Trabecular Outflow Facility and Formation Rate of Aqueous Humor During Propofol, Nitrous Oxide, and Halothane Anesthesia in Rabbits

Propofol decreases intraocular pressure (IOP) and the IOP response to laryngoscopy and intubation, but the mechanisms responsible for this effect have not been reported. The present study examined the effect of propofol on IOP, intraocular fluid formation and outflow facility, and intraocular compliance. Twenty-two white New Zealand rabbits were anesthetized with halothane (0.8%–1.0% inspired concentration) in nitrous oxide (2 L/min) and oxygen (1 L/min). Muscle paralysis was established with pancuronium, and the lungs were mechanically ventilated through a tracheal tube. Twelve rabbits examined under these conditions served as controls. In the treatment group (n = 10), 6 mg/kg propofol followed by 18 mg-kg−1-h−1 propofol intravenously was added to halothane/nitrous oxide/oxygen anesthesia. In both groups, a series of intraocular infusions was made via a 30-gauge needle in the anterior chamber, and IOP, the rate of aqueous humor formation (Fa), and trabecular outflow facility (Ctr) were determined using conventional analysis. These same measures, as well as intraocular compliance, were determined using a new method of analysis adapted from the manometric technique for determining cerebrospinal fluid dynamics. IOP was 11.3 ± 1.8 mm Hg (mean ± SD) in halothane-anesthetized controls and decreased to 9.4 ± 2.2 mm Hg when propofol was added to halothane anesthesia (P < 0.05). By conventional analysis, Fa was 2.82 ± 0.94 μL/min and Ctr was 0.121 ± 0.044 μL-min−1 mm Hg−1 in controls. After addition of propofol, Fa decreased by 24% to 2.15 ± 0.62 μL/min (P < 0.05) and Ctr decreased by 18% to 0.099 ± 0.034 μL-min−1 mm Hg−1 (P < 0.05). In both groups, values for Fa and Ctr were about 30% lower with the new method of analysis than with the conventional method. With the new method of analysis, intraocular compliance in controls, 183 ± 86 nL/mm Hg, was similar to that in the propofol group, 186 ± 113 nL/mm Hg. We concluded that adding intravenous propofol to halothane anesthesia decreases IOP to values lower than those seen with halothane, and that the decrease of IOP seen with propofol occurs in part because propofol decreases Fa to greater extent than it decreases Ctr.