Biomedical Engineering Reference
In-Depth Information
(
Figure 17.7
). It is also important to note that some
ventilators require a compressed gas source to power their
action, whereas others are electrically powered.
Most of the more complex ventilators allow the tidal
volume (the volume of each breath) and the respiratory rate
to be set. For most primates, tidal volumes of 7
e
10 ml/kg
are suitable, but these may need to be adjusted to
compensate for any leakage of gas from the breathing
system (e.g. around the cuff of the endotracheal tube). The
respiratory rates needed range from 40
e
50 breaths per
minute for small primates (
urethra. In New World primates, urethral catheterization of
females is difficult but in males it is reasonably practicable.
Infusion rates of 5
e
10 ml/kg/h are generally considered
appropriate for all species, however for nonsurgical
procedures and prolonged procedures where the fluid losses
are minimal, 2
e
5 ml/kg/h may be more appropriate in
order to avoid bladder hyperdistention (a problem if
urethral
catheterization
is
not
performed)
and/or
hypervolemia.
Body Temperature
Careful monitoring of the patient is important to allow
early detection and correction of any problems that may
arise. In all primates, respiratory and cardiovascular
function are of primary importance, but body temperature
should also be monitored and maintained. Most anes-
thetics depress thermoregulation, and this effect, coupled
with muscle relaxation, the use of cold fluids, clipping of
hair, preparation of the surgical site, and use of cold
anesthetic gases, can rapidly result in marked hypo-
thermia even before the procedure has begun. Hypo-
thermia can lead to reduced drug metabolism,
coagulopathy, an increased risk of cardiac arrhythmias,
andincreasedtissueoxygendemandinrecovery.An
external heat source should be used and if necessary the
animal covered in insulating material (e.g. bubble
packing). Body temperature should be monitored, and
suitable inexpensive thermometers can be obtained easily.
In line with human patient data (
Ng et al., 2003
), our
experience shows that the most effective means of main-
taining body temperature is to use a forced warm air
system such as the “Bair hugger.” Water-circulating
blankets, with or without a heat dissipating gel sleeve,
may also be used. Electric heating pads should be used
with caution because of the risk of burns caused by “hot-
spots” or lack of appropriate cut-off systems. Whichever
method is used, if the device is placed underneath the
animal, additional insulation material between the heat
source and the operating table will help to improve the
efficiency of the device.
1 kg) to 10
e
15 breaths per
<
minute for larger (
20 kg) animals. The rate should nor-
mally be adjusted to maintain end tidal carbon dioxide
concentrations at 4
e
4.5%. This slight hypocapnia is often
sufficient to prevent animals attempting to breath against
the ventilator. If respiratory efforts are made that do not
synchronize with the ventilator rate, then increase the rate
by 20
e
50% until spontaneous efforts cease. This is then
followed by a gradual reduction in rate until the target
carbon dioxide concentration is attained. Alternatively,
a neuromuscular blocking agent can be administered, but
these are generally not required simply to allow ventilation
of an animal. Further details of ventilators and management
of IPPV can be found in a number of anesthetic texts
(
Hartsfield, 2007a
).
>
Fluid Administration
Total blood volume in all species of primates is approxi-
mately 70 ml/kg of bodyweight. It is critically important to
minimize blood loss in smaller species by careful surgical
technique and to monitor blood loss in all animals by
accurate weighing of swabs and cotton tips (1 ml of blood
weighs approximately 1 g) and by assessing other losses at
the surgical site. Blood loss of up to 10% is fairly well
tolerated in the normal healthy adult, however an anes-
thetized individual undergoing surgery has impaired
compensatory mechanisms and such losses are likely to
cause serious problems.
In addition to blood, fluid may be lost as a result of
ongoing metabolism, via the respiratory system if inspira-
tory gases are not humidified, from the surgical site by
evaporation, and as a result of diuresis if certain drugs are
used (e.g. alpha-2 adrenergic agonists).
For any procedure in which anesthesia is likely to last
more than 30 minutes or in which blood loss could occur, it
is advisable to provide circulatory support by administering
an isotonic balanced electrolyte solution (e.g. Hartmann's/
Lactated Ringers solution). Infusion rates should be suffi-
cient to maintain urinary output to at least 1
e
2 ml/kg/h.
This can be monitored by placing a urethral catheter or, in
nonrecovery procedures, by bladder cannulation via
a laparotomy. Urethral catheterization is relatively simple
in Old World primates, although catheter passage in males
can be more challenging because of the anatomy of the
DEALING WITH EMERGENCIES
All of the measures for coping with anesthetic emergencies
described in standard anesthesia texts (
Evans and Wilson,
2007; Flecknell, 2009a
) can be used for nonhuman
primates. As with many other techniques, the small body
size of some species can limit or complicate some of these
procedures. When planning protocols for dealing with
emergencies, it is important to consider carefully the
impact of major deviations from normal physiological
function on the intended research use of the animal. For
example, excessive blood loss, prolonged periods of
hypoxia, or cardiac or respiratory arrest can all produce
