Relationship of aqueous outflow resistance to age and total volume perfused in rhesus and cynomolgus monkeys.

Purpose: The effect of total volume perfused on outflow resistance (the reciprocal of outflow facility) and the effect of age on the rate of change in resistance as a function of total volume were determined in rhesus and cynomolgus monkeys.

Methods: Outflow facility was measured under general anesthesia by two-level constant pressure perfusion in one eye of 22 rhesus and 17 cynomolgus monkeys (ranging in age, respectively, from 4 to 25 and from 3 to 12 years). Total volume perfused was calculated from data obtained during the perfusion.

Results: Resistance decreased in both cynomolgus and rhesus monkeys as total volume perfused increased (-0.085 ± 0.021 and -0.022 ± 0.011 mm Hg/μL/min/μL(tot); P = 0.001 and P = 0.047, respectively). Rate of change in resistance significantly increased in cynomolgus monkeys as total volume perfused increased (0.0018 ± 0.0.0007 mm Hg/μL/min/μL(tot), P = 0.033); however, this was not the case in rhesus monkeys. After accounting for total volume perfused, the rate of change in resistance significantly decreased with increasing age in rhesus monkeys (-0.0068 ± 0.0026 [mm Hg/μL/min]/μL(tot)/y, P = 0.017). There was no significant difference in rate of change in resistance with age, after accounting for total volume, in the cynomolgus monkeys.

Conclusions: The present study supports previous findings indicating that total washout is largely dependent on perfusion volume. However, in populations with old/elderly animals, such as our rhesus group, we found that age does play a significant role in rate of change in resistance, and may be an even more important factor to consider in the rate of resistance change than volume perfused in aged animals.