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S.K. Charisis, H.S. Ginis, G.A. Kounis, M.K. Tsilimbaris; The Effect Of Vacuum Ring Application On Iop In The Living White Rabbit Eye . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3590.
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Although it is known that application of a vacuum ring on the limbal area increases intraocular pressure (IOP) transiently, very limited knowledge exists on the magnitude of this pressure rise. Moreover the accuracy of contact tonometers during the increased stress conditions of the cornea associated to the vacuum ring is uncertain. It is the purpose of this study to validate and calibrate a commercial tonometer (TONO–PEN XL) for use in the rabbit eye and moreover to evaluate its accuracy at the presence of a suction ring.
Tonometer calibration was performed on ten eyes of ten anesthetized albino rabbits. An 22–gauge intravenous catheter needle connected to the pressure transducer and a saline column was inserted in the anterior chamber through the paracentral cornea. IOP was adjusted hydrostatically by means of the saline column and verified with the transducer. IOP was set in a range of values from 10 to 70 mmHg in steps of 10mmHg. Each known IOP was measured eight times with the TONO–PEN XL. In a separate series of measurements involving 10 eyes of ten different albino rabbits pressure was regulated at 30 and 60 mmHg using the previously described method and additionally a purposely constructed vacuum ring was applied to the measured eye. Vacuum pressure ranged from 350 to 650mmHg. IOP measurements using the tonometer were taken at each vacuum setting. Central corneal pachymetry was measured in all eyes by means of an ultrasound pachymeter (Sonogage Corneo–Gage Plus).
For IOP set in the range between 10 and 70mmHg the tonometer readings correlated well with the IOP ( IOP=1.3472*Tono_reading –7.0333, R2=0.9855). During vacuum ring application no statistically significant correlation between the measured and the true IOP was observed. Moreover, the error on the measured IOP did not correlate with the vacuum pressure or central corneal pachymetry.
The tonometer employed in this study (TONO–PEN XL) can provide reliable IOP measurements in the rabbit eye provided that a conversion formula is used in order to compensate for the anatomical differences between the human and rabbit cornea. Once a suction ring is applied the accuracy of the tonometer is dramatically reduced. This can be attributed to the increased corneal stress associated with the vacuum ring. This observation may be of particular importance for the development of instruments incorporating a vacuum ring.
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