Circle Breathing System

Question 1

What are the two disadvantages of high gas flow systems?

Answer 1

1. wasteful of anaesthetic gases, venting of expired gas.
2. compressed gases are dry with no water vapour. In order to humidify the gases, the respiratory tract dries out + heat loss due to evaporation.

Question 2

Disadvantages of low gas flow?

Answer 2

1. Re-breathing with possible CO2 retention.

2. More difficult to predict concentration of the gases present with re-breathing.

Question 3

What is considered low flow of gas?

Answer 3

When gas flow is less than minute ventilation, usually below 1L/min.

Question 4

Most common breathing system in the UK?

Answer 4

Circle breathing system.

Question 5

Advantages of using circle breathing system?

Answer 5

1. Recycling, and economical use of anaesthetic gases.
2. Expired, rebreathed gases are warmed and humidified, less heat loss.
3. Characterstics are similar for both spont and controlled ventilation.

Question 6

Potential disadvantages of the circle breathing system?

Answer 6

1. The rebreathing of certain breakdown products of anaesthetic agents
2. Potential formation of carbon monoxide
3. Complex arrangement with more potential for errors
4. Some components are difficult to clean
5. Gas monitoring is required, as the inspired gas concentration becomes difficult to predict
6. Gas leaks in the system are difficult to compensate for

Question 7

What faults can occur with the unidirectional valve and what will result from this?

Answer 7

Usually due to condensation of water vapour, valve can be stuck in one of two positions:

1. Closed valve - obstruction to breathing resulting in high resistance.
2. Open valve - increase in dead space from gas mixing.

Question 8

In a low flow closed system,, what fresh gas flow requirements must be met?

Answer 8

must meet oxygen requirements of the patient, approx 200-250mls/min, no leaks and any analysed gas returned to patient.

Question 9

Why is low flow anaesthesia not started at induction?

Answer 9

high flow - rapid onset of anaesthesia, denitrogenation of the system, and increased patient uptake of volatile agents.

Question 10

What is denitrogenation of the system?

Answer 10

Using high flow air to flush out the nitrogen in the circuit at the beginning.

Question 11

Components of soda lime?

Answer 11

94% calcium hydroxide, soddium hydroxide, silinca and zeolite added to maintain a higher pH for longer.

Older preparations use potassium hydroxide as a catalyst.

Question 12

How much CO2 can be absorbed by 100g soda lime?

Answer 12

25L CO2.

Question 13

How much water is produced for one more of CO2 absorbed?

Answer 13

One mole of water produced.

Question 14

What is the pH of soda lime?

At what pH is soda lime exhausted?

Answer 14

Soda limes pH is 13.5.

pH < 10 exhausted.

Question 15

Which anaesthetic gases react with soda lime?

Answer 15

Anaesthetic gases with CHF2 side chain can react with soda lime to form CARBON MONOXIDE.

Desflourane greatest effect, followed by enflurane, then isoflurane.

Question 16

When is production of carbon monoxide in a circuit significant?

How can it be prevented?

Answer 16

Only significant when water content in the soda lime is < 1.5%.

Prevented by the use of small adsorber canisters that are change regularly, and disconnecting unused gas pipelines.

Question 17

What substances interact to form ‘Compound A’?

Answer 17

Soda lime degrades Sevoflurane partly to form a vinyl ether called soda lime

(nephrotoxic in rats not humuans)

Question 18

What factors increase production of Compound A? (3)

Answer 18

1. increasing temperature
2. high sevoflurane concentrations
3. Low FGF

Question 19

What type of soda lime can be used to prevent formation of Compound A?

Answer 19

Non-caustic - no KOH and very low NaOH, no Compound A production.

Question 20

What type of vaporizers are used in the circle system?

Answer 20

1. VOC- vaporizer outside circle. High resistance to breathing, overcome by the pressure of fresh gas flow.
2. VIC- vaporizer inside circle. Uses patients respiratory flow to pick up vapour; needs low resistance to breathing ie. Oxford miniature vapouriser.

Question 21

Two most likely sources of resistance to breathing in circle system?

Answer 21

1. soda lime

2. unidirectional valves.

Question 22

What is the water canister system?

Answer 22

an alternative breathing system, non-circle Mapleson C circuit that carbon dioxide can be absorbed.

Question 23

In what condition is CO2 absorption unnecessary in low flow systems?

Answer 23

when FGF exceeds minute ventilation.

Question 24

Is FGF more or less than alveolar ventilation in a low flow system?

Answer 24

FGF is less than alveolar ventilation.