Intraocular pressure in anaesthetized dogs given flumazenil with and without prior administration of midazolam

This study examined the effect of flumazenil, a benzodiazepine antagonist, on aqueous humour pressure in dogs receiving either midazolam or no benzodiazepine. Twenty-four halothane-anaesthetized dogs were assigned to one of four groups. Group I (n = 6) received saline iv at 0, 45 and 90 min. Group 2 (n = 6) received saline at 0 min, flumazenil 0.0025 mg.kg-1 iv at 45 min and flumazenil 0.16 mg.kg-1 at 90 min. Group 3 (n = 6) received midazolam 1.6 mg.kg-1 at 0 min followed by continuous iv infusion (1.25 mg.kg-1.hr-1). Flumazenil was given at 45 and 90 min as in Group 2. In Group 4 (n = 6) aqueous humour pressure was elevated to about 35 mmHg then midazolam and flumazenil were given as in Group 3. Aqueous humour pressure was determined using a 30-gauge needle placed into the anterior chamber. Saline or flumazenil produced no change in aqueous humour pressure in Groups 1 and 2. In Groups 3 and 4, midazolam decreased aqueous humour pressure from 18 +/- 2 mmHg (mean +/- SD) to 14 +/- 3 mmHg (P less than 0.001) and from 34 +/- 5 mmHg to 31 +/- 3 mmHg (P less than 0.01) respectively. Flumazenil given during continuous infusion of midazolam produced increases of aqueous humour pressure of 2 +/- 1 (P less than 0.01) to 5 +/- 2 mmHg (P less than 0.01) that lasted less than or equal to 12 min. It is concluded that at both normal and elevated aqueous humour pressures flumazenil produces statistically significant but clinically unimportant increases of aqueous humour pressure in anaesthetized dogs receiving midazolam, but not in dogs given no benzodiazepine.