Biomedical Engineering Reference
In-Depth Information
severely limited. Use of electronic monitoring devices can
resolve this problem, however specially designed devices
are required (e.g. for pulse oximetry, ECG, or blood pres-
sure recording) for use with MRI. Other devices such as
side-stream capnography need to have the measuring
instrument placed a safe distance from the magnet. This
requires use of extension tubing, which can lead to instru-
ment malfunction or a loss of signal quality. It is therefore
important to establish which devices will function reliably
and without harm to the animal before attempting to use
these on an anesthetized animal during an imaging
procedure.
It is also important to note in the case of MRI that
unless specialized equipment is available, delivery of
anesthetic agents by inhalation or continuous intravenous
infusion will require use of long lengths of tubing in order
to allow the anesthetic machine or pump to be placed
a safe distance from the magnet. Long tubing introduces
hysteresis that will need to be considered when making
alterations to the dose of an anesthetic agent. If
mechanical ventilation is used the calculated tidal volume
should include a correction for the compliance of the
breathing circuit if a remote anesthetic machine is used. It
should be noted that the consequent increase in resistance
introduced if remote pumps are used to deliver intrave-
nous drugs causes some pumps to give an occlusion alarm
and fail.
Maintaining body temperature can be a particular
problem. Use of warm air blowers and magnet-compatible
heating devices coupled with use of insulating materials are
often all required to maintain normothermia.
Specialized MRI protocols utilize magnets with verti-
cally oriented bores to facilitate functional MRI of awake,
behaving nonhuman primates. Anesthesia in this environ-
ment is challenging because the positioning of the monkey
makes pooling of blood in the lower body and limbs likely
and can prevent adequate lung expansion.
Particular health and safety considerations apply to
anesthesia in an MRI environment in order to ensure
that ferromagnetic objects are not brought into the field
of the magnet and potentially cause serious or fatal harm
to the animal or to humans. This must be carefully
considered particularly with regard to planning for
management of potential anesthetic emergencies in the
MRI environment.
not be used in animals that are to undergo magnetic reso-
nance imaging (MRI).
When placing animals in ear bars, local anesthetic
cream (e.g. EMLA Astra, applied at least 20 min before)
can be applied to the ear canals with the aim of producing
local anesthesia and reducing both intraoperative and
postoperative nociceptive stimulation. Scalp blocks, such
as those used for human patients (
Osborn and Sebeo, 2010
),
may be useful in reducing pain associated with placement
in stereotaxic frames or neurosurgical head-holders. Head-
holders that place a large degree of pressure on the orbit can
be associated with vagally induced bradycardia and the
prophylactic use of muscarinic antagonists may prove
advantageous.
Volatile anesthetics dose-dependently increase cerebral
blood flow, which will increase brain volume and “tight-
ness” to the dura. When craniotomy and open neurosurgery
are being performed, often hyperventilation is carried out to
induce hypocapnia; the reduction of CO
2
levels counteracts
the effect of the volatile anesthetic on cerebral blood flow
and reduces brain volume, facilitating neurosurgery.
Macaques tolerate prolonged maintenance of end-tidal CO
2
of 25 mmHg (normocapnia is 35
e
40 mmHg) providing
normotension is maintained. However, if end-tidal CO
2
is
below 25 mmHg very little further reduction in brain
volume is achieved and the induced vasoconstriction may
prove harmful.
Corticosteroids are commonly administered in
conjunction with neurosurgical procedures to prevent
intraoperative and postoperative cerebral edema. In the
authors' experience the prophylactic administration of
corticosteroids for a brief period (less than a week post-
operatively) is without any negative systemic effects
associated with prolonged steroid use (e.g. wound break-
down or dysphagia associated with gastric discomfort). The
beneficial effects of corticosteroids include preventing
postoperative cerebral edema and swelling of temporal
muscles when these are retracted as part of the surgical
procedure as well as preventing signs that may reflect
formation of inflammatory foci as part of dural healing
(epileptiform behaviors not accompanied by drowsiness,
hemiplegia, or other signs of possible cerebral edema).
Corticosteroids should be tapered rather than withdrawn
abruptly to prevent adrenal insufficiency. Diuretics, such as
furosemide, may be used both intra- and post-operatively to
treat signs of cerebral edema. If signs of cerebral edema
are observed in the post-operative period, additional treat-
ment may include sedation with propofol in order to
ventilate the nonhuman primate (to maintain normocapnia
and reduce cerebral volume) and to enable intravenous
administration of osmotic diuretics such as mannitol or
hypertonic saline.
Volatile anesthetics are contraindicated at any point in
the treatment of nonhuman primates with suspected
Neurosurgery
When positioning an animal in a stereotaxic frame for
neurosurgical procedures, care must be taken to avoid
kinking of the endotracheal tube as the head and neck are
flexed. This risk can be avoided by use of armoured tubes,
but these are not available in the sizes needed for smaller
primates. It is important to note that armoured tubes must
