Fig. 9.
Tonography example. A tonogram from a patient with glaucoma. The initial scale reading of 4.5 with a 7.5-g weight means that the P0 (the resting pressure in the eye before applying the tonometer) was 28. The Pt (the increased pressure in the eye induced by Schiøtz tonometry) was initially 44.4 and fell gradually to 39.5 over 4 minutes, yielding an average pressure during tonograghy (Ptav) of 41.9. The increased flow induced by this extra pressure is the sum of the increased corneal indentation volume occurring during this period (Vcc) plus the decreased scleral distention volume (Vs) occurring simultaneously: in this instance, 2.6 (Vcc) + 2.4 μL (Vs) = 5 μL total volume change. Grant's Equation for C, the Facility of Aqueous Outflow Where ΔV is the volume change in μL and T is time in minutes—usually 4. Substituting, for this case, As here, 1.25 is usually added to F0 to compensate for the small increase in episcleral venous pressure induced by tonography. The P0/C ratio for this eye is 28/0.10, or 280. Stated another way, under the influence of an average increased IOP of 12.65 mmHg (induced by applying a Schiøtz tonometer with a 7.5-g weight), an extra 5 μL of aqueous humor was forced through the outflow channels during a 4-minute period. This reflects a low aqueous outflow.
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