Trabecular Outflow Facility and Formation Rate of Aqueous Humor During Intravenous Cocaine or Lidocaine in Rabbits

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Abstract
This study examined the effect of cocaine and lidocaine on intraocular pressure (IOP), fluid formation, outflow facility, and compliance. Fourteen New Zealand White rabbits were anesthetized with halothane (0.8–1.0% inspired concentration) in nitrous oxide (2 L/min) and oxygen (1 L/min). Pancuronium was given intravenously (IV) to produce neuromuscular block, and the lungs were mechanically ventilated through a tracheal tube. In one group (n = 8) cocaine 0.5 mg/kg followed by 1.0 μg·kg−1.min−1 IV was added to halothane/ nitrous oxide/oxygen anesthesia. In the second group (n = 6) lidocaine 0.5 mg/kg followed by 1.0 μg·kg−1.min−1 IV 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, trabecular outflow facility, and intraocular compliance were determined. In Group I, IOP was 15.0 ± 6.3 mm Hg prior to and 14.5 ± 7.1 mm Hg after cocaine. The rate of anterior chamber aqueous formation was 3.79 ± 1.25 μL/min and trabecular outflow facility was 0.141 ± 0.090 μL·min−1.mm Hg−1. In Group 2, IOP was 14.4 ± 4.8 mm Hg prior to lidocaine and IOP decreased (P < 0.05 compared to precocaine in Group 1 and prelidocaine in Group 2) to 11.9 ± 6.8 mm Hg after lidocaine. The rate of anterior chamber aqueous formation was 2.59 ± 0.86 μL/min and trabecular outflow facility decreased (P < 0.05 compared to Group 1) to 0.077 ± 0.040 μL·min−1.mm Hg−1. Intraocular compliance during cocaine, 36 ± 12 nL/mm Hg, was similar to that during lidocaine, 33 ± 14 nL/mm Hg. It is concluded that cocaine does not significantly change IOP, in part because cocaine does not significantly change the rate of anterior chamber aqueous formation or trabecular outflow facility. In contrast, lidocaine decreases IOP even though it does not significantly change the rate of anterior chamber aqueous formation and it decreases trabecular outflow facility, (an effect which favors increased IOP). Considering the factors known to determine IOP, lidocaine-induced decrease of IOP is presumed to result from decreased choroidal blood volume and/or extraocular muscle tone.

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