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E-lfh Basic Sciences Pharmacology > Nitrous Oxide and Xenon > Flashcards

Nitrous Oxide and Xenon Flashcards

1
Q

How is N2O produced?

A

By heating ammonium nitrate (NH4NO3) to 250 degrees C

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2
Q

What else is produced when you heat ammonium nitrate?

A
  • water
  • nitric oxide NO
  • nitrogen dioxide NO2
  • NH3 ammonia
  • N2 nitrogen
  • HNO3 nitric acid
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3
Q

How is N2O contamination minimized?

A

Careful temperature control.

The toxic impurities are removed by cooling and passing the raw products through alkaline gas washes and acidic washes (to remove NO and NH3).

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4
Q

Where is N2O stored and in what medium?

A

Stored in French blue cylinders as a liquid.

The vapour phase exists above the liquid at a pressure of 52 bar at 20 degrees C.

As the N2O vapour is used, liquid N2O will become a vapour so the gauge pressure remains constant. When there is no more N2O liquid - the gauge pressure falls.

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5
Q

What is the filling ratio?

A

It’s the ratio of the mass of liquid in the cylinder compared with the mass of water that the cylinder could hold.

Normally 0.75 in temperate regions.

0.67 in tropics.

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6
Q

Why is the filling ratio lower in the tropics?

A

Because if the filling ratio is 0.75 and the cyclinder reaches >36.5 degrees then the N2O would all convert to gas and exceed the pressure the cylinder can cope with = explosion.

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7
Q

What is the MAC of N2O?

A

103%

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8
Q

What is the B:G of N2O?

A

0.47

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9
Q

What is the critical temperature of N2O?

A

36.5 degrees C

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10
Q

What is the saturated vapour pressure of N2O?

A

5200 kPa

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11
Q

Is it possible to produce anaesthesia with N2O alone?

A

Yes. MAC is 103% but at atmospheric pressure - so if you had a hyperbaric chamber you could have 103kPa of N2O with additional O2.

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12
Q

What is the definition of the blood:gas solubility coefficient?

A

The ratio of the amount of anaesthetic in blood and gas when the 2 phases are of equal volume, pressure and at equilibrium at 37 degrees.

N2O B:G = 0.47 (so roughly half exists in blood compared to gas phase)

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13
Q

What is the critical temperature?

A

The temperature above which it’s not possible to return a substance to liquid no matter what pressure is applied.

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14
Q

What is the saturated vapour pressure?

A

The pressure that exists above it’s liquid phase at equilibrium and is dependent on temperature, but independent of other gases present.

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15
Q

What is the boiling point of N2O?

A
  • 88 degrees C

Above this N2O is a vapour and with sufficient pressure can be “squashed” into a liquid. But once the critical temp is reached (36.5) liquid state is impossible.

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16
Q

What is the MOA of nitrous oxide?

A

Inhibits the NMDA receptor within the CNS.

Effects are analgesia and sedation.

17
Q

What are the unwanted effects of N2O?

A
  • inhibits VitB12
    • oxidises the cobalt ion in the centre of B12
    • in this state it can’t act as a cofactor for methionine synthetase
    • reduced synthesis of methionine and therefore DNA
    • results in megaloblastic changes in bone barrow after a few hrs N2O exposure
  • increases cerebral blood flow (so avoid in neurosurgery)
  • CVS depression (mild but worse in heart failure pts)
  • small decrease in tidal volume which is compensated for by increased RR
18
Q

What is entonox? What is the pseudocritical temperature?

A

A 50/50 mix of O2 and N2O.

It’s the temperature below which Entonox separates back into N2O and O2 which is pressure dependent.

19
Q

What pressure is Entonox stored at?

A

137 bar in cyclinders.

4.1 bar in pipes.

20
Q

If a cyclinder of Entonox were to fall below it’s pseudocritical temperature, what would the gas withdrawn be made of?

A

More O2 than N2O, because N2O would become liquid while O2 would remain a gas. Eventually the mix would become hypoxic as all the O2 is used up.

21
Q

What is the concentration effect?

A

The disproportionate rate of rise of FA/FI when high concentrations of N2O are used compared with when low concentrations are used.

Relates only to N2O because it’s the only anaesthetic agent used in sufficiently high concentrations.

N2O is relatively insoluble but large amounts are absorbed into the pulmonary capillaries. The main driving force is the large concentration gradient generated by high concs of N2O.

22
Q

What are the assumptions of the model explaining the concentration effect?

What are the limitations?

A

50% of N2O is absorbed.

Lung volume remains constant.

Limitations -

  • gas exchange not complete (50% of augmented N2O is not reabsorbed)
  • re-circulating anaesthetic not included
  • FRC not included
23
Q

What is the second gas effect?

A

The FA/FI for other inhaled gases rises more rapidly in the presence of N2O - applies to O2 and also volatiles.

24
Q

What is diffusion hypoxia?

A

At the end of an anaesthetic with N2O/volatile/O2 if the inhaled mix is substituted for air.

2 key drivers are:

  • large concentration gradient that exists for N2O between the pulmonary capillaries and the alveoli
  • relative insolubility of N2 compared to N2O

Therefore - you get rapid movement of N2O into the alveoli, but a smaller volume of N2 enters the pulmonary capillaries. So the gases in the alveoli becomes diluted = hypoxia.

25
Q

What is Xenon?

A

Xe is a noble gas, naturally occuring has 7 stable isotopes and 2 radioactive isotopes with atomic weights ranging from 124-136.

26
Q

How is Xenon made?

A

Fractional distillation of liquid air, a by-product of O2 manufacture.

It costs 2000x that of N2O but has no greenhouse gas effects.

27
Q

How does Xenon compare with N2O in: smell, colour, combustion, boiling point and critical temperature?

A

Xenon also has no smell and is colourless.

However, it is non-flammable whilst N2O is flammable.

Boiling point is -108.1 degrees (N2O is - 88C)

Critical temp is 16.6C (N2O is 36.5C)

28
Q

What is the B:G coefficient for Xenon?

A

0.114 (faster onset than N2O)

29
Q

What is the MAC for Xenon?

A

70%

30
Q

What is the MOA of Xenon?

A

Inhibits glutamate at the NMDA receptor to produce anaesthesia and also analgesia.

It also blocks the 5HT3 receptor, although hasn’t been shown to have an effect on PONV.

31
Q

What CVS effects does Xenon have?

A

No effect on CO or SVR.

It can be vagotonic, unlike N2O.

It has ischaemic pre-conditioning effects on the heart, unlike N2O.

32
Q

What CNS effects does Xenon have?

A

It’s neuroprotective.

33
Q

What are the benefits of Xenon?

A

No toxic, allergic or mutagenic effects

34
Q
A

E-lfh Basic Sciences Pharmacology (3 decks)

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