On the Consistency of Tonography

¹ Department of Ophthalmology, State University of Iowa, and the University Hospital, Iowa City, Iowa

Sixty tonograms of 10 to 40 minutes' duration were analyzed by calculating the outflow facility estimate obtained at each successive minute and during the first 4 minutes with the use of Grant's tonographic equation. The tonographic tracing was found to be a negative exponential function, which, only rarely, is appropriately approximated by a straight line in the first 4 minute segment. It is best delineated by a smooth curve passing through the majority of points in the tracing.

This study showed that the 4 minute estimate of outflow facility is a good representative of similarly calculated estimates of outflow facility prevailing during each individual minute. It also showed that within this framework no modification or alteration can be expected by prolonging tonography for periods longer than 4 minutes, and that such modification should be sought in the basic framework of tonography.