Calibration of the TonoVet and Tono-Pen Vet tonometers in the porcine eye.

OBJECTIVE: The pig has an increasingly important role in ocular drug delivery models, but the most accurate tonometer in this species is unknown. The purpose of this study was to evaluate the accuracy of TonoVet and Tono-Pen Vet tonometers in the ex vivo porcine eye.

PROCEDURE: Four freshly enucleated normal porcine eyes were cannulated with two 25-gauge needles; one connected via tubing to a mercury manometer calibrated continuous physiologic recorder and the other connected to a reservoir of lactated Ringer's solution on an adjustable stand. Triplicate IOP readings were taken with the TonoVet and then the Tono-Pen Vet at 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80 mmHg.

RESULTS: Linear regression showed strong linear trends for both the TonoVet (r2 = 0.969) and Tono-Pen Vet (r2 = 0.983). The TonoVet slightly underestimated IOP at lower pressures and slightly overestimated IOP at higher pressures (y = 1.092x - 4.0, where y = tonometer reading, x = manometer reading, and 4.0 = intercept). The Tono-Pen Vet consistently underestimated IOP (y = 0.773x - 2.1). These differences were statistically significant (P = <0.001, one-way repeated-measures ANOVA).

CONCLUSION: As in other species, both the TonoVet and Tono-Pen Vet tonometers do not measure true IOP in the porcine eye; however, the TonoVet more closely approximated true IOP in the normal porcine eye than the Tono-Pen Vet and may be the tonometer of choice for this species.