Biomedical Engineering Reference
In-Depth Information
taken not to “oversteer” in response to a change in physi-
ological parameters following surgical stimulus. Recovery
after discontinuation of isoflurane is relatively rapid.
However, it is often accompanied by ataxia (depending on
other drugs given), so the recovery area should be designed
to take this into account and/or sedation used to enable
a more gradual recovery (see sections “Sevoflurane” and
“Monitoring” below). Along with ketamine, isoflurane has
been shown to cause neuronal apoptosis in neonatal
macaques ( Brambrink et al., 2010 ). Whether apoptosis
occurs when animals are exposed at other life stages and
whether neurocognitive deficits result from the apoptosis is
not yet known. Until these questions are answered and
other anesthetic agents also investigated the advantages of
isoflurane for many experimental (and clinical) situations
would still appear to outweigh any potential risk associated
with its use.
high that it would boil at normal room temperatures, and so
a heated and pressurized vaporizer that relies on an elec-
trical power supply must be used. Unlike sevoflurane des-
flurane is pungent, and although signs of airway irritation
are not seen during induction of dogs and cats with des-
flurane ( Hammond et al., 1994; McMurphy and Hodgson,
1994 ), in humans it causes airway irritation and has the
potential for sympathetic stimulation with rapid increases
in alveolar concentration ( Whitton et al., 1993 ). For these
reasons and because emergence from desflurane anesthesia
may be more rapid than is desirable, the use of desflurane
may not afford advantage over sevoflurane for many
procedures.
Nitrous Oxide
The gaseous anesthetic nitrous oxide (N 2 O), a NMDA
antagonist, is commonly used as a carrier gas in addition to
oxygen (between 70:30 and 50:50 N 2 O:O 2 depending on
blood oxygen saturation) because of its “MAC-sparing”
effects. The anesthetic and, in particular, analgesic prop-
erties of N 2 O allow for a reduction in the concentration of
other anesthetic agents. Unlike volatile agents, the use of
N 2 O is not associated with hypotension. However, concerns
about the use of nitrous oxide in humans include its
potential neurotoxic effects ( Culley and Crosby, 2008 ), as
well as possible retardation of wound healing after surgery
( Myles et al., 2007 ). It is not clear whether these latter
effects are due to an active property of N 2 O or a facilitation
of wound healing by increased oxygen delivery when N 2 O
is not used. N 2 O may be desirable in particular procedures
where administration of a higher concentration of volatile
anesthetics is cause for concern, e.g. due to the hypotension
associated with higher doses. Because of its ability to
diffuse into gas filled cavities, N 2 O has the potential to
cause gastrointestinal dilatation and to worsen any condi-
tion involving the trapping of gas, e.g. pneumothorax if
used while closing a thoracotomy and pneumocephalus if
used while closing a craniotomy. The degree of risk of these
complications in nonhuman primates is uncertain.
However, since these problems occur in humans and
a range of other species, it would seem advisable to use
nitrous oxide with caution in nonhuman primates consid-
ered at risk of these complications. For the same reason it is
important that 100% oxygen be administered to all animals
for 10 minutes after cessation of N 2 O, in order to avoid
diffusion hypoxia.
Sevoflurane
Sevoflurane has a lower blood/gas solubility coefficient
than isoflurane, so physiological changes in response to
modifications of sevoflurane concentration are more rapid
(of the order of tens of seconds). This quality enables
more precise control of depth of anesthesia, which is
particularly useful where the level of surgical stimulation
varies throughout the procedure. It is less pungent than
isoflurane and may be more useful for mask or chamber
induction of smaller primates. Because of the lower
solubility, recovery after discontinuation of sevoflurane is
more rapid than after isoflurane. The time to extubation
following cessation of agent is significantly shorter for
sevoflurane than isoflurane ( Murphy and Baxter, 2009 ).
Indeed, in the authors' experience, emergence from
anesthesia can be too rapid in some animals, and the
authors have found that it is advisable to taper the sevo-
flurane over 10 e 15 minutes after the surgical procedure is
completed and in some cases to administer a low dose of
benzodiazepine (e.g. midazolam 0.1 mg/kg for macaques)
(also see section “Monitoring” below for an alternative
method). This reduces or eliminates the ataxia induced
hypermobility (that can lead to injury during recovery)
and produces a smooth recovery without marked sedation
or respiratory depression.
Desflurane
The solubility characteristics (low blood:gas partition
coefficient) and low potency of desflurane enable very rapid
induction and emergence from anesthesia ( Eger, 1992 )
(more rapid than sevoflurane) as well as precise control of
depth. Of the commonly used volatile anesthetic agents it
appears to be the least vulnerable to metabolism. However
there a number of drawbacks associated with the use of
desflurane. The vapor pressure of desflurane is sufficiently
Neuromuscular Blocking Agents
Neuromuscular blocking agents (NMBAs) have no anes-
thetic or analgesic properties. They block transmission at
the neuromuscular junction by competitive inhibition of
acetylcholine, leading to paralysis of skeletal muscles.
They are useful for some types of abdominal or thoracic
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