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D. M. Silver; Analysis of the Pneumatic Tonometer for Intraocular Pressure and Pulsatile Ocular Blood Flow Measurement. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4131.
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Capturing the magnitude and time variations of intraocular pressure (IOP) pulsations requires accurate and repetitive IOP measurement per pulse. This is necessary to enable a determination of pulsatile ocular blood flow (POBF) with high fidelity. The principle of the pneumatic tonometer involves a flowing column of air directed toward a thin membrane that is in contact with the surface of the cornea. The air stream exerts a force against the cornea that depresses its surface against the opposing force of the IOP. The efficacy of the pneumatic tonometer depends on the specific probe tip configuration and air flow. The purpose of the present analysis is to assess the validity of several pneumatic tonometer probe designs for measuring IOP.
Using geometry, thermodynamics, fluid mechanics and the principles of elasticity, mechanical models were constructed, consisting of a cornea and several pneumatic probe tip designs. Differential equations were solved for the elastic deflections of the cornea subject to the opposing intraocular and pneumatic forces.
The probe of a pneumatic tonometer consists of two concentric cylindrical hollow tubes. A critical element is the shape and position of the inner tube and its juxtaposition with the membrane against the cornea. The end of the inner tube was modeled as (1) flat, round or pointed and (2) coplanar or elevated a distance from the end of the outer tube. Over the range of these parameters, the numerical modeling simulated a probe operating either against a spherical corneal segment or against a flattened central corneal depression of ~150 microns. The pneumatic effect was found to be a stretching deformation of the elastic cornea of ~30 microns. The fluid mechanical analysis permitted the pressure in the upstream chamber of the pneumatic tonometer to be related through first principles to the IOP for each probe tip design.
The quantitative validation of the pneumatic tonometer is significant for providing a basis for calculating the average net POBF from IOP measurements using pneumatic tonometers with alternative probe tip designs.
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