The anti-nociceptive and cardiopulmonary effects of extradural fentanyl-xylazine in sheep.

OBJECTIVE

To compare the anti-nociceptive effects of extradural xylazine, fentanyl and a xylazine-fentanyl combination in sheep, and to measure the cardiopulmonary effects of the xylazine-fentanyl combination.

STUDY DESIGN

Prospective, randomized study.

ANIMALS

Twenty-five half-merino ewes 2-4 years of age and body mass 54.2 ± 1.1 kg.

METHODS

Six sheep in group 1 received 0.2 mg kg xylazine by extradural injection, six in group 2 received fentanyl 1.5 μg kg and 13 in group 3 received the combination of both treatments. In all groups, drugs were mixed with saline (0.15 mL kg before injection). Pulmonary and carotid arterial catheters were placed in seven sheep of group 3 which were used to evaluate cardiopulmonary effects. Anti-nociception was determined by the response to electrical stimulation (40 V for 1.5 milliseconds) of the left flank and by superficial and deep muscular 'pinpricking' stimulation of the pelvic and thoracic limbs and thoracolumbar region.

RESULTS

Lack of response to electrical stimulation at the left flank was present in 10 ± 1.1 minutes (mean ± SEM) (group 1) and in 4.5 ± 0.5 minutes in group 3. The duration of lack of response to electrical stimulation at the left flank was 96 ± 6 minutes in group 1 and 315 ± 6 minutes in group 3. Responses persisted in group 3. Significant decreases (p < 0.05) in cardiac output 30, 45, 60 and 90 minutes after injection, and in cardiac work at 30 and 45 minutes were observed in the seven animals of group 3. Arterial blood pH was lowest at 90 minutes, arterial bicarbonate was lowest at 60 minutes and values for both arterial and mixed venous base excess increased significantly at 60 and 90 minutes. There was no significant change from baseline values in heart rate, mean arterial blood pressure, respiratory rate, body temperature, systemic vascular resistance, arterial and mixed venous PO, PCO, oxygen saturation, blood oxygen content, haemoglobin concentration, mixed venous blood bicarbonate and pH.

CONCLUSIONS

Fentanyl decreases the onset time and prolongs the duration of anti-nociception produced by xylazine. The combination decreases cardiac output but is without significant respiratory effects.

CLINICAL RELEVANCE

Further studies are required to show that surgery is possible in sheep after extradural xylazine-fentanyl injection.