Biomedical Engineering Reference
In-Depth Information
To solve for the time required to return back to 5% of the steady state normal pressure
ð
e
2
t
=
RC
:
Þ
1
05p
norm
5
p
norm
1
5 mmHg
05p
norm
5 mmHg
5
e
2
t
=
RC
0
:
t
RC
5
05p
norm
5 mmHg
0
:
0
:
05
Q
in
R
5 mmHg
2
ln
5
ln
0
:
05
Q
in
R
5 mmHg
t
52
RCln
!
2
ln
2
μ
min
5
mmHg min
μ
0
:
05
5
mmHg min
μ
L
mmHg
μ
L
t
52
:
8
L
5 mmHg
t
5
32
:
2 min
Tonometry is a method to directly measure the intraocular pressure. By assuming that
the eye is composed of elastic tissue that forms a perfect spherical shell, the effect of
increases in external pressure, which causes deformations to the eye, can be used to esti-
mate the intraocular pressure. Increases in intraocular pressure would make it harder to
deform the eye. There are two general approaches to conduct tonometry: the first is to mea-
sure the force required to deform a fixed area of the eye (Goldmann tonometry), and the
second is to measure the area deformed by a fixed force (air-puff tonometry). In this ideal
situation, the intraocular pressure can be quantified as
F
A
p
IO
5
ð
10
:
8
Þ
where F is either the force applied or measured and A is either the covered area or the
measured deformed area. Although this formulation can be used to estimate intraocular
pressure, it is important to note that the eye is not a thin-walled, homogeneous, purely
elastic, spherical shell (of uniform thickness) and that the deformed area would not be the
same as anticipated because the fluid that bathes the exterior portion of the eye can dissi-
pate some of the load applied to the eye.
10.6 DISEASE CONDITIONS
10.6.1 Glaucoma
Glaucoma is one of the leading causes of blindness and occurs when the intraocular
pressure rises significantly. Typically the intraocular pressure rises slowly during glau-
coma, but any increase can eventually lead to blindness, if the force exerts itself on the eye
structures for enough time. An increase in intraocular pressure to about 30 mmHg is com-
mon during glaucoma. This level of pressure causes a significant decrease in vision and
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