Biomedical Engineering Reference
In-Depth Information
Assessing Anesthetic Depth
For an interesting discussion regarding the assessment of
anesthetic depth in laboratory animal species see
Antognini
et al. (2005)
. In all species, onset of a surgical plane of
anesthesia is accompanied by onset of muscle relaxation,
notably relaxation of jaw tone. Withdrawal responses to
pinching of the interdigital skin folds, digit, or tail are lost.
The palpebral blink reflex is lost, but the eye remains
approximately central in many species, and does not rotate
downwards.
When neuromuscular blocking drugs are used, these
somatic reflex responses are no longer useful and other
methods of assessing the adequacy of anesthesia that rely
on autonomic reflexes (heart rate and blood pressure) are
required (see section “Neuromuscular blocking agents”
above).
The analysis of electroencephalography (EEG) data is
commonly used as an additional depth of anesthesia
monitor in humans. The bispectral (BIS) monitor is the
most extensively studied and the use of BIS is associated
with a lower incidence of awareness in high risk patients,
faster recovery, and a lower requirement for adjunct
medication in humans (
Punjasawadwong et al., 2007
). Both
the assessment of EEG and the use of BIS has been reported
in nonhuman primates (
Prior et al., 1978; Izrailtyan et al.,
2004
). However the BIS algorithm is derived from human
data and although it may be useful for following trends in
EEG data, absolute values should be interpreted in this
context. The BIS monitor does not work well with some
anesthetics such as ketamine (
Sakai et al., 1999
) and nitrous
oxide (
Barr et al., 1999
), and information obtained when
those drugs are used is of unknown value.
been demonstrated in nonhuman primates when 100%
oxygen is delivered over a period of days (
Robinson et al.,
1974
). In order to avoid this potential problem occurring
during anesthesia, it is advised that the delivered oxygen
concentration be limited to 30
e
50% during prolonged
procedures (
12 h). This can be achieved either by the
addition of nitrous oxide to the gas mixture (but see above)
or the gas mixture from the anesthetic machine can be
blended with medical air. A pulse oximeter should be used
to ensure that the concentration used is adequate to main-
tain oxygen saturation
>
95%. The delivered concentration
of oxygen can be monitored using an oxygen analyzer. This
is often provided as a standard monitoring device on
modern multifunction monitors.
>
Ventilation
Adequate ventilation not only involves the intake of
sufficient oxygen but also the expulsion of sufficient
carbon dioxide, and hence optimizing respiratory function
may involve the use of intermittent positive pressure
ventilation (IPPV). Mechanical ventilation of animals
may be required from the outset for certain procedures,
e.g. thoracotomy, some imaging or neurosugical proto-
cols, or to deal with unexpected respiratory depression.
Ventilators designed for veterinary use or for use in people
can be used with nonhuman primates. The choice of
instrument depends upon the size of the animal and the
range of functions required. A “mechanical thumb”
ventilator controls the gas delivered to the patient in the
inspiratory phase until a set airway pressure is reached.
This type of pressure cycled ventilator is compact
(
Figure 17.7
), intuitive to operate, and has no lower limit
to the size of the animal. However, for procedures
involving large nonhuman primates, nonhuman primates
with lung pathology, or requiring control of multiple
ventilation parameters, a more complex ventilator
utilizing either a pressurized gas driving mechanism or
piston will be needed. A particularly versatile veterinary
ventilator at moderate cost is available from Vetronic
Services LTD (“Merlin” ventilator,
www.vetronic.co.uk
)
Intraoperative Support
Oxygenation
All anesthetic agents lead to some degree of respiratory
depression. The administration of a high inspired concen-
tration of oxygen can therefore be beneficial for main-
tainence of tissue oxygen delivery. Oxygen toxicity has
FIGURE 17.7
Ventilators suitable for
use with nonhuman primates. The
“mechanical thumb” ventilator (SAV03,
right) is intuitive to use and suited to
smaller NHPs (
<
15 kg). The piston driven
ventilator (Merlin, left) is a particularly
versatile ventilator for all species. (Cour-
tesy of Vetronic Services Ltd.)
