Anaesthetic circuits

Samantha Morgan

Aim

• To provide an overview of anaesthetic circuits

Objectives

• By the end of the session students should be able to;– Describe Endotracheal intubation– List advantages and disadvantages of Endotracheal

intubation– Recognise the common anaesthetic circuits– Calculate the fresh flow rate of patients on the

different circuits– Perform a safety test on an anaesthetic machine

Endotracheal intubation

• Ensuring a patent airway• Ideally the tube should extend from the incisor

to a point level with the spine of the scapula.• Surplus dead space is minimised by cutting off

the projecting tube.• Choose the maximum tube diameter

appropriate to the patient to minimise resistance to air flow

Intubation

• The jaws must be relaxed and laryngeal reflexes suppressed before intubation is attempted.

• Laryngeal reflexes in cats persist to relatively ‘deep’ level of anaesthesia, and laryngospasm is not uncommon.

• Lignocaine is used to depress this spasm.

Advantages of Endotracheal intubation

• Airway protection• Allows positive pressure ventilation• Reduces waste-gas pollution• Reduces anatomical dead space.

Disadvantages of Endotracheal intubation

• Airway resistance• Kinking or occlusion possible• Traumatic laryngitis• Chemical/ischemic tracheitis• Apparatus dead space• Endobronchial intubation• Surgical interference from the tube.

Endotracheal tubes

• Magill tube• 3 mm to 40 mm internal diameter• plain or with an inflatable cuff• Can also be made from PVC (Portex)

Endotracheal tubes

• Endobronchial intubation– Inserted into one of the main stem bronchi– arterial hypoxemia– cyanosis – laboured breathing– uptake of the inhalation

anaesthetic agent may be impaired

Endotracheal tubes

• Impaction– Tip of the tube against the tracheal wall– The Murphy eye

Endotracheal tubes

• Herniation of the cuff

• Compression of the lumen

Endotracheal tubes

• Stretching of the tracheal wall– Tracheitis– Pressure necrosis of the tracheal wall– Tracheal rupture.

Endotracheal tube check

• Correct size• Check patency• Check cleanliness• Check cuff and balloon are functioning• Ensure adequate lubrication to avoid

mucosal trauma.

Laryngeal Masks

• 1980s• Alternative to endotracheal intubation• Difficulties in producing a gas-tight seal in

animal patients• Considerable cost

Mask inductions-advantages

• Do not damage the airway• Produce smooth induction when patients are

depressed or heavily sedated.

Mask inductions-disadvantages

• Causes resistance/anxiety to poorly sedated patients

• Mask increases mechanical dead space• Limited ventilator support during IPPV• Greater atmospheric pollution.

Circuits

• Used to deliver anaesthetic agents• Can be classified as rebreathing and non-

rebreathing• Ventilation – Inspiration– Expiration– End expiratory pause

Calculations

• Tidal volume– 10-15mls x Bodyweight (kgs)

• Respiratory minute volume– Tidal volume x respiratory rate (BPM)

How do we end up with circuit factors?....

• By calculating the resistance of the circuit• Tube diameter alters the resistance• Corrugated tubing has a greater pull• Flushing the system

Reservoir bags

• The volume should be 3-6 times the patients tidal volume.

• Oversized bags increase circuit volume• Inadequately sized bags collapse during large

breaths and over distended during expiration.• For small animal use, 2,4 and 6 litre bags are

required.

Circuits

Ideal breathing system

• Simple, safe and inexpensive• Delivers intended inspired gas mixture• Permits spontaneous, manual and controlled

ventilation in all age groups• Efficient, allows low fresh gas flow• Protects patient from barotrauma• Sturdy, compact, lightweight• Permits easy removal of gases

Rebreathing circuits

• Exhaled gases are collected and the exhaled CO2 is removed– Sodalime

• Flow rate is approximately 5-10mls/kg/min

Advantage of re-breathing circuits

• Low flow rate• Cheaper to run• Lower theatre contamination• Less anaesthetic agent requirements.

Disadvantages of re-breathing circuits

• Respiratory tract irritation• Heavy• Need to check soda lime• Heat produced• Higher resistance.

De-nitrogenation • Patients expire considerable volumes of

nitrogen• Lowers circuit oxygen levels• Use high flow rates for the first 10-15 mins

of anaesthesiaor• Empty the reservoir bag every 3 minutes

for the first 15 minutes.

To and Fro

• Bidirectional flow improves CO2 scrubbing efficiency

• Greater heat conservation• Lower low circuit volume• Can roll off table• Suitable for patient over 7kgs• Valve position is inconvenient for IPPV

Circle circuit

• High gas efficiency • Less circuit inertia unlike to and fro• Can be expensive and cumber some• For animals over 5-10kgs• Unidirectional flow is dependant on the

valves.

Soda lime

• Baralyme• Soda lime– 94% calcium hydroxide– 5% calcium hydroxide– 1% potassium hydroxide

• Water • pH sensitive dye

Soda lime

• Two types– White to purple– Pink to white

• Exhausted after about 8 hours– Colour change will disappear!

Non-rebreathing circuits

• Rely on adequate gas flow rate

– remove carbon dioxide from the circuit– avoid re-breathing of expired gas.

Ayres T’piece

• Jackson-Rees modification• Circuit factor 2.5-3 x minute volume

Ayres T Piece

• Minimal apparatus, dead space and resistance• Simple and inexpensive• Good for IPPV• Fresh gas flow (FGF) is high• Need the modified system to scavenge

effectively • Afferent reservoir system

Magill

• Circuit factor 1-1.5 x minute volume

Magill

• For use in dogs greater than 8kgs• If no end expiratory pause you can get mixing

of gases• Cumbersome at patient end as scavenging

attaches there• Inexpensive • Efferent reservoir system

Bain

• Circuit factor 2.5-3 x minute volume• Useful circuit for IPPV• Basically same as Ayres• Watch frequently the inner tube becomes

disconnected, kinked and leaks!• Afferent reservoir system

Lack

• Circuit factor 1-1.5 x minute volume

Lack

• For dogs over 10kgs• Similar to Magill but with the valve more

conveniently placed• Should not be used for prolonged IPPV• Afferent reservoir system

Mini Lack

• Smaller version of standard• Alternative to Ayres• Bodyweight range 1-10kg

• Circuit factor 1-1.5 x minute volume

Humphrey ADE

• Has three different modes– Semi-closed system for under 7-10kg– Recycling for over 7-10kgs– Ability to connect a ventilator

Fresh gas flow recommendations (after induction)- MINIMUM 300ml/min

Cats 70-100 ml/kg/min semi closed without absorber

Dogs under 10kgs 70-100 ml/kg/min semi closed without absorber

Dogs over 10kgs 30ml/kg/min induction recycling with soda lime canister10ml/kg/min maintenance

Anaesthetic machine check

• Open the oxygen cylinder valve• Slowly anticlockwise

Anaesthetic machine check

• Check the registered quantity of oxygen on the pressure dial

Anaesthetic machine check

• Turn on the oxygen flow meter control to check smooth function

• Turn it off after this

Anaesthetic machine check

• Press the emergency oxygen flush button

Anaesthetic machine check

• Attach the “in use” label to oxygen cylinder

Anaesthetic machine check

• Check the vaporiser percentage dial to ensure it turns easily

Anaesthetic machine check

• Check the level of the volatile agent in the vaporiser