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Exam prep tutorials (Physics and Equipment) > Gas Supplies, Breathing Systems and Ventilators > Flashcards

Gas Supplies, Breathing Systems and Ventilators Flashcards

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1
Q

What is critical temperature?

A

The temperature above which, a substance cannot be liquefied irrespective of the amount of pressure applied

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2
Q

What is critical pressure?

A

The minimum pressure required to liquefy a gas at it’s critical temperature

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3
Q

What is a gas?

A

A compressible fluid where intermolecular spacing is so large that intermolecular forces are negligible

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4
Q

What is a vapour?

A

A gas below it’s critical temperature, thus compression to the liquid is possible

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5
Q

What is a fixed gas?

A

A gas above it’s critical temperature

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6
Q

What is latent heat?

A

The energy change associated with a change in state of a substance in either direction, with no change in temperature.

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7
Q

How is O2 supplied in hospital?

A
  • One or more VIEs (vacuum insulated evaporators) with a cylinder manifold in back-up.
  • A network of high-integrity copper pipes throughout the hospital
  • supply maintained by hospital estates
  • Chief pharmacist is legally responsible for certifying the quality of the gas supplied
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8
Q

What is the VIE?

A

A double-skinned steel tank with a vacuum of 0.16 - 0.3 kPa existing between skins.

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9
Q

What is the temperature inside the VIE?

A

Between -160 to -180 C.

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10
Q

Why is the temperature in the VIE low?

A

The VIE is below (-160 to -180C) the critical temperature of O2 (-118 C). This means O2 is a liquid inside with a saturated vapour at a pressure of 10 bar above the liquid.

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11
Q

How is O2 taken out of the VIE?

A

Oxygen vapour is drawn from the top of the VIE. Latent heat of evaporation is required to vaporize O2 to re-establish equalibrium.

Supplying latent heat from liquid ensures no external cooling system is needed.

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12
Q

How does low demand/warm weather affect the VIE?

A
  • reduces the cooling effect of the vaporizing process
  • pressure increases
  • safety valve opens at 15bar
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13
Q

How does high demand/cold weather affect the VIE?

A
  • insufficient energy to sustain adequate evaporation to meet demand
  • liquid O2 can be tapped from the bottom of the system and warmed by the environment to restore vapour supply (Pressure-raising vaporizer)
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14
Q

How is the supply pressure of O2 maintained constantly at 4.1 bar?

A

By a system of pressure regulators

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15
Q

What is nitrous oxide inhaled and exhaled as?

A

It is inhaled as a vapour (below it’s critical temperature of 36.4 degrees) and exhaled as a fixed gas (above it’s critical temperature).

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16
Q

What are isotherms?

A

The curves of constant temperature

eg nitrous oxide

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17
Q

What law does the isotherm of a gas above critical temperature follow?

A

Boyle’s Law - the substance is a fixed gas above critical temp so can’t be liquefied by increasing pressure

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18
Q

What does the isotherm of nitrous oxide look like at it’s critical temperature (36.4C) ?

A

Demonstrates a sharp inflection at 73 bar (critical pressure) where it becomes a liquid.

When any vapour is at critical temperature, it’s on a “knife edge” of temperature and pressure. Any tiny change in either will cause liquefaction or transformation to a fixed gas so very little vapour exists.

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19
Q

What is the filling ratio of cylinders?

A

It’s based on WEIGHT not volume. It should be 75% of the weight of water that would completely fill the cylinder (67% in warmer climates).

Differing densities of N2O (1222 kg/m3) and water (1000 kg/m3) so 0.75 filling ratio does not give a cylinder 75% full by volume.

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20
Q

Why can standard Bourdon pressure gauges not be used to measure N2O cylinder content?

A

Because they measure the pressure of the vapour but not the remaining liquid at steady state.

The only true measurement is weight minus tare weight (empty weight of the cylinder).

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21
Q

What would happen to the Bourdon pressure gauge when a nitrous oxide container is used constantly?

A

Due to the cooling effect of the latent heat of vaporization during sustained use, a steady pressure drop will be observed.

When the cylinder is switched off and allowed to re-warm to ambient temperature, equalibrium is restored and a pressure of 53 bar is regained.

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22
Q

What is entonox?

A

A 50/50 mix of O2 and N2O by VOLUME

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23
Q

What is the pseudo-critical temperature of entonox?

A

*MINUS*

- 5.5 degrees C

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24
Q

What form does entonox exist in within a cylinder?

A

Only gaseous form.

25
Q

What pressure is Entonox stored at in cylinders?

A

137 bar

26
Q

What happens to entonox below it’s pseudocritical temperature?

A

The O2 and N2O separate

27
Q

How often are gas cylinders checked?

A

Every 5 years

28
Q

What do the coloured plastic rings around cylinder necks signify?

A

The date of when the cylinder was last checked

29
Q

How much pressures above working pressure can cylinders withstand?

A

65 -70% above working pressure

30
Q

Can molybdenum steel be used in an MRI scanner?

A

No, only specialist composite or aluminium cylinders can be used in MRI scanners

31
Q

Which Mapleson systems are commonly used in paediatric practice?

A

Mapleson E + F

32
Q

Which Mapleson system is often seen in resuscitation or when moving ventilated patients between critical areas?

A

Mapleson C

33
Q

Which Mapleson systems are most typically encountered in routine anaesthetic practice?

A

Mapleson A + D

34
Q

What is the Lack system breathing circuit?

A

The Magill system (mapleson A) with the weight of the APL valve removed from the face mask

It is available as parallel and coaxial (expiratory limb within the inspiratory limb) varients

35
Q

What is the Mapleson D?

A

A T-piece system with the fresh gas next to the patient and a bag and APL valve on the end.

36
Q

What is the Bain system?

A

A co-axial version of the Mapleson D for compact design (inspiratory limb within the expiratory limb)

37
Q

When is the Mapleson A most efficient?

A

Spontaneous respiration

38
Q

What % of minute ventilation is needed if using the Mapleson A?

A

FGF of ~70% of minute ventilation is needed

39
Q

Why is the Mapleson A extremely inefficient for controlled ventilation?

A

Because the APL valve will be partially closed, at the beginning of expiration the reservoir bag tends to be almost empty.

During the expiratory pause, alveolar gas will reach the reservoir bag before APL valve opens, very high FGF is required to vent this gas mix prior to inspiration.

Then, on inspiration, when the bag is squeezed, the positive pressure will open the APL valve and cause FGF and dead space gas are wasted = inefficient.

40
Q

What are the Key Points about the Mapleson A?

A
  • Examples are the Magill and Lack systems
  • tubing is 110-180cm long
  • APL valve at the patient end (In Magill)
  • FGF runs in the outer tube of coaxial variants
  • efficient (low FGF to prevent rebreathing) for spontaneous ventilation
  • requires FGF equal to alveolar ventilation (approx 70mls/kg/min)
  • inefficient for controlled ventilation
  • dead space is too great to use in kids < 30kg
41
Q

When is the Mapleson D most efficient?

A

For controlled ventilation. Very inefficient for spontaneous respiration.

42
Q

Why is Mapleson D inefficient for spontaneous breathing?

A

On expiration, FGF, dead space and alveolar gas pass to the bag.

During the expiratory pause the FGF will purge the alveolar gas out the APL valve (if high enough).

During inspiration the patient will inhale the FGF but if this is not high, extra flow will draw on the bag - which is a mix of dead space and alveolar gas- leading to rebreathing of CO2

43
Q

What are the key points about the Mapleson D?

A
  • the Bain is a co-axial version of the Mapleson D
  • tubing is normally 180cm long, but increasing it’s length does not affect it’s performance
  • APL valve at machine end
  • FGF in the inner tube
  • efficient for controlled ventilation
  • requires FGF equal to alveolar ventilation (70mls/kg/min)
  • inefficient for spontaneous ventilation
44
Q

What are the essential elements to a circle system?

A
  • reservoir bag
  • APL valve (never between patient and inspiratory valve)
  • CO2 absorber cylinder
  • inspiratory and expiratory tubing with unidirectional valves each
  • source of FGF and anaesthetic vapour (never between patient and expiratory valve)
45
Q

What are the advantages of the circle system?

A
  • minimal equipment dead space
  • converves heat and humidity
  • ability to remove CO2 and recycle gas
  • use in a closed circuit configuration to minimize FG consumption
  • reduction in pollution
46
Q

What are the disadvantages of the circle system?

A
  • multiple components to maintain and test
  • high potential for leaks
  • potential for circuit to empty if used in closed configuration with inadequate FGF
47
Q

What is soda lime made from?

A

Calcium hydroxide – 75%

Water – 20%

Sodium hydroxide – 4%

Potassium hydroxide – 1%

Indicator dye

48
Q

What does the reaction in the soda lime do?

A
  • exothermic
  • produces 1 mole of H2O for each mole CO2 removed (humidifies)
  • granules are “4-8 mesh” size (small enough to fit through a 1/4 inch opening but not an 1/8 inch opening)
49
Q

What is the chemical reaction in the soda lime?

A

The chemical reaction in the cannister:

  1. CO2 + H2O → H2CO3 (slow, rate-limiting step)
  2. H2CO3 + NaOH → H2O + NaHCO3
  3. NaHCO3 + Ca(OH)2 → NaOH + CaHCO3 + H2O

(Note NaOH is a catalyst only - it is regenerated in step three)

50
Q

What does fracture of the inner tube in the Bain circuit cause?

A

Massive increase in dead space within the system

51
Q

What does the coaxial version of the Mapleson A have?

A

An inner EXPIRATORY and outer INSPIRATORY tube

52
Q

How much CO2 will 1kg of soda lime absorb?

A

120 L of CO2

53
Q

What is a Cardiff Aldasorber?

A

It’s an adsorber of anaesthetic volatiles onto activated charcoal. It does not employ soda lime or remove CO2.

54
Q

What dangerous gas is soda lime associated with production of?

A

Carbon monoxide

55
Q

How does volume control ventilation work?

A
  • fixed tidal volume is set
  • constant flow generates an increasing pressure throughout inspiration
  • if a high inspiratory flow rate is used - the preset volume is reached quickly
  • an inspiratory pause follows with no flow until the inspiratory time is complete
  • for a given airways resistance, if a higher flow is applied, a higher pressure will result as the lungs require time to expand - resulting in barotrauma
56
Q

What are the advantages/disadv of volume control ventilation?

A
  • Allows optimization of PaCO2, which can be particularly important eg in brain injury.
  • newer ventilators decelerate the flow once a pressure limit is reached resulting in the target tidal volume at a safe pressure
  • unable to compensate for leaks
    • eg using uncuffed ETT in paeds
57
Q

What are the adv/disadvantages of pressure control?

A
  • a decelerating flow generates a tidal volume which is dependent on compliance and resistance of the airways
  • reduces control over PaCO2 but less likely to worsen/cause lung injury
  • more able to compensate for leaks in breathing system
  • decelerating flow pattern also improves gas exchange and homogeneity of ventilation
  • because the pressure is constant throughout the breath - the mean airway pressure (MAwP) is greater than a volume control breath for a given tidal volume - this improves oxygenation
58
Q
A

Exam prep tutorials (Physics and Equipment) (8 decks)

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