anaesthetic circuits

1. Anaesthetic circuits Samantha Morgan

3. 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

4. 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

5. 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.

6. Advantages of Endotracheal intubation Airway protection Allows positive pressure ventilation Reduces waste-gas pollution Reduces anatomical dead space.

7. 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.

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

9. 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

10. Endotracheal tubes Impaction Tip of the tube against the tracheal wall The Murphy eye

11. Endotracheal tubes Herniation of the cuff Compression of the lumen

12. Endotracheal tubes Stretching of the tracheal wall Tracheitis Pressure necrosis of the tracheal wall Tracheal rupture.

13. Endotracheal tube check Correct size Check patency Check cleanliness Check cuff and balloon are functioning Ensure adequate lubrication to avoid mucosal trauma.

14. Laryngeal Masks 1980s Alternative to endotracheal intubation Difficulties in producing a gas-tight seal in animal patients Considerable cost

15. Mask inductions-advantages Do not damage the airway Produce smooth induction when patients are depressed or heavily sedated.

16. Mask inductions-disadvantages Causes resistance/anxiety to poorly sedated patients Mask increases mechanical dead space Limited ventilator support during IPPV Greater atmospheric pollution.

17. Circuits Used to deliver anaesthetic agents Can be classified as rebreathing and non-rebreathing Ventilation Inspiration Expiration End expiratory pause

18. Calculations Tidal volume 10-15mls x Bodyweight (kgs) Respiratory minute volume Tidal volume x respiratory rate (BPM)

19. 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

20. 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.

21. Circuits

22. 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

23. Rebreathing circuits Exhaled gases are collected and the exhaled CO2 is removed Sodalime Flow rate is approximately 5-10mls/kg/min

24. Advantage of re-breathing circuits Low flow rate Cheaper to run Lower theatre contamination Less anaesthetic agent requirements.

25. Disadvantages of re-breathing circuits Respiratory tract irritation Heavy Need to check soda lime Heat produced Higher resistance.

26. De-nitrogenation Patients expire considerable volumes of nitrogen Lowers circuit oxygen levels Use high flow rates for the first 10-15 mins of anaesthesia or Empty the reservoir bag every 3 minutes for the first 15 minutes.

27. 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

28. 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.

29. Soda lime Baralyme Soda lime 94% calcium hydroxide 5% calcium hydroxide 1% potassium hydroxide Water pH sensitive dye

30. Soda lime Two types White to purple Pink to white Exhausted after about 8 hours Colour change will disappear!

31. Non-rebreathing circuits Rely on adequate gas flow rate remove carbon dioxide from the circuit avoid re-breathing of expired gas.

32. Ayres T�piece Jackson-Rees modification Circuit factor 2.5-3 x minute volume

33. 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

34. Magill Circuit factor 1-1.5 x minute volume

35. 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

36. 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

37. Lack Circuit factor 1-1.5 x minute volume

38. 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

39. Mini Lack Smaller version of standard Alternative to Ayres Bodyweight range 1-10kg Circuit factor 1-1.5 x minute volume

40. Humphrey ADE Has three different modes Semi-closed system for under 7-10kg Recycling for over 7-10kgs Ability to connect a ventilator

42. Anaesthetic machine check Open the oxygen cylinder valve Slowly anticlockwise

43. Anaesthetic machine check Check the registered quantity of oxygen on the pressure dial

44. Anaesthetic machine check Turn on the oxygen flow meter control to check smooth function Turn it off after this

45. Anaesthetic machine check Press the emergency oxygen flush button

46. Anaesthetic machine check Attach the �in use� label to oxygen cylinder

47. Anaesthetic machine check Check the vaporiser percentage dial to ensure it turns easily

48. Anaesthetic machine check Check the level of the volatile agent in the vaporiser