Breathing Systems

Question 1

What is IPPV?

Answer 1

Intermittent Positive pressure ventilation

Question 2

Dead Space?

Answer 2

Volume of gas that does not eliminate carbon dioxide

Question 3

Tidal Volume?

Answer 3

Volume of gas entering the lung with each inspiration

Question 4

Minute volume?

Answer 4

Volume of gas entering the lungs in each minute

Question 5

Metabolic oxygen requirements?

Answer 5

Amount of oxygen required each minute for metabolic processes

Question 6

Rebreathing?

Answer 6

Rebreathing occurs when the inpired gas(es) reaching the alveoli contain more carbon dioxide than can be accounted for by mere re-inhalation from the patient deap space gas (should be negligible).

Question 7

Functions of a breathing system?

Answer 7

-Provide oxygen +/- anaethetic agent
-Enable IPPV or spontaneous ventilation
-Enable scavenging of expired gases: remove carbon dioxide (Adrenaline release, Tachycardia, Tachypnoea), remove waste anaethetic gases.

Question 8

Reservoir Bag and APL Valve?

Answer 8

-Bag= 3-6 x tidal volume: Common sizes; 0.5, 1, 2, and 3 Litre. Larger ones are available.
-Perishes over time.
APL valve attaches to the scavenging system (should be open!).

Question 9

Breathing system tubing?

Answer 9

• 2x radius= 16x less resistance.
  -2x length= 2x resistance
  -Coaxial vs parallel.

Question 10

Breathing System Tubing: Coaxial and Parallel Configurations?

Answer 10

-Parallel configurations may increase drag (pull on the endotracheal tube) but generally have less resistance.
-Beware of inner hose disconnection with coaxial systems
-Warming of inspired air theoretically possible with coaxial configuration though unlikely to have a clinical impact.

Question 11

Soda Lime-Circle?

Answer 11

-Soda lime absorbs carbon dioxide: exothermic reaction.
-94% Ca(OH)2 and 5% NaOH, silica, and dye: water and heat are produced when soda lime reacts with carbon dioxide. The dye changes colour when the soda lime is exhausted.
-One kg of soda lime absorbs more than 120L of carbon dioxide.
-Caustic (wear gloves when changing it!!).

Question 12

Types of breathing system?

Answer 12

-Non-rebreathing Breathing systems: T-piece, Bain, and Magill (Magill rarely used). Fresh gas flow removes expired carbon dioxide.
-Rebreathing systems: Soda lime removes expired carbon dioxide. Circle (nad To and Fro though this is rarely used).

Question 13

Non-Rebreathing?

Answer 13

-Higher fresh gas flow: increased pollution risk, heat and moisture lost, and more expensive to run.
-The inspired agent should be the same as that on the vaporiser.
-Low resistance and lightweight.
-Some suitable for IPPV: T-piece and Bain.
-Cheap to purchase.
-Minute volume: Tidal volume x respiratory rate.
-Tidal Volume: use 10 mL/kg as a starting point
-Fresh gas flow (mL/kg/min)= minute volume x circuit factor
(Circuit factor is multiple of minute volume, T-piece and Bain 2-3 and Lack 1).

Question 14

Rebreathing?

Answer 14

-Lower fresh gas flow: lower pollution risk, heat and moisture retained (soda lime), and less expensive to run.
-Slow changes in inspired anesthetic agent concentration.
-Can be used for ventilation.
-Higher resistance.
-Can be used for ventilation (IPPV).
-Minimum FGF: metabolic oxygen consumption (large animals 5mL/kg, small animals 10mL/kg): the closer the flow is to these values, the lower the margin for error. Accurate flow meters and a vaporiser are required at low flows.
-Semi-closed GFG: >metabolic oxygen consumption and valve is open to allow excess gas to pass into the scavenging system
-In practice, we use a high FGF initially and then reduce to 1L/min in small animals up to 100kg.

Question 15

Ayres T-pice with Jackson Rees Modification?

Answer 15

• Greater than or equal to 10kg, preferably less than 7.5kg
  -FGF: minute of volume x 2-3
  -Can be used for IPPV
  -Low resistance and dead space.
  -Modest drag due to 2 tubes
  -Difficult to scavenge: a version with an APL valve is available.

Question 16

Bain?

Answer 16

• > 8-10kg with valve, smaller bain without a valve may be suitable for small animals.
  -FGF: min volume x2-3
  -Can be used for IPPV
  -Low drag and dead space
  -Easy to scavenge

Question 17

Lack?

Answer 17

-Patients >10kg (minimum versions available)
-FGF: min volume x 0.8-1 (remember 1)!
-Not suitable for prolonged IPPV
-Moderate drag, resistance, and dead space.
-Valve position facilitates scavenging and operation

Question 18

Circle?

Answer 18

Variety of Sizes: Human adult circles patients >20kg. Small animal veterinary systems patients >15kg. Even smaller ones are available for cats and small dogs with paediatric tubing. Large animal versions for horses, cattle, etc.
-Unidirectional valves, sode lime canister, and APL valve: resistance.
-FGF set at more than metabolic oxygen requirement: 1L/minute oxygen adequate for most small animals up to 100kg. Horses and cattle (0.5-1L oxygen per 100kg).

Question 19

How do you select a breathing system for a clinical case?

Answer 19

-Size of animal: resistance, dead space, economy.
-Valve position and IPPV requirements
-Ease of scavenging
-Cleaning and sterilisation
-Whether you’re using nitrous oxide.
-Heat and moisture retention.

Question 20

APL Valve accidentally left closed?

Answer 20

-Reservoir bag distends
-Reduced thoracic movements.
-Possibly leaking rough ET tube cuff
-Tachycardia, hypoxia
-Potential for pneumothorax/ pneumomediastinum: rupture of lung tissue or trachea.
-Potentially fatal.

Question 21

How do I recognise excessive resistance in the breathing system?

Answer 21

-Altered respiratory rate: low or occasionally fast
-Decreased tidal volume
-Hypoventilation and hypercapnia: increased end-tidal carbon dioxide
-Hypoxia
-Reduced alveolar ventilation may lead to “light” plane anaesthesia.
-Altered respiratory pattern: paradoxical ventilation, increased effort.
-Unpredictable! Remember that weight ranges for breathing systems are only guidelines, and an animal may not be able to cope with the resistance in a system despite being in the “correct” weight range.

Question 22

Apparatus Dead Space?

Answer 22

-May be an integral part of the breathing system or an excessively long endotracheal tube protruding from an animals mouth
-In small animals, increasing the dead space to tidal volume ratio: increased PaCO2. Increased the work of breathing as minute volume needs to increase to maintain PaCO2 at normal levels.

Question 23

Magill- non-rebreathing system?

Answer 23

-Patients 5+kg
-FGF: minute volume x1
-Increased drag due to position of valve
-Moderate dead space and resistance.
-NOt for prolonged IPPV

Question 24

Humphrey ADE system?

Answer 24

-Hybrid system: Can be used with or without a soda lime canister. Functions like a lack, t-piece, bain, or circle. Coaxial and parallel versions exist.
-Can be used for spontaneous and positive pressure ventilation.
-Efficient system: Charts for FGF come with the system, though not fully evaluated in veterinary species.
-Expensive to purchase.

Question 25

To and Fro Rebreathing system?

Answer 25

-Size variable: small animal and large animal versions available. -FGF is similar to a circle. - Bronchiolitis if soda lime is inhaled. -Drag as heavy - Scavenging can be challenging -Very uncommon.

Question 26

Define the term Tidal Volume?

Answer 26

Volume of gas entering the lung with each inspiration

Question 27

What effect increasing apparatus dead space have?

Answer 27

Increase PaCO2

Question 28

You're anaesthetising a 1kg kitten and anticipate needing to ventilate the lungs throughout the procedure. What would be the most appropriate breathing system to use?

Answer 28

Ayres T-piece

Question 29

You're anaesthetizing a 2kg rabbit which you anticipate will be breathing spontaneously through. What is the most economical breathing system to use that is suitable for this animal?

Answer 29

Mini-Lack

Question 30

You're anaesthetizing a 10kg and are using a Lack breathing system. The dog is hypoventilating, so you decide to start intermittent positive pressure ventilation (IPPV). Can you continue using the same breathing system?

Answer 30

No.

Question 31

What is the approximate tidal volume of a 20kg animal?

Answer 31

200mL

Question 32

What is the approximate minute volume of a 5kg cat?

Answer 32

1L/minute

Question 33

What is an appropriate fresh gas flow for a LAck when used for a 10kg dog?

Answer 33

2L/minute

Question 34

What fresh gas flow is required for an Ayres T-Piece when used for anaesthesia of a 5kg cat?

Answer 34

3L/minute