Breathing Systems

Question 1

Define dead space.

Answer 1

Dead space = volume of gas that does not eliminate carbon dioxide

Question 2

Define tidal volume.

Answer 2

Tidal volume = volume of gas entering the lungs with each inspiration

Question 3

Define minute volume.

Answer 3

Minute volume = volume of gas entering the lungs in each minute

Question 4

Define rebreathing.

Answer 4

Rebreathing occurs when the inspired gases reaching the alveoli contain more CO2 than can be accounted for by mere re-inhalation from the patient’s dead space gas

Question 5

How big should a reservoir bag be?

Answer 5

3-6x tidal volume

Common sizes = 0.5, 1, 2 and 3 Litre

Question 6

How do radius and length of breathing system tubing affect resistance?

Answer 6

2x radius = 16x less resistance

2x length = 2x resistance

Question 7

What are the two configurations of breathing system tubing?

Answer 7

Parallel and coaxial.

Question 8

What extra feature does a circle system have and what does it do?

Answer 8

Soda lime - absorbs carbon dioxide and produces water and heat.

Question 9

Give some examples of non-rebreathing systems.

Answer 9

T-piece
Bain
Lack

Question 10

Give an example of a rebreathing system.

Answer 10

Circle

Question 11

What are the advantages/disadvantages of a non-rebreathing system?

Answer 11

A = inspired agent same as on vaporiser, low resistance, lightweight, suitable for IPPV, cheap to purchase

D = higher fresh gas flow so increased pollution risk / lost heat and moisture / more expensive to run

Question 12

What are the advantages/disadvantages of rebreathing systems?

Answer 12

A = lower fresh gas flow so lower pollution risk / retained heat and moisture / less expensive to run, can be used for IPPV

D = slow changes in inspired anaesthetic agent concentration, higher resistance

Question 13

What are the calculations for a non-rebreathing system?

Answer 13

Minute volume = tidal volume x resp rate (~200ml/kg/min)
Tidal volume = use 10ml/kg
Fresh gas flow (ml/kg/min) = minute volume x circuit factor
Circuit factors = multiples of minute volume (T-piece/bain = 2-3, lack = 1)

Question 14

Describe the T-piece.

Answer 14

<10kg, preferably less than 7.5kg
FGF = minute volume x2-3
Can be used for IPPV
Low resistance/dead space
Modest drag due to 2 tubes
Can be difficult to scavenge

Question 15

Describe the Bain.

Answer 15

>8-10kg with valve
FGF = minute volume x2-3
Can be used for IPPV
Low drag and dead space
Easy to scavenge

Question 16

Describe the Lack.

Answer 16

Standard lack suitable for more than 10kg
Mini-lack suitable for more than 1kg
FGF = minute volume x0.8-1
Not suitable for prolonged IPPV
Moderate drag/resistance/dead space

Question 17

Describe the Circle.

Answer 17

Variety of sizes
Unidirectional valves, soda lime canister and APL valve - resistance
FGF set at more than metabolic oxygen requirement

Question 18

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

Answer 18

Size of animal - resistance, dead space, economy
Valve position and IPPV requirement
Ease of scavenging
Cleaning and sterilisation
Use of nitrous oxide
Heat and moisture retention

Question 19

What can go wrong with a breathing system?

Answer 19

APL valve left closed
Excessive resistance
Apparatus dead space

Question 20

What happens if the APL valve is accidentally left closed?

Answer 20

Reservoir bag distends
Reduced thoracic movements
Possibly leaking round ET tube cuff
Tachycardia, hypoxia
Potential for rupture of lung tissue/trachea

Question 21

How can we recognise excessive resistance in the breathing system?

Answer 21

Altered resp rate (low or occasionally fast)
Decreased tidal volume
Hypoventilation/hypercapnia
Hypoxia
Reduced alveolar ventilation - light plane of anaesthesia
Altered resp pattern

Question 22

What happens if there is excessive apparatus dead space?

Answer 22

From breathing system or excessively long ET tube
Increasing dead space to tidal volume ratio increase PaCO2
Increased work of breathing as minute volume needs to increase to maintain PaCO2 at normal levels