Purpose: The purpose of this study was to evaluate the Tono-Pen 2 tonometer for measuring intraocular pressure (IOP) in the living rat eye.
Methods: One eye from each of 20 adult, anesthetized brown Norway rats (group 1) was cannulated and simultaneously connected to a syringe and a pressure transducer with a chart recorder. We increased IOP from 15 to 45 mmHg in 5-mmHg increments and obtained 15 consecutive readings (ignoring instrument-generated averages) at each pressure increment with a Tono-Pen 2 tonometer. To test the tonopen's ability to measure unknown IOP, transducer pressures were varied randomly in 20 additional animals (group 2), and tonopen readings were obtained in masked fashion.
Results: Plotting the mean tonopen readings for each animal against transducer IOP produced a regression formula of y = 4.54 + 0.79x (r = 0.98). Mean group 2 tonopen values plotted against transducer IOP yielded a regression formula of y = 4.75 + 0.78x (r = 0.94). A method comparison analysis showed that the tonopen significantly overestimates pressures at low IOP (< or = 15 mmHg), and it significantly underestimates pressures at high IOP (> or = 30 mmHg). Using two-way analysis of variance, it was determined that the group 2 data did not differ significantly from the group 1 data (P > or = 0.76). Because of this consistency, we generated a correction factor with 95% prediction intervals for Tono-Pen readings.
Conclusions: The Tono-Pen 2 can be used reliably to measure IOP in the normal rat eye.