Biomedical Engineering Reference
In-Depth Information
the operator and on whether an assistant is available to hold
the animal in the required position or to assist with opening
of the jaw. If no assistance is available, then placing
animals in dorsal recumbency (i.e. on their back) enables
easy positioning for intubation. In this position the operator
may find a Winsconsin laryngoscope blade the most useful
type.
After intubation, the animal should be connected to an
appropriate breathing system for its size
e
suitable systems
for spontaneously breathing primates are listed in
Table
17.6
, along with recommended fresh gas flow rates. Smaller
primates would usually be connected to a nonrebreathing
circuit such as T-piece. Rebreathing circuits such as the
circle circuit enable the recycling of expired gases, which
are passed through soda lime (or equivalent) to remove
expired carbon dioxide. This arrangement is more
economical but resistance of the unidirectional valves
limits use to the larger species. For more details see
Hartsfield, 2007b
. For smaller primates (
Endotracheal Intubation
Visualization of the larynx is easy to achieve in primates
using a Macintosh or Wisconsin laryngoscope blade
(
Figure 17.4
), but the larynx is relatively mobile, so use of
a stylet can make insertion of an endotracheal tube simpler,
especially in smaller primates (
Morris et al., 1997
).
Nonhuman primates appear to be relatively prone to trauma
of laryngeal and tracheal tissue when compared to some
other species, therefore, regardless of the technique used, the
utmost care should be taken during the procedure to avoid
tissue trauma. Once placed it is essential that the tube does
not move with respect to the larynx and trachea, which may
be of particular concern during transport or repositioning.
In all species it is advisable to administer 100% oxygen
for 1
e
2 minutes before attempting intubation. The larynx
should be sprayed with local anesthetic (e.g. xylocaine) to
reduce the incidence of laryngospasm. The face-mask
should then be replaced and oxygen administered for
a further 30
e
60 seconds. The tube should be measured by
holding it against the exterior of the animal's head and neck
so that it is passed no further than midway between the
caudal aspect of the occipital ridge and the points of
the scapulae in order to avoid trauma to the carina with the
distal end of the tube (Old World nonhuman primates have
a relatively short trachea). “Dead space” is a term used to
refer to the space within breathing apparatus where no gas
exchange takes place and is composed of anatomical dead
space (the volume of the trachea and bronchi) and breathing
circuit dead space (e.g. the volume of the endo-tracheal tube
that is exteriorized). If dead space is too high then
rebreathing of carbon dioxide may lead to hypercapnia, and
so, unless armoured tubes are used, endo-tracheal tubes
should be cut (at the breathing circuit end) to the level of the
incisors. Insertion of the tube, past the epiglottis and
between the laryngeal folds, should always be carried out
slowly and with care. If difficulty is encountered, the tube
can be withdrawn, repositioned, and a further attempt made.
When learning the technique, it is helpful to monitor the
animal with a pulse oximeter during this process, so that
attempts can be discontinued and oxygen administered if the
animal becomes hypoxic (oxygen saturation
2 kg), low
volume connectors between the endotracheal tube and
breathing circuit (as shown in
Figures 17.5 and 17.6
) will
help to reduce dead space.
<
Ocular Protection
Anesthetized animals lose their protective blink reflexes,
and the eyes should be protected both from physical
damage and from drying. Ophthalmic ointment can be
placed in the eyes, or the lids can be taped closed with
surgical tape.
Positioning
When positioning the animal it is important to ensure that
limbs are not either over-extended or excessively flexed and
85%). If
respiratory effort increases and/or oxygen saturation
decreases shortly after intubation, consider whether the tube
has been placed either in the esophagus or a bronchus. In
order to reduce the risk of trauma the relatively stiff red
rubber intubation tubes should be avoided in preference for
softer silicon or PVC tubes. The use of a laryngeal mask is
associated with less airway trauma than an endotracheal tube
and offers an alternative for fasted, larger nonhuman
primates undergoing short procedures (
Johnson et al., 2010
).
Intubation can be carried out with the animal in sternal
(
Figure 17.4
A), dorsal (
Figure 17.4
B), or lateral recum-
bency. Choice of position depends upon the preference of
<
FIGURE 17.5
Intubated marmoset connected to a low dead space
breathing circuit via which volatile anesthetic agent and/or oxygen can
be supplied and expired gases expelled. A pulse oximeter probe is
positioned on the hind foot (top right) and measures blood oxygen satu-
ration of 99% and a pulse rate of 240 bpm. (Courtesy of P. M. Taylor.)
