You are here

Home » Community » Blogs

Pressure II and what is more to say on IOP???

Submitted by Marcel on Thu, 10/04/2007 - 9:33pm

When writing on altitude I was surprised to see the influence on barometric pressure and researched the web a bit more.

I did find some more information on altitude:

and stating completely the opposite:

David did a blog on this subject too:, this might reply to some of his questions.

In my personal opinion (looking at it from mechanical engineering point of view) the "IOP" would increase when going up the mountain, internal pressure remains the same while external pressure reduces.
During a period of acclimatization the internal eye pressure would decrease as all other internal (blood) pressures would also return to base line during pressure equalization.

This however would mean that our internal system might take some time to get stabilized with the changed environment and sudden exposure to high or low environmental pressure could put a lot of stress on an already elevated IOP! Main risk for a person with a high "IOP" is getting to great heights in short time without a pressure chamber!?

Apart from this issue:

Is there a correct understanding of measuring pressure, i.e. is there an understanding of measuring pressure gauge, pressure absolute and delta P? I.e. do we all talk the right pressure talk?

In my personal opinion what we now call "IOP" is not really a correct term for what we measure.
Or the unit of measure (mmHg) is not correct.

Why not?
The pressure inside the eye: Intraocular Pressure (IOP).
The pressure outside the eye: Periocular Pressure (POP).

We currently measure the pressure which we call IOP at the cornea which is exposed to the atmospheric pressure, this is the POP.

Let us say barometric pressure is 765 mmHg. And we measure an "IOP" of 25 mmHg for example, whether we use GAT or NCT makes no difference.

This does not mean the POP is 765mmHg and the IOP is 25mmHG, in that case the eye would collapse!
Measuring 25mmHg A is near absolute vacuum.

The IOP is actually 765 + 25 mmHg = 790mmHg A.

What we really measure the delta P between IOP and POP at the cornea! A better way to express the current IOP is "delta IOP" or "∆ IOP"
Alternatively we may value IOP (for above example) at 790mmHg A. The capital A stands for absolute pressure.
Or we may say the IOP is 25mmHg G which stands for gauge.

Actually the term mmHg as a unit of pressure should be replaced by Pascal under the SI system. One millimeter of Hg pressure is equivalent to approximately 133, 3 Pascal. Our example of 25mmHg is 3333 Pa G, please check for further conversion tables.
As I have mentioned in my previous blog, 25 mmHg is such a low pressure to measure, 25 mmHg is identical to 0.48 PSI.
Normally your car tyres should be around 30 PSI. The unit of measure is very small and it requires very good instruments to measure.

Till next tech talk :-)


Subscribe to RSS Feed