Biomedical Engineering Reference
In-Depth Information
In addition to the above, each tonometer has an inherent variability
which is typically
4.1 Tonometer-
Associated Factors
2 mmHg. As with many instruments, individual
tonometers of the same model may also give somewhat different
measurements [
38
]. Although it is often considered more reliable
to collect and average three IOP estimates at each interval, this
approach does not always make the data more accurate, especially
with certain tonometers which already collect multiple measure-
ments to generate the single displayed reading. For example, the
potential errors in IOP estimates with the Tono-Pen and many
other tonometers are not normally distributed, but instead these
errors almost always result in overestimation of true IOP [
39
]. This
means that averaging three readings is likely to increase the error in
those IOP estimates.
Just as there are inter-instrument variations in IOP measurements
there are inter-tonometrist differences as well. These can be several
mm Hg or more, and may be due not only to differences in
individual technique but also to variations in the animal's acclima-
tion to one individual over another. Therefore the ideal situation
is to have one experienced individual collect all measurements. It is
also important that the tonometrist (or the assistant) avoids com-
pressing the jugular veins or eyelids, ensures that the instrument
cleanly contacts the cornea, and is aware of confounding factors
that may affect the accuracy of an individual reading (globe retrac-
tion, excessive eye movements, panting, contacting paraxial cornea
or the cornea at an angle, nonparallel to the ground positioning of
the TonoVet, etc.). As the animal's acclimation to tonometry can
rapidly fade, it is important on long-duration studies to continue to
periodically collect measurements from the animals so as to main-
tain acclimation even if this data is not to be used for data interpre-
tation purposes. For example, acclimation is likely to be lost, and
IOP values become more variable, if measurements are collected
once every 4 weeks in a typical 3 or 6 month efficacy study. Most
species do best with collection two to three times a week, although
this varies by tonometrist, species, the extent of the time the animal
has been acclimated to the procedure, and to some extent the
device being used.
Eyelid Manipulation
. Manipulation of the eyelids can result in
marked alterations in IOP. In one study retracting the eyelids of a
dog laterally or in a dorsoventral direction resulted in IOP estimates
that were increased by 16.5 mmHg and 6.4 mmHg respectively
[
40
]. The low scleral rigidity of rabbits suggests that this effect
would be even greater in rabbits.
4.2 Tonometrist-
Associated Factors
Compression of the External Jugular Veins
. Compression of the
external jugular veins frequently occurs in dogs during restraint
for IOP measurements and may also occur in some primate studies
in which the animal
is wearing a rigid neck collar to facilitate
