INHALATION ANAESTHESIA BREATHING SYSTEMS Prepare and monitor anaesthesia in animals INHALATION ANAESTHESIA BREATHING SYSTEMS

INHALATION ANAESTHESIA BREATHING SYSTEMS

Prepare and monitor anaesthesia in animals

INHALATION ANAESTHESIA

BREATHING SYSTEMS



Inhalation Anesthesia

• Can be used for both induction & maintenance

• Delivers O2

• Removes CO2

• Vaporizes volatile anesthetic liquids– Controls amount of anesthetic delivered

• Can assist ventilation


2 Basic Systems

• Rebreathing

• Non-Rebreathing


Flow of oxygen

• Re-breathing– To & Fro flow (horses)– Circular flow (humans, pets>7-10kg)

• Vaporiser within the circle• Vaporiser outside the circle

• Non-rebreathing– One-way flow (pets<10kg)

• T-piece• Baines• Magill


Advantages of Rebreathing

• Less waste of oxygen

• Less waste of anaesthetic

• Gas warmedwarmed by patient

• Gas humidified by patient

$


Advantages of Non-Rebreathing

• Less dead space– Smaller tubes

• Less pressure required (low resistance)– No valves to move– No need for animal to force gas through

the circle


Principles of Design

• Resistance to moving air– Tubing surface friction– Valve movement friction– Head on gas flows– Gas flow around corners– Soda lime crystals

• Dead space– Machine & tubing dead space– Physiological (airway) dead space

• Pressure relief valve (=pop-off valve)• Rebreathing bag (=reservoir bag)


Why T-piece needs high O2 flow…

• O2 is used to flush the tubing of expired gas– Avoids rebreathing CO2


2 Vaporiser Locations

• Re-breathing Vaporiser – Vaporiser in circle

• Stephens Machine

• Non-rebreathing Vaporiser outside any system– Vaporiser ‘outside’ any system

• ‘Tec’ Series


Vaporiser outside circle

V

O2 O2+ V

Soda LimeSoda Lime

10mL/kg/min O2 metabolised

Anaesthetic vapour absorbed

CO2 removed


Vaporiser inside circle

V

O2

V

Soda LimeSoda Lime

10mL/kg/min O2 metabolised

Anaesthetic vapour absorbed

CO2 removed


VOC


VIC


Non-Rebreathing Configurations

→Bain(1)

Magill

→ Lack

Jackson-ReesAyre

→ Bain(2)→ Modified to…

→ Bain(3)


Magill Configuration

• Used in animals > 7-10kg – Due to expiratory valve resistance– Use 200 mL/kg/minute

• Forced ventilation can be difficult


Lack Configurations

• Tube-in-tube variation (of Magill)

• Parallel tube variation (of Magill)


Ayre’s (T-Piece) Configuration

• Low resistance (no valves)– So ideal for animals < 10 kg


The T-Piece

• Simply describes the shape of the junction

patientexpired

Fresh Gas


Ayre’s (T-Piece) Configuration

• O2-in tube is completely separate from exhaust tube


Bain’s (tube-in-tube) Configuration

• Bain modification of Mapleson D or E non-rebreathing configuration– O2-in tube hidden inside exhaust tube– Ok for forced (positive pressure) ventilation– With or without valves/bag


Bain Variations

Without Valves (<10kg)

Parallel Bain

Bain with bag

Bain connected directly to O2 outlet

With Valves (>10kg)Bain (1)

Bain (2)

Bain (3)


Damaged Bain

• Undetected internal leaks cause high CO2


Bag or No bag?

• Breathing Bag permits positive pressure ventilation (emergencies etc)


Stephens Circle

• Increased or Forced breathing → increased vaporizer output


CircleExpiratory One-way valve

Inspiratory One-way valve

Soda Lime Canister

Rebreathing bagRebreathing bag

Pop-off valve

Waste Anaesthetic Gases(WAG) scavenge tube


One-way Valves

• Attached to inspiratory & expiratory tubes

• Many mechanical types– Vary with degree of force required to move

them• Light plastic flaps

– For smaller patients want low force requirement

• Heavy plates– For heavy duty use on larger patients


One-way valvesOne-way valves






Circle Tube-in-Tube(Universal F-Circuit,King modification)

Parallel Tubes•Large (>10kg)•Paediatric (7-10kg)

Rebreathing Hoses

Y-Piece

‘F-Piece’


Note: 2 kinds of tube-in-tube (Coaxial)

• Rebreathing– ‘King’ modification of circle

• Non-Rebreathing – ‘Bain’ modification of T-piece

Bain King


?


To and Fro

• Carbon dioxide absorber is very close to endotracheal tube attachment

• Lower resistance• Can result in

superheated air being breathed in as it passes over soda lime- there have been cases reported where respiratory tract damage has resulted


Soda Lime



Soda Lime

• 94% calcium hydroxide

• 5% sodium hydroxide

• 1% potassium hydroxide


Soda Lime Chemistry

CO2 + H2O H2CO3 H+ + HCO3-

NaOH + H2CO3 NaHCO3 + H2O

2NaHCO3 + Ca(OH)2 2NaOH + H2O + CaCO3


CO2 Calcium Carbonate

CO2 CaCO3

•calcium carbonate•lime (gardening)•calcite•chalk (older types)•seashell•antacid


Soda Lime Exhaustion & Confusion

White → Purple Pink → White

OR

• 2 colour changes possible– According to brand of soda lime

• Loses colour change if not replacedheat heat


Soda Lime Canisters

• Transparent, or

• Opaque Stephens Machine


Soda lime exhausted if

• Colour change (as indicated)

• Does not generate heat when in use

• Loses crumbly texture


Pop-Off Valve

• Also = ‘Pressure Relief Valve’

• Valve tension adjusted by a screw mechanism

Weak Spring


Pop-Off Valve

Waste gases


Pop-Off ValvePop-Off Valve


Flush Valve

• Also known as– ‘Quick Flush’ valve– ‘Oxygen Flush’ valve– ‘By-Pass’ valve

• By-passes vaporiser (VOC type) & Flowmeter

• Only use to flush out tubing– If patient connected risks pressure burst into lungs

• Delivers a very high 50-70L/min (@50 psi)

– Never use with Bain configuration


Flush Valve


Flush Valve


Pressure valve

• Some machines– Backup in case of regulator failure


Flowmeter


Flowmeter

• Bobbin or Ball

Bobbin flowmeter,

reading 2 l/minBall-float flowmeter,

reading 2 l/min

Read at top Read at middle


Rebreathing Bag

= Reservoir bag• Bag size ~ Animal size

– Adjust pop-off valve and O2 flow rate to keep ½-3/4 full– Approx size = tidal vol (10 mL/kg) x 6 (eg 20 kg dog = 1.2 L)

• Required when O2 supply doesn’t match an inspiration– Most important in a circle system– Not really required in non-rebreathing system

• Also used for – Monitoring tidal volume– Assisted ventilation

• Thoracic surgery• In emergency resuscitation


Anaesthetic tubing

• May be rubber, plastic• Usually corrugated

– Prevents kinking– Collects moisture

• Transports gases• Often condensation from expired gases

– Clean and hang to drain each day

• Avoid kinking• Tube Size ~ Animal Size


O2 Pressures

Psi Atm Cm H2O

kPa

Bottle out 2000 136 13790

Regulator out 50 3.4 340

Circuit 0.43 0.03 30 3

1 atm = approx 100kPa


Which system to use?

< 7-10 kg > 7-10 kg

Non-RebreathingAyres T-piece

Bain (?)

RebreathingCircle

To and Fro

Non-RebreathingBain

Magill

Lack


What about 7-10 kg animals?

• Some practices use a paediatric (ie small diameter) circle tubing system


To-and-Fro Configuration

• Horse anaesthesia– For large animal practices where

occasional longer anaesthetics are required

– Cheap alternative to an anaesthetic machine

– Highly portable & suitable for field use

– Robust - no moving parts – Can store for long periods w/o

need for maintenance


The End


Summary

• What are the 2 basic kinds of ‘breathing system’?– Rebreathing

• Use soda lime to absorb CO2– Circle

» VIC (e.g. Stephens)» VOC (e.g. Tec Series)

– To-and-Fro

– Non-Rebreathing• Use high flow rates to flush out CO2

– T-Piece– Baines


O2 Flows

• Rebreathing (e.g. Circle)

–10 mL/kg/min (range 5-30)• At induction - 20• Usual spay maintenance, most spays -10• Long surgery maintenance - 5

• Non-rebreathing (e.g. T-Piece)

–200 mL/kg/min


N2O Flows

• Non-Rebreathing (T-Piece)– Make up 1/2 or 2/3 of total flow, eg

• O2 100 mL/kg/min

• N2O 100 mL/kg/min

• Rebreathing (Circle)


Re-Breathing Bag

• Should be kept full enough to accommodate each breath– Because rate of gas inflow may not be as

fast as an animals inspiration• This most important for closed/semiclosed

circle systems

• Monitors respiration depth & rate• Allows for positive pressure

ventilation if required


Non-rebreathing

• Advantages– Known inspired concentration – No soda lime to change – Small dead space – Low resistance

• Disadvantages– Expensive to use – Increased use of natural resources – Pollution – Loss of water vapor – Loss of heat


Forced Breathing

• Also known as– Positive Pressure Ventilation (PPV)– Intermittent Positive Pressure Ventilation (IPPV)

• Different machine configurations tolerate forced breathing differently– Valve pressure tolerances may be affected

• Anaesthetic depth can increase rapidly if using VOC machine (Stephens)

• Can cause lung injury (pulmonary barotrauma) if too much pressure



Activity 1

• What is an example of a rebreathing system?

• What is an example of a non rebreathing system?

• What is the basic difference between rebreathing and non rebreathing systems?

• Why are low resistance systems used?


Activity 2

• Give three examples of low resistance systems?


Activity 2 answers

• Mask

• T piece

• Chamber

• To & Fro system


Activity 3

• What are the two categories of vaporisers?


Activity 3 Answers

• Simple– Stephens machine

• Precision– Tec Series vaporisers


Activity 4

• What factors do Precision Vaporisers compensated for?


Activity 4 Answers

• Ambient temperature

• Flow rates

• Back pressure from patient (circuit resistance)


Activity 5

• How do T pieces work?


Non-rebreathing (e.g. Bain-no-valve)


Activity 6

• What do the following parts of an anaesthetic machine do?– Cylinder– Regulator– Flowmeter– Oxygen flush valve– Vaporizer– One way valves– Gas tubing– Soda lime canisters– Pop off valve, escape valve– Scavenger system– Endotracheal tube


The Real End

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INHALATION ANAESTHESIA BREATHING SYSTEMS Prepare and monitor anaesthesia in animals INHALATION ANAESTHESIA BREATHING SYSTEMS