Understanding Anesthesiology - The Global Regional Anesthesia ...

Drugs for Maintenance of Anesthesia
Inhaled Agents
The term “inhaled agent” refers to the volatile agents
(desflurane, isoflurane, sevoflurane) and nitrous oxide
(N2O). While the development of intravenous agents
has largely eliminated the use of inhaled agents for induction
except in the pediatric population, they continue
to be the mainstay for maintenance of anesthesia.
The volatile agents are so called because with vapour
pressures below atmospheric pressure, they exist in
equilibrium between liquid and gaseous states at room
temperature. N2O is a gas at room temperature and atmospheric
pressure and is therefore not a volatile agent
though it is, of course, an inhaled agent.
Mechanism of Action
The mechanism of action of inhaled agents is not well
understood.
Dose
While intravenous agents are given in mg (or mcg)/kg
doses, the inhaled agents are given in volume percent
concentrations. The volatile agents can also be termed
“potent” vapours, because concentrations in the range
of 0.3-6% are clinically effective while N2O (not potent)
must be given in concentrations of between 30% and
70% to have any effect.
alveoli creates an alveolar partial pressure of gas which
in turn reflects its partial pressure in the active site
(brain). MAC refers to the concentration of the inhaled
agent in alveolar gas necessary to prevent movement of
50% of patients when a standard incision is made. This
definition hints at the fact that a specific concentration
of gas does not correlate to a predictable clinical effect.
Many factors influence MAC, and therefore influence
the concentrations required to maintain anesthesia.
Some of the known factors are listed in Table 12.
The MAC values of the commonly used agents are
shown in Table 13. Note that N2O, with a MAC>100%,
can never be used as a sole agent to provide anesthesia.
Table 12 Factors affecting MAC
FACTORS WHICH
DECREASE MAC
advanced age
pregnancy
hypothermia
acute alcohol intoxication
FACTORS WHICH
INCREASE MAC
childhood
hyperthyroidism
hyperthermia
chronic alcohol use
This brings us to the concept of Minimum Alveolar
Concentration (MAC). The concentration of a gas in the
drugs: benzodiazepines, opioids,
muscle relaxants, central-acting
antihypertensives
drugs: amphetamine, cocaine
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