spill valve opens
• Breathing system gases out to scavenger

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Anesthesia Ventilators

• Control Module
• Ventilator controls on the front of the
  Anesthesia Machine to
• Select the volume, rate and limit the pressure of
  the gas to be delivered to the patient airway
• Control components that allow inhalation or
  exhalation and to respond to high pressure in the
  patient airway

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Anesthesia Ventilators

• Control Module (cont.)
• Display information about percentage of oxygen
  in the gas supplied to the pt, and the volume of
  gas exhaled from the patient
• Select alarm limits for oxygen percentage, airway
  pressure and exhaled volume
• Display and sound alarms when alarm limits are
  exceeded
• The control module includes
• User controls and a display panel
• A microprocessor and electronic circuits
• Pressure regulators and transducers
• Electronically controlled pneumatic valves that
  open and close to allow inhalation and exhalation

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Anesthesia Ventilators

• Control Module Rear Panel
• Electric power cable
• Supply gas inlet (either O2 or medical grade
  compressed air). Before changing from one gas to
  another, must be recalibrated!!

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Anesthesia Ventilators

• Tidal volume (50 to 1500ml per breath)
• Rate in completed cycles (2 to 100 breaths per
  minute)
• Flow (controls the flow rate of drive gas at 10
  to 100 l/min)
• Pressure limit
• Inspiratory pause
• Mechanical Vent (on-off button)

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Anesthesia Ventilators

• IE ratios
• InspiratoryExpiratory ratios use a 1 for the
  inhalation time and an appropriate number
  expressing the relative length for the exhalation
  time. (IE ratio controlled by inspiratory flow
  knob)
• Can you think of pathology that would benefit
  from prolonged expiratory time?
• Tidal Volume
• Usually 10-15ml/kg, depending on age, etc.Most
  providers adjust depending on ETCO2 and PIP

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Anesthesia Ventilators

• Control Module Display (Ohmeda7800)
• Top line provides numeric information about
  exhaled tidal volume, exhaled breath rate,
  exhaled minute volume, and inhaled O2
  concentration
• Bottom line displays alarms and control settings

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Ohmeda 7900 (Smart Vent)

• Microprocessor control delivers set VT, in spite
  of changes in fresh gas flow, small leaks, and
  absorber or bellows compliance losses (compliance
  losses in the breathing circuit corrugated hoses
  are not corrected).
• It uses flow sensors (in the inspiratory and
  expiratory limbs) and pressure sensors to
  accomplish this.
• Compliance losses in the breathing circuit
  corrugated hoses are not corrected for, but these
  are a relatively small portion of compliance
  losses. The first "modern" ventilator- it offers
  such desirable features as integrated electronic
  PEEP control, and pressure-controlled ventilation
  mode

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Ohmeda 7900
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Drager AV

• Pneumatically and electrically powered, double
  circuit, pneumatically driven, ascending bellows,
  time cycled, electronically controlled, VT-preset
  vent.
• Incorporates Pressure Limit Controller (PLC)
  which allows maximum peak inspiratory pressure
  (PIP) adjustment from 10-110 cm H2O.
• Inspiratory flow control must be set properly
  (like the Ohmeda 7800), so that driving gas flow
  does not create an inspiratory pause. Standard on
  Narkomed 2A, 2B, 2C, 3, 4

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Drager AV
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Anesthesia Ventilators

• Pneumatic manifold assembly
• Contains drive chamber and exhaust chamber
• Flow control valve
• Precisely meters gas flow to bellows housing
  during inspiration
• Exhalation valve
• The exhaust chamber opens to atmosphere
• Free breathing valve
• If patient creates negative pressure in circuit,
  valve will open to allow ventilation (even though
  the vent is on)

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Anesthesia Ventilators

• Inspiratory pause
• If Inspiratory pause is on, the exhalation valve
  stays inflated at the end of inspiration for an
  additional 25 of the inhalation time

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Anesthesia Ventilators

• Safety
• Airway Pressure Transducer
• PAP is continuously monitored during the
  anesthetic
• The AW pressure monitor compares the sensed
  pressure to the setting on the control panel
• If the pressure reaches the pre set limit, the
  ventilator stops the inhalation phase and begins
  the exhalation phase

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Anesthesia Ventilators

• Safety
• High Pressure Limit Switch
• Works independent of the Airway Pressure
  Transducer. Opens at 110 cm H2O
• Regulated Pressure Transducer
• Monitors drive pressure and alarms if it falls
  below 22 psig. Automatically fails vent if 30
  psig is sensed. This ensures that driving
  pressure is within a set safe parameter

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Anesthesia Ventilators

• Alarms
• 2 LEDs located by the alarm silence
• YELLOW indicates alarms that require prompt
  operator response or awareness. This can
  indicate incorrect settings, an indirect affect
  on the patient, or potential harm to the
  equipment
• RED indicates alarms that require immediate
  attention of the operator. These are high
  priority alarms that warn of possible danger for
  the patient

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Anesthesia Ventilators

• Alarms (cont.)
• When more than one alarm condition is present,
  the display alternates between them
• The priority of audible alarms on the Ohmeda 7800
  are
• Warble
• Continuous
• Intermittent
• Single Beep

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Anesthesia Ventilators

• Alarms (cont.)
• Some alarms can be silenced permanently, such as
• Power failure
• Ventilator failure
• Oxygen sensor failure
• Oxygen calibration error
• Volume senor failure

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More on Alarms

• High pressure
• Pressure below threshold for 15 to 30 seconds
  (apnea or disconnect)
• Continuing high pressure
• Subatmospheric pressure
• Low tidal volume
• High respiratory rate
• Reverse flow (may indicate incompetence of
  expiratory unidirectional valve in the breathing
  circuit)
• Apnea/disconnect alarms may be based on
• Chemical monitoring (lack of end tidal carbon
  dioxide), or
• Mechanical monitoring
• Failure to reach normal inspiratory peak
  pressure, or
• Failure to sense return of tidal volume on a
  spirometer

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Anesthesia Ventilators

• Electronic monitor circuits
• Percentage of inspired oxygen
• High O2, low O2, or limit set error can be
  displayed
• These can be adjusted with the push wheel
  switches
• Oxygen sensor assembly
• Sensor cartridge inside assembly. Unscrew and
  change if machine will not calibrate

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Anesthesia Ventilators

• Typically O2 sensor cartridges are kept
  refrigerated to extend their shelf life. Do not
  break them open, as they contain caustic
  chemicals that can burn your skin

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Anesthesia Ventilators

• Calibration of the Oxygen Sensor
• Accomplished EVERY TIME you do your morning
  anesthesia machine check
• Check that it will calibrate to 21 (room air) by
  removing it from the circuit and exposing it to
  room air for 3-5 minutes.
• Rotate the thumb wheel to adjust to 21 if
  needed.
• 100 check needs to be accomplished once a month
  or so

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Anesthesia Ventilators

• Tidal Volume Monitor
• Turbine vane transducer located at the expiratory
  limb of the circuit
• Sensor clip snaps onto transducer cartridge
• Due to effects of patient circuit fresh gas flow
  from the anesthesia machine adding delivered
  volume, the measured tidal volume can
  significantly differ from the set tidal volume

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Anesthesia Ventilators
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Anesthesia Ventilators

• A few assorted tid bits
• In the monitor mode, the high pressure alarm and
  beep will sound whenever the preset maximum PAP
  is exceeded.
• If sustained pressure is sensed (I.E. you forget
  to open the APL while on manual mode) the alarm
  will sound
• Low pressure alarm will sound when a defined
  change is not sensed for 20 sec.

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Anesthesia Ventilators

• Bellows Assembly
• Base
• Housing
• Housing Gasket (u cup seal)
• Bellows
• Pop off valve

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Anesthesia Ventilators

• PEEP Valve
• Positive End Expiratory Pressure can be added
  manually
• When the PEEP valve is adjusted, the set amount
  of pressure is seen at the end of the exhalation
  phase of the next breath
• Can be set between 3-30 cm H2O

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Anesthesia Ventilators
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Anesthesia Ventilators
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Problems/Hazards with Vents

• Disconnection
• Most common site is Y piece. The most common
  preventable equipment-related cause of mishaps.
  Direct your vigilance here by precordial ALWAYS
  if you turn the vent off, keep your finger on the
  switch use apnea alarms and dont silence them.
• The biggest problem with ventilators is failure
  to initiate ventilation, or resume it after it is
  paused.
• Be extremely careful just after initiating
  ventilation- or whenever ventilation is
  interrupted observe and listen to the chest for
  a few breathing cycles. Never take for granted
  that flipping the switches will cause ventilation
  to occur, or that you will always remember to
  turn the ventilator back on after an Xray.

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Monitors for Disconnection

• Precordial monitor (the most important because
  its "alarms" can't be inactivated)
• Capnography
• Other monitors for disconnection
• Ascending bellows
• Observe chest excursion and epigastrium
• Airway Pressure monitors
• Exhaled Volume monitors

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Occlusion/obstruction of breathing circuit

• Besides inability to ventilate, obstruction may
  also lead to barotrauma. Obstruction may be
  related to
• Tracheal tube (kinked, biting down, plugged, or
  cuff balloon herniation). "All that wheezes is
  not bronchospasm".
• Incorrect insertion of flow-direction-sensitive
  components (PEEP valves which are added on
  between the absorber head and corrugated
  breathing hoses)
• Excess inflow to breathing circuit (flushing
  during ventilator inspiratory cycle)
• Bellows leaks
• Ventilator relief valve (spill valve) malfunction
  
• APL valve too tight during mask ventilation or
  not fully open during preoxygenation.

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Misconnection

• Much less of a problem since breathing circuit
  and scavenger tubing sizes have been standardized
  
• Tubing sizes- scavenger 19 or 30 mm, common gas
  outlet (CGO) 15 mm, breathing circuit 22 mm

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More Problems

• Failure of emergency oxygen supply May be due to
  failure to check cylinder contents, or driving a
  ventilator with cylinders when the pipeline is
  unavailable. This leads to their rapid depletion,
  perhaps in as little as an hour, since you need
  approximately a VT of driving gas per breath,
  substantially more if airway resistance (RAW) is
  increased).
• Infection Clean the bellows after any patient
  with diseases which may be spread through
  airborne droplets, or dont use the mechanical
  ventilator, or use bacterial filters, or use
  disposable soda lime assembly, or use a Bain
  Circuit.

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Protocol for Mechanical Ventilator Failure

• If the ventilator fails, manually ventilate with
  the circle system.
• If 1 is not possible, then bag with oxygen (if a
  portable cylinder is available) or room air.
• If 2 is not possible, then try to pass suction
  catheter through the tracheal tube.
• If 3 is not possible, then visualize the
  hypopharynx and cords, or reintubate (?).

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Advantages of Ventilators

• Hands free
• More regular ventilation (rate, rhythm, TV) than
  manual ventilation
• Anesthesia Vents simpler in design fewer
  controls than ICU vents

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Disadvantages of Ventilators

• Loss of contact b/w provider pt.
• Manual (bag) can detect disconnections changes
  in resistance and compliance, continuous positive
  press spont. Vent.
• False sense of security
• Some vents cannot develop high enough
  inspiratory pressure, flow or PEEP to ventilate
  certain pts (ICU vent to OR)

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Scavenging Systems Guidelines

• NIOSH recommendation to OSHA Workers should not
  be exposed to an eight hour time-weighted average
  
• of gt 2 ppm halogenated agents
• (not gt 0.5 ppm if nitrous oxide is in use) or
  gt 25 ppm nitrous oxide.

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Scavenging Systems

• Waste Gas Scavenging systems remove gases vented
  from the breathing system. This helps minimize
  venting of waste gases into the operating room
• There are 2 types of scavenging systems
• Active
• Passive

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Scavenging Systems Components

• Gas collection assembly, (tubes connected to APL
  and vent relief valve)
• Transfer tubing (19 or 30 mm, sometimes yellow
  color-coded)
• Scavenging interface
• Gas disposal tubing (carries gas from interface
  to disposal assembly)
• Gas disposal assembly (active or passive - active
  most common, uses the hospital suction system)

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SCAVENGER INTERFACE

• The scavenger interface is the most important
  component. It protects the breathing circuit from
  excess positive or negative pressure.
• Positive-pressure relief is mandatory to vent
  excess gas in case of occlusion distal to
  interface. If active disposal system used, must
  have negative pressure relief as well. Reservoir
  highly desirable with active systems

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OPEN INTERFACE

• Open interface (a Dräger option) has no valves,
  and is open to atmosphere (allows both negative
  and positive pressure relief). Should be used
  only with active systems. Keep the suction
  indicator between the white etched lines.
  Remember that hissing from an open interface is
  normal (there is no audible indication of waste
  gas leaks)

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OPEN INTERFACE
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CLOSED INTERFACE

• Closed interface (Dräger or Ohmeda) communicates
  with atmosphere only through valves. Should
  adjust vacuum so that reservoir bag neither flat
  not over-distended.

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CLOSED INTERFACE
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Scavenging Systems

• Excess (or waste) gas from the breathing circuit
  is vented through the APL valve when the selector
  switch is on bag
• Waste gas is vented from the ventilator belows
  when the selector switch is on ventilator

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Scavenging Systems
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Scavenging Systems

• Active
• Connects to hospital suction system
• Positive and negative pressure relief valves
  prevent fluctuation to the patient
• A 3 liter reservoir bag is present (holds excess
  gas until it can be removed)
• The positive relief valve opens at 5cm H20 ( with
  exhalation by the patient or ventilator)
• At -.25cm H2O the negative pressure valve opens
  to prevent suctioning from the circuit

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Scavenging Systems
Active System
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Scavenging Systems

• Passive
• Interfaces with hospital ventilation duct
• Relies on the build up of gases in the bag to
  passively empty into the hospital ventilation
  system
• May see both active and passive systems in the
  same institution

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Scavenging Systems
Passive System