The goal of the presented research was to assess the influence of continuously administered fentanyl on the intraocular pressure, pupil size and aqueous tear production in dogs. A prospective, randomised, double “blind” clinical study was performed. Twenty-five non-painful dogs, 13 breeds, a body weight of 10.0 ± 5.4 kg (mean ± SD) and age of 6.5 ± 3.3 years, 12 males and 13 females with no ocular abnormalities were randomly allocated into two groups receiving an intravenous injection of saline (SAL) 0.3 ml/kg followed by an infusion 2 ml/kg/h or an intravenous injection of fentanyl (FEN) 0.005 mg/kg (diluted in 0.3 ml/kg) followed by an infusion 0.005 mg/kg/h (diluted in 2 ml/kg/h). The intraocular pressure (IOP), pupil size (PS), pulse rate (PR), respiratory frequency (f_R) and systolic and diastolic arterial pressures (SAP, DAP) were measured before (baseline) and at 2, 5, 10, 20 and 30 minutes after the premedication. The Schirmer Tear Test I (STT-I) was measured prior to and at 30 min after the premedication. The data were analysed by Bartlett’s, Anderson-Darling and Dunnett’s tests, the t-test and an analysis of variance (ANOVA) (P < 0.05). Relative to the baseline, in the fentanyl group, the PS was significantly decreased at all time points, the PR was significantly decreased at T₃₀ and the f_R was significantly decreased at T₅, T₁₀, T₂₀ and T₃₀. There were no other significant changes in the IOP, STT-I, SAP and DAP relative to the baseline. Compared to the control group, in the fentanyl group, the PS was significantly smaller at T₂, T₅, T₁₀, T₂₀ and T₃₀, the PR was significantly lower at T₂, T₂₀ and T₃₀ and the f_R was significantly higher at T₂₀. Within thirty minutes of a constant rate infusion of fentanyl in the healthy non-painful dogs, the intraocular pressure and aqueous tear production were not affected. However, the fentanyl significantly decreased pupil size. This fact should be considered, when planning analgesia where miosis is undesirable.

Fifty-five healthy conscious dogs were included in a prospective randomised double-blinded clinical study. The dogs allocated to one of four groups received intravenous bolus followed by infusion of fentanyl (FEN-group), or ketamine (KET-group), or lidocaine (LID-group), or saline (SAL-group). The intraocular pressure (IOP), pupil size (PS), heart rate (HR) and mean arterial pressure (MAP) were measured prior to and at 2, 5, 10, 20 and 30 min after initiation of the drug administration. The data were analysed using an analysis of variance and the Steel-Dwass test. No significant difference in the IOP within or between the groups was detected. In the FEN-group, the PS decreased significantly at all the measured times. In the KET-group, the PS increased significantly at 2, 5 and 10 minutes. The PS was significantly smaller in the FEN-group compared to the KET-group at 2, 5, 10 and 20 min, compared to the SAL-group at 5, 10, 20 and 30 minutes. In the FEN-group the HR significantly decreased compared to the baseline and was significantly lower compared to the KET-group and LID-group. Fentanyl, ketamine or lidocaine administered at the doses studied as a bolus followed by a 30-min infusion seem to cause no effect on the IOP in healthy conscious non-painful dogs without ocular abnormalities. Fentanyl decreased and ketamine transiently increased the PS.

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