The effect of intravenous hydromorphone alone or in combination with midazolam or dexmedetomidine on intraocular pressure in dogs

Objective

To evaluate the effect of intravenous (IV) hydromorphone alone or in combination with midazolam or dexmedetomidine on intraocular pressure (IOP) in dogs.

Study design

Prospective, randomized, blinded, crossover study.

Animals

A group of seven healthy, ophthalmologically normal, adult Beagle dogs.

Methods

A total of four IV drug combinations were evaluated: hydromorphone 0.1 mg kg^–1 (H); hydromorphone 0.1 mg kg^–1 and dexmedetomidine 0.001 mg kg^–1 (HD); hydromorphone 0.1 mg kg^–1 and midazolam 0.2 mg kg^–1 (HM2); and hydromorphone 0.1 mg kg^–1 and midazolam 0.4 mg kg^–1 (HM4). Treatment order was randomized, with a 2 week washout period between treatments. IOP and sedation scores were obtained before (T0) and 3, 30, 60, 240 and 480 minutes after drug injection. To account for repeated measurements for each dog across treatments and time points, mixed models were used to compare IOP at T0 by eye and to describe changes from T0 in IOP (averaged across eyes) and sedation scores.

Results

In treatment H, IOP increased significantly from baseline levels [predicted mean increase of 5.5 mmHg [95% confidence interval (CI): 3.7–7.3] at T3 (p < 0.001) and 2.7 mmHg (95% CI: 0.9–4.5) at T30 (p = 0.005)]. In treatment HD, mean IOP increased from baseline by 2.3 mmHg (95% CI: 0.5–4.1) at T30 (p = 0.014). In treatment HM2, mean IOP increased by 2.5 mmHg (95% CI: 0.2–4.9) at T30 (p = 0.035). In treatment HM4, IOP did not change significantly from baseline at any time point. Sedation scores over time did not differ significantly between treatments.

Conclusions and clinical relevance

Injection of IV hydromorphone alone (0.1 mg kg^–1) caused a transient increase in IOP and might not be appropriate if an acute increase in IOP is undesirable. Addition of dexmedetomidine or midazolam to hydromorphone, at the doses studied, appears to attenuate this increase in IOP.