Circle Breathing System Flashcards

1
Q

What are the two disadvantages of high gas flow systems?

A
  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.
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2
Q

Disadvantages of low gas flow?

A
  1. Re-breathing with possible CO2 retention.

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

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3
Q

What is considered low flow of gas?

A

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

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4
Q

Most common breathing system in the UK?

A

Circle breathing system.

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5
Q

Advantages of using circle breathing system?

A
  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.
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6
Q

Potential disadvantages of the circle breathing system?

A
  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
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7
Q

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

A

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.
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8
Q

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

A

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

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9
Q

Why is low flow anaesthesia not started at induction?

A

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

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10
Q

What is denitrogenation of the system?

A

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

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11
Q

Components of soda lime?

A

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

Older preparations use potassium hydroxide as a catalyst.

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12
Q

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

A

25L CO2.

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13
Q

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

A

One mole of water produced.

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14
Q

What is the pH of soda lime?

At what pH is soda lime exhausted?

A

Soda limes pH is 13.5.

pH < 10 exhausted.

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15
Q

Which anaesthetic gases react with soda lime?

A

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

Desflourane greatest effect, followed by enflurane, then isoflurane.

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16
Q

When is production of carbon monoxide in a circuit significant?

How can it be prevented?

A

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.

17
Q

What substances interact to form ‘Compound A’?

A

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

(nephrotoxic in rats not humuans)

18
Q

What factors increase production of Compound A? (3)

A
  1. increasing temperature
  2. high sevoflurane concentrations
  3. Low FGF
19
Q

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

A

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

20
Q

What type of vaporizers are used in the circle system?

A
  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.
21
Q

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

A
  1. soda lime

2. unidirectional valves.

22
Q

What is the water canister system?

A

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

23
Q

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

A

when FGF exceeds minute ventilation.

24
Q

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

A

FGF is less than alveolar ventilation.