Understanding Anesthesiology - The Global Regional Anesthesia ...

Even the potent agents are often not administered at 1
MAC concentrations during general anesthesia. This is
because other agents (such as opioids) are also being
given. It is generally felt that 0.5 MAC of inhaled agent
is the minimum level to provide adequate hypnosis and
amnesia.
Onset of action, duration of action and elimination
The solubility of a gas in blood determines its rate of
onset and offset of effect. An agent such as N2O, which
is relatively insoluble in blood, will build up its alveolar
partial pressure (and therefore brain partial pressure)
quickly and consequently will have a faster onset
and offset of effect. Conversely, a soluble gas such as
Table 13 Characteristics of inhaled agents
N 2 O ISOFLURANE SEVOFLURANE DESFLURANE
isoflurane will equilibrate slowly throughout body
stores and therefore onset and offset lags. You may notice
this theory being put into practice in the operating
room: a soluble potent agent is often discontinued 5 or
10 minutes prior to the end of surgery while N2O is delivered
until moments before the desired emergence or
wake-up. Table 13 summarizes these pharmacokinetic
properties. The termination of effect of inhaled agents
depends only on its exhalation from the lungs. The exception,
of historical interest, is halothane of which up
to 20% can undergo metabolism in the liver.
Effects of the inhaled agents
The effects of the volatile agents are quite distinct from
those of nitrous oxide. Inhaled agents have effects on
almost every organ system and the reader is referred to
Chapter 6 for a detailed summary. Several key effects
are highlighted below.
ODOUR
MAC%
BLOOD/GAS
PARTITIONING
odourless pungent sweet pungent
104 1.15 1.8 6.6
0.47 1.4 0.65 0.42
All volatile anesthetics are triggers of malignant hyperthermia
while nitrous oxide is not. N2O expands the
volume of gas-containing spaces as N2O diffuses
across membranes more readily than nitrogen can diffuse
out. Thus the size of a pneumothorax, an emphysematous
bleb or a distended bowel loop will increase
when N2O is used.
VAPOUR
PRESSURE 20°C
MMHG
38,770 238 157 669
67