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J.H. K. Liu, R. Li, T.R. Nelson, R.N. Weinreb; Study Retrobulbar Blood Flow in Rabbits After Topical Treatment With Timolol Using Color Doppler Imaging . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4721.
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Purpose: Color Doppler imaging (CDI) of the retrobulbar orbit allows real–time assessment of blood circulation in individual vessels. Using this technique, we studied the resistance of blood flow in the posterior ciliary artery in laboratory rabbits after topical treatment with timolol, a beta–adrenergic antagonist. Methods: Eight adult New Zealand albino rabbits received a series of 20 microlitter topical timolol (vehicle, 0.01%, 0.1%, 0.33%, and 1%) on the right eye between 9 AM and 10 AM. Measurements of intraocular pressure (IOP) were taken in both eyes using a pneumatonometer before the treatment and at 0.5 hr, 1 hr, 1.5 hr, 2 hr, and 3 hr after the treatment. At the same time points, color Doppler images of the posterior ciliary artery were captured in the right eye using a 12 MHz linear transducer (GE Logiq 9 ultrasound system). The angle between the ultrasound beam and the posterior ciliary artery was less than 20 degrees. Blood flow resistive index was calculated using the peak systolic velocity and the end diastolic velocity and the corresponding heart rate was recorded. A control experiment was performed with CDI data collected from the right eye after the left eye was treated with 1% timolol. Repeated–measures ANOVA and post–hoc Bonferroni t–test were used to analyze data obtained from different time points. Results: There was no change of IOP between the treated eye and the untreated eye in all cases. At doses of 0.1%, 0.33%, and 1%, topical timolol caused a significant reduction in heart rate. An increase in resistive index was observed in the right eye at 0.5 hr and 1 hr after topical treatment with 1% timolol in the right eye. However, there was no change of resistive index in the right eye after the left eye was treated with 1% timolol. Bradycardia occurred when either the right eye or the left eye was treated with 1% timolol. Conclusions: Topical treatment with 1% timolol in rabbits causes a short–term increase of resistive index in the posterior ciliary artery, indicating a decrease of retrobulbar blood flow. This effect is caused by a local mechanism, and is unrelated to the physiological reactions due to systemic absorption of timolol, such as reduced heart rate.
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