Factors affecting outflow facility calculations

Abstract

Outflow facility (C), a ratio of outflow rate (F) to relevant pressure, is an important indication of outflow resistance in the ocular drainage pathway. The higher the C, the lower the outflow resistance. According to the classic Goldmann equation, C=F/(IOP-Pe)(IOP, intraocular pressure; Pe, episcleral venous pressure). Since anterior chamber (AC) perfusions directly measure outflow and are less affected by scleral creep and rigidity or changes in uveal blood content than repeated tonography in living eyes, they are widely used to determine C in living animals or cultured anterior segments from enucleated eyes. Barany (1964) initially developed the 2-level constant pressure perfusion technique for living monkey eyes to avoid possible influences of changes in eye parameters (e.g., Pe, spontaneous IOP [P0] or aqueous humor formation [AHF]) on C measurements. However, since these eye parameters do not exist in cultured anterior segments, 1-level constant pressure perfusion (Erickson-Lamy et al., 1991) and constant flow rate perfusion (Johnson and Tschumper, 1987) are typically used to determine C in organ culture. Based on different perfusion techniques, two basic formulas derived from the Goldmann equation are used to calculate C: (A) C=(F2-F1)/(P2-P1), which is usually used in 2-level constant pressure perfusion either in living animals or in cultured anterior segments (F1 or F2 represents F at P1 [lower perfusion pressure] or P2 [higher perfusion pressure]); (B) C=F/P, which is usually used in 1-level constant pressure perfusion or constant flow rate perfusion in cultured anterior segments (P, perfusion pressure; Pe is ignored in this formula because it is assumed to be zero in the cultured anterior segments). A comparison of C values calculated using Formula B and the data obtained during 2-level constant pressure perfusion in vivo or in vitro requires the following modifications: C=F1/(P1-P0) or C=F2/(P2-P0) where F1 or F2 is due to the change in pressure from P0 to P1 or P2 (P0 is assumed to be stable during short-term perfusions in vivo and is zero in vitro). C values from the same eye and the same perfusion are often quite different when calculated by the two formulas, since Formula A calculates C for the change in pressure from P1 to P2, whereas Formula B calculates C either for the change in pressure from P0 to P1 or P2 in living eyes or for a given pressure (P1 or P2) in cultured segments. However, the difference has not been clearly stated in previous studies, so that confusion might occur when comparing C values obtained by the two formulas. Additionally, C at different pressures may also be affected by factors physiologically unrelated to the formulas. To determine factors that may affect C values and to evaluate the significance of these factors in aqueous humor dynamics research, we compared C values calculated by Formulas A and B based on some previous perfusion experiments.