A comparison of the haemodynamic effects of epidurally administered medetomidine and xylazine in dogs

ABSTRACT

Alpha₂ agonists have a significant role in epidural anaesthetic techniques. However, there are few reports regarding epidural administration of these drugs especially in small animals (Greene et al. 1995; Keegan et al. 1995; Vesal et al. 1996). This study compared the haemodynamic effects of xylazine and medetomidine after epidural injection in dogs.

Six dogs (four females and two males) weighing 27.5 ± 3.39 kg, aged 5.6 ± 1.42 years were studied on two separate occasions one month apart. Dogs were sedated with 0.5 mg kg⁻¹ diazepam IM and 0.1 mg kg⁻¹ acepromazine IM. After 20 minutes, a lumbosacral epidural injection of 0.25 mg kg⁻¹ xylazine was administered (group X). One month later, following the same sedation, 15 µg kg⁻¹ medetomidine was administered epidurally (group M). Haemodynamic variables (ECG and indirect blood pressure (Doppler)), respiratory rate and rectal temperature were recorded before (baseline) and then every 5 minutes after the epidural injection, up to 60 minutes. Differences between groups were compared by a paired t-test. Within group changes were compared to basal values by anova. A p-value of < 0.05 was considered statistically significant.

Both groups showed significant reductions in heart rate (106.3 ± 7.7 beats minute⁻¹ baseline versus 67.7 ± 7.6 (group M); 91 ± 3.8 baseline versus 52.3 ± 9 (group X)) and mean arterial blood pressure (113.1 ± 12.3 mm Hg baseline versus 87 ± 11 (group M); 118 ± 7 baseline versus 91 ± 14 (group X)). There were no differences between groups in these variables. After epidural injection, first degree atrioventricular block was recorded significantly more often in group X (50% against 33%) but second degree block was significantly more frequent in group M (66% against 33%). Also 50% of dogs in group X and 66% in group M showed sinus arrest. Respiratory rate decreased significantly in both groups following the epidural injection (20.66 ± 0.66 minute⁻¹ baseline versus 16.33 ± 4.77 (group M); 37.66 ± 0.56 baseline versus 16.33 ± 1.81 group X), but no differences between groups were observed. Rectal temperature decreased significantly in group X (38.16 ± 0.21) with respect to the basal measurement (39.30 ± 0.14 °C). In group M, there was no significant reduction in temperature, however, no statistical difference in rectal temperature was found between groups.

This study shows that 0.25 mg kg⁻¹ xylazine and 15 µg kg⁻¹ medetomidine produce similar, significant cardiovascular and respiratory changes following lumbosacral epidural administration in dogs.