Tono-Pen XL calibration curves for cats, cows and sheep

Abstract

The objective of this study was to provide calibration curves for correcting intraocular pressure (IOP) measurements obtained using the Tono-Pen XL tonometer in cats, cows and sheep. Twelve eyes from 9 cats, 13 eyes from 7 cows, 10 eyes from 5 sheep were used. The anterior chamber of the eye was cannulated in vivo, in situ (immediately post mortem) or ex vivo with a fine needle and IOP was varied from 10 to 90 mmHg in steps of 10 mmHg by adjusting the height of a saline reservoir connected to the needle. For each pressure setting, several readings of IOP were made using the tonometer. The relationship between Tono-Pen reading and manometer setting was linear over the full range of measurement. However, the slope of the data regression line deviated significantly from 1 and indicated that the instrument systematically underestimated IOP. For cats the average slope was 0.62 and for cows and sheep it was 0.72 and 0.69, respectively. For the latter animals, the regression line also had a nonzero intercept of approximately 4.5 mmHg. Similar results were obtained from in vivo and ex vivo eyes and with different Tono-Pen XL tonometers. Although developed for use on humans, the Tono-Pen XL can provide reproducible and accurate measurement of IOP in cats, cows and sheep when suitably calibrated by manometry. The calibration curves provided here, and by implication those reported for other animals using this tonometer, differ in slope from those measured with earlier models of the Tono-Pen. The reproducibility of the curves we obtained implies that they can be used to correct IOP readings from the Tono-Pen XL when manometry is not possible.

Figure 1.

Schematic diagram of the experimental setup. A 25-gauge needle was inserted into the peripheral cornea of one eye. Connected to the needle via tubing were a mercury manometer and a saline reservoir. By varying the height of the reservoir IOP could be lowered or raised. Care was taken to remove air bubbles from the tubing and to position zero pressure on the manometer level with the center of the eye (dotted line).

Figure 2.

Tono-Pen XL readings of IOP as a function of manometric pressure setting for an in vivo (A) and ex vivo (B) cat eye. Measurements on the excised eye were taken at 24 hrs post mortem. Thick lines are linear regression fits of the data. The intercept and slope of the regression line are provided for each eye. Thin lines with a slope of one indicate what an appropriately calibrated tonometer would read. r² = correlation coefficient.

Figure 3.

Tono-Pen XL readings of IOP from excised cow and sheep eyes. The pressure readings were taken using our tonometer (tonometerA) and a loaner tonometer (tonometerB). Thick lines are linear regression fits of the data. All measurements were taken within one day of enucleation. r² = correlation coefficient.