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D. De Brouwere, H. S. Ginis, A. Dastiridou, G. Kounis, M. Tsilimbaris, I. Pallikaris; A Mathematical Model for the Dynamic Equilibrium of Intraocular Pressure Based on Manometric Measurement in the Living Human Eye. Invest. Ophthalmol. Vis. Sci. 2008;49(13):355.
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The Intraocular Pressure (IOP) is determined by a dynamic equilibrium of aqueous humor secretion and outflow. The outflow rate as a function of IOP is determined by the outflow facility coefficient. In this study, we employ a manometric instrument for in vivo measurement of the pressure-volume relationship (ocular rigidity) and outflow facility in the living human eye.
Fifty four cataract patients (60 eyes) were enrolled in the study. A previously described device for intraoperative IOP measurements consisting of a dosimetric pump and a pressure transducer was connected to the anterior chamber trough a 22gauge catheter. The measurement was held under topical anesthesia with a combination of proparacaine and lidocaine drops. After catheter insertion, the system was used to initialize the IOP to 15mmHg. Following, the pump infused saline in steps of 4ul steps to increase IOP to 45mmHg. In each step the IOP was recorded for a time interval of 2 seconds at a sampling rate of 200Hz. At 45mmHg, we registered the outflow by recording the IOP decay for 60 seconds. Rigidity was calculated by the volume-IOP relation in the stepping sequence. A mathematical model based on Friedenwald’s equations for rigidity and Brubaker’s equations for outflow facility was developed to describe the decay of the IOP in the outflow sequence. Outflow facility was obtained by fitting the outflow measurement sequence in the model. The study was performed under institutional board approval. An informed consent was obtained from each patient.
The average rigidity coefficient of Friedenwald was of 0.0137ul-1(SD=0.0058). We estimated the outflow facility to be 0.343 ul/min/mmHg (SD=0.215). The results are in correspondence with values for outflow facility obtained in the literature based on tonography and with the values for ocular rigidity based on Friedenwald’s studies. There is a linear correlation between ocular rigidity and outflow facility (R2=0.37).
We presented a mathematical model of the ocular volume-IOP relation and for the time decay of the IOP based on existing equations. We developed data acquisition and processing algorithms as well as measurement procedures suitable for the measurement of the above-mentioned parameters in the living human eye. These methods may provide intraoperative feedback to the surgeon during various anti-glaucomatic surgical procedures attempting to increase outflow facility.
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