Breathing and Scavenger Systems

Question 1

What is the function of an anaesthetic breathing system

Answer 1

TO allow normal respiratory exchange whilst permitting controlled delivery of our anaesthetic gases.

Question 2

What are the prime requirements for the anaesthetic breathing system

Answer 2

1. Predictable delivery AA and O2
2. Removal CO2
3. Monitor / control Ve
4. Economy of gases and anaesthetic vapours
5. Simplicity and convenience
6. Ease of cleaning
7. Humidity and thermal control
8. Minimise theatre pollution
9. Cheap
10. Lightweight and compact

Question 3

What is an APL valve

Answer 3

Adjustable Pressure Limiting Valve is a valve which allows exhaled gases excess fresh gas flow to leave the breathing system

One-way, adjustable, spring loaded valve. Spring is used to adjust the pressure required to open the valve

Question 4

What errors can occur with the APL valve

Answer 4

1. Malfunction scavnging system –> keep APL valve open –> excessive deadspace in breathing system
2. Closed APL during spontaneous ventilation (Pressure relief at 60cmH20 present in most designs)
3. Water vapour –> sticking valve (Hydrophobi material used)

Question 5

What is a reservoir bag?

Answer 5

Ellipsoidal shaped antistatic rubber or latex bag that acts as a reservoir for gas in multiple different types of breathing circuits

Question 6

What are the functions of the reservoir bag?

Answer 6

1. Accommodate FGF during expiration acting as a reservoir available for the following inspiration.
2. Monitor of ventilatory pattern during spontaneous breathing (Inaccurate guide to Vt)
3. Assisted / controlled ventilation
4. Limits pressure build up in the breathing system (LaPlace: P = 2t/r)

Question 7

What are the standard adult and paediatric sizes of reservoir bags and what is the range of sizes available. What are the benefits of smaller and also larger reservoir bags

Answer 7

Standard adult: 2 L
Paediatric: 0.5 Litres

Range: 0.5 L –> 6L

Larger bags assist with inhalational induction in adults

Smaller bags allow for more accurate observation of Vt in paediatrics and lower mechanical dead space.

Question 8

What is the Mapelson classification of breathing systems –> include modified versions and eponyms. Include the Advantages A and disadvantages D of each system

Answer 8

Mapelson A: FGF | RB | T | APL | Patient
1. Magil
2. Lack's (Coaxial and parallel)
A: FGF = Ve for spontaneous ventilation
D: FGF = 3Ve for controlled ventilation

Mapelson B: RB | T | FGF | APL | Patient
Obsolete circuit

Mapelson C: RB | FGF | APL | Patient
1. Waters To and Fro
A: FGF = 1.5 - 2Ve for controlled ventilation
D: FGF = 3Ve for spontaneous ventilation

Mapelson D: RB | APL | T | FGF | Patient
1. Bain’s (coaxial)
A: FGF = Ve for controlled ventilation
D: FGF = 2 Ve for spontaneous ventilation

Mapelson E: T | FGF | Patient
1. Ayres’ T piece
A: Valveless, minimal dead space, low pressure, paediatric (<30kg), assisted / controlled ventilation
D: FGF = 3 Ve / Scavenging (No APL) / No CPAP

Mapelson F: Open RB | T | FGF | Patient
1. Jackson Rees
A: Valveless, minimal dead space, low pressure, paediatric (<30kg), bag assisted / controlled ventilation / CPAP
D: FGF = 3 Ve / Scavenging (No APL) /

Question 9

Summarise the components of a Hmphrey ADE breathing system

Answer 9

1. Parallel (and coaxial) 2 x 15 mm (id) smooth (non-corrugated) tubing: 1) FGF and 2) exhaled gas. Distal end to patient via y-connector and proximal end to Humphrey block.
2. Humphrey block
   a) APL valve with visible indicator
   b) 2 L reservoir bag
   c) Lever: controlled / spontaneous
   d) ventilator port
   e) safety pressure relief valve (60cmH2O)
   f) New design has CO2 cannister

Question 10

Describe the mechanism of action of the Humphrey ADE breathing system

Answer 10

With lever in spontaneous mode –> Forms Mapelson A (Magill circuit)

With lever in controlled mode –> Forms Mapelson E circuit

Question 11

How does smooth tubing and narrower diameter of the Humphrey ADE circuit affect spontaneous and controlled ventilation

Answer 11

Smooth tubing 15 mm vs corrugated 22mm tubing
–> Laminar flow more likely –> possibly lower resistance to flow.

Controlled ventilation
- low Vt possible during controlled ventilation

Spontaneous ventilation
- Less energy is needed to overcome inertia of gases during spontaneous ventilation

Question 12

What FGF are required during controlled and spontaneous ventilation using the Humphrey ADE system

Answer 12

Spont FGF = Ve. Kids < 25 kg –> FGF should = 3L/min

Controlled FGF = Ve Kids < 25 kg –> FGF should = 3L/min

Question 13

What is soda lime

Answer 13

Calcium Hydroxide Ca(OH)2 ——> 94%
Sodium Hydroxide NaOH ——> 5%
Potassium Hydroxide KOH ——> < 1%

Silica is added to prevent disintegration of granules into powder

Dye: Ethyl Violet

Question 14

Name 5 dyes used in soda lime and indicate their colour change when exhausted with CO2

Answer 14

Ethyl violet: White –> Purple

Phenolphthalein: White –> Pink

Clayton Yellow: Red ——> Yellow

Ethyl Orange: Orange —-> Yellow

Mimosa 2: Red ——–> White