Biomedical Engineering Reference
In-Depth Information
or a digit (
Figures 17.5, 17.6
). Use of an angled probe
placed in the mouth has proven particularly reliable in
macaques.
Measurement of end-tidal carbon dioxide is usually
straightforward in larger (
“Respiratory function” above) to measure blood oxygen
saturation. Measurement of arterial blood pressure using
noninvasive systems is possible in nonhuman primates
using an appropriate sized cuff, placed on the upper
(
Figure 17.6
) or lower limb, proximally or distally,
depending upon the size of the animal and the cuff. It is
possible to measure blood pressure in this way in marmo-
sets using a tail cuff, but many instruments are insuffi-
ciently sensitive to achieve this. Cuff width should be
approximately 40% of the limbs' circumference, as cuffs
wider or narrower than that can lead to falsely low or high
blood pressure readings respectively. During prolonged
procedures, it is advisable to remove the cuff, massage the
limb, and replace or reposition on the contralateral limb, to
avoid partial constriction of limb circulation. An oscillo-
metric system will provide systolic, mean and diastolic
blood pressure measurements and can be automated to
ensure regular readings. Doppler systems are manual and
provide a single blood pressure reading, however they do
provide an audible pulse signal that can free the anesthetist
from pulse palpation and be used to “fine tune” the depth of
anesthesia because a change in the audible signal can be
detected as vasomotor tone increases in response to noxious
stimuli. In some species the Doppler system reading is
equivalent to systolic blood pressure, however the authors
have observed that, at
2 kg) primates and both main-
stream and side-stream methods can be used successfully.
In marmosets and other small primates the volume of gas
sampled by side-stream capnographs may be very large in
relation to the animal's tidal volume (e.g. 100
e
200 ml/min
sample rate) which can equal or exceed the minute volume
when anesthetized (and lead to dilution of the sample by
fresh gas flow, recognized as blunting of the end tidal
carbon dioxide reading). Mainstream capnographs may
introduce too much equipment dead space into the anes-
thetic breathing system (recognized as high inspired carbon
dioxide readings
e
a high base line on the trace). Lower
sample rate (50 ml/min) capnographs are available and,
when used with low dead space connectors that enable
sampling close to the animal (
Figure 17.6
), can be used to
reliably monitor the smaller nonhuman primates.
Arterial blood gas analysis allows assessment both of
the adequacy of oxygenation and measurement of carbon
dioxide tensions, as well as measurement of pH. Interpre-
tation of blood gas data can be complex (for more details
see
Martin, 1999
), but simply establishing that P
CO
2
, P
O
2
,
and pH are within acceptable limits is often of significant
benefit. Many analysers also provide measurements of
major electrolytes (e.g. Na and K) and blood glucose. This
information can be particularly useful when managing
long-term anesthetics. Arterial blood samples for analysis
can be obtained relatively easily in larger primates via
either percutaneous puncture or cannulation of vessels, but
surgical cut-down may be needed in small primates. The
volume required for analysis is typically 100 microliters,
enabling this technique to be used for even the smallest
animals.
>
least
in macaques,
it provides
a reading much closer to mean blood pressure.
Invasive blood pressure monitoring is possible in all
species, but surgical exposure of the vessel may be needed
in smaller primates (
1 kg), depending upon the technical
skill of the operator, which tends to limit use of this tech-
nique to nonrecovery procedures. In larger primates the
femoral artery or radial artery can be catheterized using
a Seldinger technique or an “over-the-needle” catheter/
cannula can be placed percutaneously into the anterior
tibial artery.
Cardiac rate and rhythm can be monitored easily
using an electrocardiogram (ECG), and most commer-
cially available devices function well in all species of
primate. In general, heart rate decreases when anesthetic
agents are administered. However, heart rate should not
be used in isolation as an indicator of depth as
a counterintuitive decrease in heart rate can occur in
response to noxious stimuli, and heart rate can be affected
by other physiological parameters such as blood carbon
dioxide and oxygen levels and body temperature.
Disturbances in rate and rhythm are a reasonably
common occurrence during very prolonged periods of
anesthesia, and use of an ECG enables appropriate
corrective measures to be implemented (e.g. use of anti-
arrhythmics). It should be noted that an ECG only gives
information about the electrical activity of the heart
which can remain relatively normal in situations where
tissue perfusion is compromised.
<
Cardiovascular Function
Clinical monitoring of the cardiovascular system is easy to
undertake in larger (
2 kg) primates but more difficult in
smaller animals. A peripheral pulse is easy to palpate
(either over the radial, brachial, femoral or medial tarsal
artery) in larger animals but difficult or impossible to
palpate in small primates other than by ausculation or
palpation of the thorax.
Other clinical assessments, such as use of capillary refill
time, are practicable and useful in larger primates. In all
species, assessment of the color of the mucous membranes
allows some assessment of peripheral perfusion and
adequacy of tissue oxygenation. It is important to note,
however, that appearance of cyanosis indicates very severe
hypoxia requiring emergency intervention, and it is there-
fore preferable to use a pulse oximeter
>
(see section
