What equipment do we need for anaesthesia? Breathing Systems.

Question 1

1. Define dead space.
2. Define tidal volume.
3. Minute volume.
4. Define metabolic oxygen requirements.
5. What is rebreathing?

Answer 1

1. Volume of gas that does not eliminate carbon dioxide.
2. Volume of gas entering the lung w/ each inspiration.
3. Volume of gas entering the lungs in each minute.
4. Amount of O2 required each minute for metabolic processes.
5. Occurs when the inspired gas(es) reaching the alveoli contain more CO2 than can be accounted for by mere re-inhalation from the patient’s dead space gas (should be negligible).

Question 2

Functions of a breathing system.

Answer 2

• Provide O2 +/- anaesthetic agent.
• Enable IPPV (intermittent positive pressure ventilation) or spontaneous ventilation.
• Enable scavenging of expired gases.
  – Remove CO2 to avoid:
  –> adrenaline release.
  –> tachycardia and potentially arrhythmias.
  –> tachypnoea.
  –> Sweating in spp that can sweat.
  –> May have brick-red MMs.
  – Remove waste anaesthetic gases.

Question 3

1. What is the connector for on the Ayres T-Piece?
2. What is situated immediately near the connector for the scavenging tube on the Ayres T-Piece.
3. What is situated between the 2 lengths of tubing on the Ayres T-Piece.

Answer 3

1. To attach the breathing system to the common gas outlet of the GA machine.
2. Adjustable Pressure Limiting (APL) valve.
3. The connector for the ET tube / mask. Also attach a capnography connector here.

Question 4

1. What is an identifying feature of the circle breathing system.
2. What features of the circle are the same as T-Piece?

Answer 4

1. Unidirectional valves and a soda lime canister.
2. Common gas outlet connector, scavenging tube connector, tubing, APL, ET tube / mask connector, reservoir bag.

Question 5

1. What should the reservoir bag be size wise?
2. What are the common sizes of reservoir bags?
3. Why is it important to check reservoir bags regularly?
4. At what position should the APL be at all times during an anaesthetic?

Answer 5

1. 3-6 x tidal volume of animal.
2. 0.5L, 1L, 2L, 3L. (larger ones are available).
3. Perishes over time.
4. Open.

Question 6

1. How much is resistance reduced by when tubing radius is doubled?
2. How much is the resistance increases by when the length of the tubing is doubled?
3. What 2 configurations does tubing come in on breathing systems?
4. Why is it important to think about resistance?

Answer 6

1. 16X.
2. 2X.
3. Coaxial and parallel.
4. Excessive resistance can either result in hypoventilation, so an increase in CO2, or increased work of breathing maintain normal CO2 levels.

Question 7

1. Parallel tubing configurations disadvantage?
2. Parallel tubing configurations advantage?
3. Something to be aware of w/ coaxial tubing configuration?
4. Coaxial tubing configuration advantage?

Answer 7

1. May increase drag (pull on the ET tube).
2. Less resistance due to wider diameter.
3. Inner hose disconnection.
4. Theoretical warming of inspired air though unlikely to have clinical impact as air moves through so rapidly.

Question 8

1. Purpose of soda lime in the canister of a Circle?
2. Constituents of the soda lime in Circle.
3. How can we tell that the soda lime is exhausted and needs changing?

Answer 8

1. Absorbs expired CO2 via reaction that is exothermic and produces moisture.
2. 94% Ca(OH)2 and 5% NaOH, silica and dye.
3. The dye changes colour.

Question 9

1. How much CO2 does soda lime absorb?
2. Precaution when changing out the soda lime?

Answer 9

1. 1kg of soda lime absorbs >120L CO2.
2. Wear gloves and face mask as it is caustic.

Question 10

2 main types of breathing systems?

Answer 10

Non-rebreathing and rebreathing.

Question 11

1. Non-rebreathing types of breathing system?
2. What is a non-rebreathing system?
3. Rebreathing types of breathing systems.
4. What is a rebreathing system?

Answer 11

1. T-Piece, Bain and Lack (Magill rarely used).
2. Where fresh gas flow removes expired CO2.
3. Circle.
4. Where soda lime removes expired CO2.

Question 12

Compare non-rebreathing and rebreathing breathing systems.

Answer 12

• Non-rebreathing requires higher fresh gas flow w/ increased pollution risk, loss of heat and moisture and higher expenses. Rebreathing requires a lower fresh gas flow w/ lower pollution risk, retention of heat and moisture (by soda lime) and lower expenses.
• Non-rebreathing system inspired agent should be same as that on vaporiser, rebreathing system slow changes in inspired anaesthetic agent conc. (consider turning up the O2 flow briefly to get the anaesthetic gas to the patient more rapidly).
• Non-rebreathing is low resistance and lightweight (useful for smaller patients) but rebreathing is higher resistance (impractical for smaller patients).
• Some non-rebreathing suitable for IPPV – T-Piece and Bain. Rebreathing systems can also be used for IPPV.
• Non-rebreathing systems cheap to purchase.

Question 13

1. How do you calculate minute volume?
2. Roughly what is tidal volume expected to be?
3. How do you calculate fresh gas flow?
4. Which rebreathing system is more economical?

Answer 13

1. tidal volume x respiratory rate. – if animal panting or RR unknown, good approximation = 200ml/kg/min.
2. 10ml/kg.
3. minute volume x circuit factor – circuit factors = multiple of minute volume, T-Piece and Bain 2-3, and lack 1.
4. The lack.

Question 14

1. What must the minimum fresh gas flow be equal to for rebreathing systems?
2. Why is it difficult to have the actual minimum fresh gas flow equal to the metabolic oxygen consumption in practice?
3. What is actually done in practice?

Answer 14

1. The metabolic oxygen consumption of the patient.
2. The GA machine does not always have that measurement marked on the oxygen tube as it is too small.
3. Use high fresh gas flow initially and then reduce to 1L/min in SA up to 100kg.

Question 15

1. What weight of patient is the T-Piece used for?
2. Fresh gas flow for T-Piece?
3. Can the T-Piece be used for IPPV?
4. Resistance?
5. Dead space?
6. Drag? – why?
7. Scavenging?

Answer 15

1. Patients <10kg but preferably <7.5kg.
2. Minute volume x 2-3.
3. Yes.
4. Low.
5. Low.
6. Modest – there are 2 tubes.
7. Difficult to scavenge but there is a version w/ an APL valve available.

Question 16

1. What size of animal should a Bain be used for?
2. Fresh gas flow for Bain?
3. IPPV w/ Bain?
4. Drag?
5. Dead space?
6. Scavenging?
7. Main consideration when using a Bain.

Answer 16

1. > 8-10kg w/ valve but a smaller Bain w/o valve may be suitable for smaller animals.
2. Minute volume x 2-3.
3. CAN be used for IPPV.
4. Low.
5. Low.
6. Easy.
7. Inner tube can become disconnected so vital integrity of inner tube must be checked when setting up the breathing system:
   - Connect the breathing system to the GA machine.
   - Occlude whole end of patient connector w/ thumb.
   - Close APL valve.
   - Turn on O2.
   - Reservoir bag should fill w/ O2.
   - Open APL to release pressure w/in system.
   - Then take the connector that comes with the breathing system or the end of a 5ml syringe plunger.
   - Set FGF to 4L/min.
   - Use connector or syringe plunger to occlude inner tube of breathing system only.
   - Will see O2 flow drop and hear hissing as high pressure relief valve opens in the back of the GA machine.
   - Remove plunger or connector. The Flowmeter reading will jump before restoring to 4L/min.

Question 17

1. What weight of patient can you use a standard lack for?
2. FGF for lack?
3. IPPV?
4. Drag?
5. Resistance?
6. Dead space?
7. Scavenging?

Answer 17

1. > 10kg. (mini lacks available and suitable for animals >1kg).
2. Minute volume x 0.8-1 (remember 1).
3. Not suitable for prolonged IPPV.
4. Moderate.
5. Moderate.
6. Moderate.
7. Valve position facilitates scavenging and operation.

Question 18

1. What weight of patient is…
   - Human adult circle use appropriate for?
   - SA veterinary circle “ “?
   – What others are available?
2. Why is there so much resistance in the circle?
3. SA FGF?
4. Horse and cattle FGF?

Answer 18

1. • Human adult circle = >20kg.
     - SA vet circle = >15kg.
     – Smaller ones for cats and small dogs w/ paediatric tubing.
     – Large animal versions for horses, cattle etc.
2. Unidirectional valves, soda lime canister and APL valve.
3. Tend to set it higher than the metabolic O2 consumption of the patient and for most SA patients up to 100kg, can set it to 1L/min, and 500ml/min for SA patients up to 50kg.
4. 0.5-1L per 100kg.

Question 19

1. What factors of the breathing system are impacted by the size and proportion of the patient?
2. What else must we consider when selecting the appropriate breathing system for our patient?

Answer 19

1. Resistance, dead space, economy.
2. Valve position and the patient’s requirements or not for IPPV.
   Ease of scavenging.
   Cleaning and sterilisation.
   Whether you’re using nitrous oxide.
   Heat and moisture retention.

Question 20

Outline the effects of accidentally leaving the APL valve closed.

Answer 20

Distension of reservoir bag.
Reduced thoracic movements due to increased pressure.
Possible leaking around the ET tube cuff.
Tachycardia.
Hypoxia w/ potential drop in spo2.
Potential pneumothorax / pneumomediastinum due to rupture of lung tissue or trachea.
Potential fatality.

Question 21

How to recognise excessive resistance in the breathing system.

Answer 21

Altered RR – low or occasionally fast.
Decreased tidal volume.
Hypoventilation and hypercapnia – increased end-tidal CO2.
Hypoxia.
Altered respiratory pattern – paradoxical ventilation, increased effort.
** weight ranges only guidelines and animals are individuals so unpredictable!

Question 22

1. What can cause apparatus dead space?
2. Effects of increasing dead space to tidal volume ratio in small animals.

Answer 22

1. An integral part of the breathing system or an excessively long ET tube protruding from an animal’s mouth.
2. Increased PaCO2.
   Increased work of breathing as minute volume need to increase to maintain PaCO2 at normal levels.