Abstract
Purpose: :
The intraocular pressure (IOP) is determined by a dynamic equilibrium between the production and the outflow of the aqueous humor. The dynamics of this equilibrium and particularly the dependence of outflow rate on IOP may be related to the biomechanical properties of the eye. The relationship between the intraocular pressure and the outflow rate can be described by the outflow facility coefficient. The purpose of this study is to determine the outflow facility as a function of IOP for the living human eye using an invasive manometric device.
Methods: :
Fifty four eyes from fifty four cataract patients (24 men and 30 women, aged 63 years, sd 13) were enrolled in the study. The study was approved by the Institutional Review Board. An intraoperative invasive manometric device was used to measure the intraocular pressure (IOP). The measurements were performed before cataract surgery. The anterior chamber of the eye was cannulated to a microstepping pump and the pressure was recorded by a pressure transducer through special developed computer software. The IOP was artificially increased to 40 mmHg by infusion steps of known volume of BSS in the anterior chamber of the eye. At 40 mmHg the infusion stops and the sensor reads the IOP decay curve over time. The pressure volume relation was taken in order to calculate the outflow facility coefficient for various pressures. An appropriate mathematical model was developed to calculate the outflow facility coefficient.
Results: :
The average outflow facility coefficient was 0.328 (SD 0.093) µl/min/mmHg. From the data analysis the outflow facility coefficient was proved to have a non linear correlation with outflow IOP, especially in the range of higher IOPs. This non linear behavior of outflow facility was described by an exponential mathematical model. At narrow range of IOPs the outflow facility can be described by Brubaker’s equation for outflow resistance (the outflow resistance is the inverse of outflow facility). The average rigidity coefficient K was of 0.0211 (sd 0.0045) µl-1 and the mean axial length was 24.5 (sd 2.56) mm. There is a negative correlation between rigidity and axial length (r=-0.603, p=0.000).
Conclusions: :
The invasive method of measuring in vivo outflow facility coefficient employed in this study, avoiding the errors of tonography may provide accurate data on the dependence of outflow facility coefficient on IOP. These data, in conjunction with ocular rigidity measurements may facilitate our understanding of the relationship between the biomechanical properties of the eye and the anatomical changes in the trabecular area at elevated IOP levels.
Keywords: outflow: trabecular meshwork • aqueous • intraocular pressure