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Section IICoredrugs in anaesthetic practiceUnless the temperature is carefully controlled N 2 Omay contain the following
contaminants: NH 3 ,N 2 ,NO,NO 2 and HNO 3. These impurities are actively removed
bypassage through scrubbers, water and caustic soda.Storage
Nitrous oxide is stored as a liquid in French blue cylinders (C = 450 litres up to G =
9000 litres) with a gauge pressure of 51 bar at 20◦C, which therefore bears no cor-
relation to cylinder content until all remaining N 2 Oisinthe gaseous phase. The
filling ratio (mass of N 2 Oincylinder/mass of water that the cylinder could hold) is
0.75 in temperate regions, but it needs to be reduced to 0.67 in tropical regions to
avoid cylinder explosions. Its critical temperature is 36.5◦C; its critical pressure is 72
bar.Effects
Respiratory – it causes a small fall in tidal volume that is offset by an increased
respiratory rate so that minute volume and PaCO 2 remain unchanged.
Cardiovascular – although N 2 Ohas mild direct myocardial depressant effects, it
also increases sympathetic activity by its central effects. Therefore, in health the
circulatory system is changed very little. However, for patients with cardiac failure
who are unable to increase their sympathetic drive the direct myocardial depres-
sant effects may significantly reduce cardiac output. It does not sensitize the heart
to catecholamines.
Central nervous system – N 2 Oincreases cerebral blood flow and is sometimes
avoided in patients with a raised intracranial pressure. Despite a MAC of 105%, its
potential to cause anaesthesia in certain patients should not be ignored.Concentration effect, second gas effect and diffusion hypoxia
The concentration effect
The concentration effect is an observed phenomenon that describes the dispropor-
tionate rate of rise of the alveolar fraction compared with the inspired fraction when
high concentrations of N 2 Oareinspired (Figure8.8a). The rate of rise is dispro-
portionate when compared with the situation where low concentrations of N 2 Oare
inspired. The concentration effect only applies to N 2 Obecause N 2 Oisthe only agent
used at sufficiently high concentration. Various models have been used to explain
the phenomenon; all have limitations.
However, the fundermental driving force for the process is the large gradient which
the high concentrations of N 2 Ogenerate. As a result large amounts of N 2 O(50%
is assumed in the model shown in Figure8.8b) are absorbed into the pulmonary
capillaries despite the fact that it is usually considered an insoluble agent (blood:gas
solubility coefficient 0.47). In order for the alveolar volume to remain constant, gas