17. Nitrous Oxide Flashcards
How is nitrous oxide produced?
Ammonium nitrate
is heated to 250 °C
causing it to decompose:
NH4NO3 ————> N2O + 2H2O
If temperature is
not controlled carefully
during production,
contaminants may accumulate
in the gas
e.g. NH3, N2, NO, NO2 and HNO3).
Any impurities are removed prior to storage.
How is nitrous oxide stored?
> N2O is stored in cylinders coloured ‘French blue’.
> I t is stored as a liquid, below its critical temperature (36.5 °C)
List the physicochemical properties of N2O.
Boiling point
Critical temperature
Crit Pressure
BlGas
Oil.Gas
Mac
> Boiling point −88 °C
> Critical temperature 36.5 °C
> Critical pressure 72 bar
> Blood/gas partition coefficient 0.47
> Oil/gas partition coefficient 1.4
> MAC 105%
(only under hyperbaric conditions can N2O produce full anaesthesia)
List its pharmacodynamic properties.
Cardiovascular:
> Reduces myocardial contractility
but increases sympathetic outflow,
resulting in a minimal change in blood pressure.
> Increases pulmonary vascular resistance
and should be avoided in patients
with known pulmonary hypertension.
Respiratory:
> Causes a reduction in tidal volume and an increase in respiratory rate, resulting in maintenance of minute ventilation.
> Blunts the ventilatory responses to
both hypoxia and hypercarbia.
Neurological:
> Increases cerebral blood flow,
cerebral metabolic requirement for oxygen
and intracranial pressure.
> Effects are more pronounced in patients
with loss of cerebral autoregulation,
e.g. traumatic brain injury.
How is N2O used clinically?
Anaesthesia
General anaesthesia:
1
> As a carrier gas.
2
> To reduce the amount of volatile agent
used because of its MAC-sparing effect
(e.g. 0.5 MAC N2O + 0.5 MAC sevoflurane = 1 MAC).
This is due to its inhibitory action at NMDA (glutamate) receptors and agonist activity at dopamine receptors.
> To give faster onset of inhalational anaesthesia using the ‘concentration effect’ and the ‘second gas effect’.
N2O is 30 times more soluble in blood than nitrogen and therefore diffuses more rapidly across the alveolar membrane into the blood than nitrogen can diffuse out into the alveoli.
This results in reduced
alveolar volume
and a rise in alveolar partial pressure
and concentration of the remaining gases.
This is the ‘concentration effect’.
Because the concentration gradient
of volatile agent between
alveoli and blood is now increased,
there is faster diffusion of volatile agents
into the blood and therefore
faster onset of anaesthesia.
This is the ‘second gas effect’.
Analgesia:
.
> N2O is a good analgesic, exhibiting agonist activity at opioid receptors and acting at a spinal cord level through modulation of descending noradrenergic pain pathways.
> It is mixed with oxygen to form
Entonox (50% O2 and 50% N2O),
which is used for pain relief,
predominantly during labour
What are its adverse effects?
1
Post-operative nausea and vomiting (PONV):
> The use of N2O during general anaesthesia is associated with an increased incidence of PONV. The exact aetiology is unclear but may involve bowel distension, middle ear or opioid effects.
2. Expansion of nitrogen-containing cavities: > This occurs because N2O is more soluble than nitrogen and so it diffuses from the blood into air-filled cavities more quickly than nitrogen already in the cavity can diffuse back into the blood.
> Use of N2O results in increased
pressure in air-filled spaces,
e.g. middle ear, pneumothoraces,
endotracheal cuffs (during prolonged surgery)
and bowel.
It is therefore contraindicated
in certain types of surgery.
3
Bone marrow toxicity and CNS toxicity:
> N2O oxidises the cobalt ion in the vitamin B12 complex, impairing its ability to act as a co-factor for the enzyme methionine synthase.
> This causes bone marrow suppression and therefore reduced DNA, methionine, thymidine and tetrahydrofolate synthesis.
The result is megaloblastic anaemia and subacute degeneration of the spinal cord (dorsal columns) leading to neuropathy.
4
Teratogenicity:
> Has occurred in rats, but never in humans.
> The exact mechanism is likely to be
multi-factorial, but includes impaired
DNA synthesis and
α1-adrenoceptor agonist activity,
which is associated
with situs inversus.
5 Environmental pollutant: > N2O is a greenhouse gas; however, anaesthetic emissions account for a tiny proportion of total nitrous oxide emissions, especially with low-flow anaesthesia.
Describe some properties
of Entonox.
What is the mixture
How is it stored
What is a full pressure in cylinder
What is the effect
what is this
Pseuocritical temp
What happens below this
whats it called
Whats the problem
How do you avoid this problem
Entonox is the trade name
for the mixture 50:50 N2O:O2.
> It is stored as a gas in
cylinders with a French blue body
and blue and white striped shoulders.
> A full cylinder has a pressure of 137 bar.
> W hen the N2O and O2 dissolve
into each other, the resulting gas takes
on properties unpredictable from its constituents.
This is called the ‘Poynting effect’.
> The pseudocritical temperature
of the mixture is −7 °C.
Below this temperature the N2O
will convert to its liquid phase,
in a process called lamination.
Anyone using this cylinder will receive
only O2,
followed by a
mixture of O2 and N2O,
and finally only the hypoxic N2O.
If Entonox has been stored below its pseudocritical temperature, the cylinder should be warmed and inverted several times to ensure mixing before use.
