Outflow facility was measured by using a pneumatonometer with a tonography option. The subjects were placed in a supine position. Subjects were then asked to breathe normally and fixate on a target on the ceiling approximately 2 m from their eyes during the procedure. The eyelids were gently retracted by the examiner's fingers without applying additional pressure on the globe. The corneas were anesthetized by instilling proparacaine 0.5% topically. The pneumatonometer probe (tip diameter of 5.3 mm) with an added 10-g weight (
Fig. 1) was placed on the center of the cornea and held perpendicularly to the corneal surface. Intraocular pressure was recorded for 2 minutes and the probe was removed from the eye. The right eye was always measured before the left eye.
Each 2-minute pressure tracing was printed by the pneumatonometer (
Fig. 2), scanned, and digitized by using CurveSnap 1.1 (
Fig. 3;
http://xoofee.com/2012/12/curvesnap/). A second-order polynomial line was fitted to the pressure curve by least-squares regression and the IOP at the beginning (
P1) and end of the interval (
P2), while the eye was exposed to the weighted probe, was determined from the value of the line at 0 and 2 minutes. The outflow facility coefficient,
C, was determined by using Grant's formula
9:
where Δ
Vs is the intraocular volume change from relaxation of tension in the sclera and corresponding to the pressure change during tonography, Δ
Vc is the change in intraocular volume by corneal indentation,
P0 is pressure in the supine position,
Pc is a correction of the steady-state pressure, and
t is the duration of the contact, typically 2 or 4 minutes. Langham et al.
10 have estimated that
Pc is equal to −4 mm Hg because of an assumed indentation effect by the pneumatonometer, while Friedenwald
11 has estimated a value of 1.25 mm Hg to account for episcleral venous pressure change during Schiøtz tonography, based on the measurement made by Linner.
14
The relationship between pressure and indentation volume is based on the value of ocular rigidity for each instrument. A constant value for ocular rigidity is implicit in the tables by Langham et al.
10 for pneumatonography (0.0126 μL
−1) and Friedenwald
11 for the Schiøtz tonometer (0.0215 μL
−1). In addition, we calculated the ocular rigidity coefficient (
K) from measurements of pressure without and with the weight added to the pneumatonometer probe and by using the equation of Friedenwald
11:
where
V1 and
V0 are the volumes indented in the eye by the tonometer when the pressure was
P1 and
P0, respectively. For pneumatonography, these volumes were extracted from data tabulated by Langham et al.
10