R1

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3. Breathing systems

• R1 ???

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Definition

• Assembly of components which connects the
  patients airway to the anesthetic machine
• Providing the final conduit for the delivery of
  anesthetic gases to the patient

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Classification

• Insufflation
• Open drop anesthesia
• Draw-over anesthesia
• Mapleson circuits
• The circle system
• Resuscitation system

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Insufflation

• The blowing of anesthetic gases across a
  patients face
• Avoids direct connection between a breathing
  circuit and a patients airway
• Because children resist the placement of a face
  mask or an IV line, insufflation is valuable
• CO2 accumulation is avoided with insufflation of
  oxygen air at high flow rate(gt10 L/m) under H
  N draping at ophthalmic surgery
• Maintain arterial oxygenation during brief
  periods of apnea

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Open- drop anesthesia

• Not used in modern medicine
• A highly volatile anesthetic is dripped onto a
  gauzecovered mask applied to the patients face
  (vaporization-gtlowers mask temp.gtmoisture
  condensation-gtdrop in anesthetic vapor pr.)

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Draw-over anesthesia
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Draw-over anestheaia

• Nonrebreathing circuits
• Use ambient air as the carrier gas
• Inspired vapor and oxygen concentrations are
  predictable controllable
• Advantage simplicity, portability
• Disadvantage absence of reservoir bag -gt not
  well appreciating the depth of TV during
  spontaneous ventilation

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Disadvantages of the insufflation draw-over
systems

• Poor control of inspired gas concentration
  depth of anesthesia
• Inability to assist or control ventilation
• No conservation of exhaled heat or humidity
• Difficult airway management during head neck
  surgery
• Pollution of the operating room with large
  volumes of waste gas

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Mapleson circuits

• Incorporating additional components
• - breathing tubes
• - fresh gas inlets
• - adjustable pressure limiting (APL) valves
• - reservoir bag

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A. Breathing tubes

• Corrugated breathing tubes
• - connect the components of the Mapleson
  circuit to the patient
• Large diameter of tubes (22mm)
• - low resistance pathway
• - potential reservoir for anesthetic gases

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B. Fresh gas inlet

• gases(anesthetics with oxygen or air) from the
  anesthesia machine
• -gt enter the circuit through the fresh gas
  inlet
• the relative position of fresh gas inlet
• - a key differentiating factor in Mapleson
  circuit performance

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C. Adjustable pressure limiting
valve(pressure-relief valve, pop-off valve)

• Allowing gases to exit the circuit through an APL
  valve
• -gt control pressure build up
• Fully open the APL valve during spontaneous
  ventaillation !
• -gt for remaining circuit pressure negligible
  throughout inspiration and expiration
• Partial closure of the APL valve limits gas exit
• -gt permitting positive circuit pressure during
  reservoir bag compressions

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D. Reservoir bag(breathing bag)

• Reservoir of anesthetic gas and a method of
  generating positive-pressure ventilation
• Phase ? the nominal 3L capacity of an adult
  reservoir bag is achieved
• Phase ? Pr rises rapidly to a peak
• Phase ? plateau or even a slight decrease in Pr
  --- ceilling effect !
  -gt help to protect the patients lungs
  against high airway pressures

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Performance characterisitcs of MC

• Lightweight, inexpensive, simple
• Breathing circuit efficiency
• - depends on the fresh gas flow required
• eliminating CO2 rebreathing
• Mapleson A
• CO2 exhaled into breathing tube or directly
  vented through open APL valve
• Most efficient for spontaneous ventilation.
• Mapleson D
• Interchanging APL valve and fresh gas inlet
• -gtFGF forces alveolar air away from pt.
  
• toward the APL valve
• -gt efficient during controlled
  ventilation
• Bain circuit
• Retains heat and humidity
• Decrease the circuits bulk

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The circle system
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A. carbon dioxide absorbent

• Eliminating CO2 in exhaled gas to prevent
  hypercapnea
• CO2 absorbent(sodalime, barium hydroxide lime)
• -gt containing hydroxide salt
• -gt neutralizing carbonic acid

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A. carbon dioxide absorbent

• Increasing hydrogen ion concentration
• - Color conversion of a pH indicator dye
• -gt 50-70 has changed color
• -gt absorbent should be replaced
• Absorbent granules can absorb and later release
  significant amounts of volatile anesthetic
• -gt delay induction or emergence
• The drier the soda lime, the more absorb and
  degrade volatile anesthetics
• Desflurane be broken down to carbon monoxide
• -gt clinically significant carbon monoxide
  poisoning

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B. Carbon dioxide absorbers

• Bulky, double canister permit
• -more complete CO2 absorption
• -less frequent changes
• -lower gas flow resistance
• Patients TV should not exceed the air space
  between absorbent granules(50 of the absorbers
  capacity)
• Dust trap - collect dust and moisture

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C. Unidirectional valves

• Function as check valves
• Contain a ceramic or mica disk resting
  horizontally on an annular valve seat
• Forward flow -gt disk upward -gt gas proceed
  through the circuit
• Reverse flow -gt push disk against its seat -gt
  preventing reflux

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C. Unidirectional valves

• Inhalation
• - Open the inspiratory valve
• - Breath a mixture of fresh and exhaled gas
• - Pass through the CO2 absorber
• - Simultaneously, expiratory valve close
  (preventing rebreathing of exhaled gas)
• Exhalation
• - Open the expiratory valve
• - Vented through the APL valve
• or rebreathed by the Pt after passing through
  the absorbers
• - Closure of the inspiratory valve (preventing
  expiratory gas from mixing with fresh gas)

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Optimization of circle system design

• Unidirectional valves
• Close to the pt prevent back flow into
  inspiratory limb
• Not placed in the Y-piece
• The fresh gas inlet
• Between the absorber and the inspiratory valve
• APL valve
• Immediately before the absorber conserve
  absorption capacity, minimize venting of fresh
  gas
• Reservoir bag
• Locating in the expiratory limb resistance to
  exhalation is decreased

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Performance characteristics of the circle system-
A. Fresh gas requirement

• At low fresh gas flows(lt1L)
• With absorber- preventing rebreathing of CO2
• At fresh gas flowsgt5L
• rebreathing so minimal absorber is unnecessary
• the greater FGF , the less time for a change in
  fresh gas anesthetic concentration
• Higher flows
• - speed induction, recovery
• - compensate for leaks in the circuit
• - decrease the risks of unanticipated gas
  mixture

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Performance characteristics of the circle system-
B. Dead space

• Part of TV that does not undergo alveolar
  ventilation
• Unidirectional valve
• Apparatus dead space is limited to the area
  distal to the point ins-exp gas mixing at the
  Y-piece
• Breathing-tube length does not affect dead space

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Performance characteristics of the circle
systemC. Resistance

• Unidirectional valves and absorber
• - increase circle system resistance
• (at high RR, large TV)

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Performance characteristics of the circle
system-D. Humidity and heat conservation

• Medical gas dehumidified gases at room temp.
• Exhaled gas saturated with water at body temp.
• The heat and humidity of inspired gas
• - depend on the relative proportion of
  rebreathed gas to fresh gas
• Absorbent granules- significant source of heat
  and moisture in the circle system

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Performance characteristics of the circle
system-E. Bacterial contamination

• Slightly risk of microorganism retention in
  circle system component
• -gt respiratory infections in subsequent Pt
• Bacterial filters are incorporated into the
  breathing tubes or at the Y- piece

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Resuscitation breathing systems

• Resuscitation bags(AMBU bags or bag-mask units)
• - used for emergency ventilation
• - simplicity, portability, ability to deliver
  almost 100 oxygen
• - contain nonrebreathing valve

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• The Pt valve open
• Allow gas flow
• from the ventilation bag to the pt.
• Exhalation ports in this valve
• Venting exhaled gas to atmosphere
• Rebreathing is prevented
• Intake valve
• Close during bag compression, permitting positive
  ventilation
• Connecting a reservoir-gtprevent the entrapment of
  room air
• Reservoir valve assembly-two unidirectional
  valves
• Inlet valve allow ambient air to enter the
  ventilation bag if FGF is inadequate to maintain
  reservoir filling
• Outlet valve venting oxygen if FGF is excessive

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• Disadvantage
• Require high fresh gas flows to achieve a high
  FIO2
• Proportional to the oxygen concentration
• Proportional to flow rate of the gas mixture
  supplied to the resuscitator
• Inversely proportional to the MV delivered to the
  Pt.
• Exhaled moisture cause valve sticking
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  -The end-