Analgesic, behavioral, and hemodynamic and respiratory effects of midsacral subarachnoidally administered ropivacaine hydrochloride in mares

Abstract

ObjectiveTo determine the analgesic, behavioral, hemodynamic and respiratory effects of midsacral subarachnoid administration of ropivacaine hydrochloride solution in mares. Study designRandomized, blinded study. AnimalsTen healthy mares, weighing from 470 to 560 kg. MethodsIntravascular and subarachnoid catheters were placed after infiltration of the skin and subcutaneous tissues with 2% lidocaine. Ropivacaine (0.2%, 5 mL) or 0.9% NaCl was then administered subarachnoidally at the midsacral (S2–S3) vertebrae. Analgesia was determined by lack of sensory perception to electrical stimulation (>40 mA) and absence of response to needle pricks extending from coccygeal to S1 dermatomes. Numerical scores of sedation, change in pelvic limb position, sweating in analgesic zones, urination, behavior, response to noise, and compliance with restraint were determined. Two-way anova with repeated measures and Dunnett's t-tests were used to evaluate differences between the listed numerical scores, and cardiovascular and respiratory variables before and during a 5-hour testing period. ResultsSubarachnoidally administered ropivacaine-induced variable analgesia extending bilaterally from the coccyx to S1, with minimal sedation and change in pelvic limb position in standing mares. Perineal analgesia was attained at 7.5 ± 2.6 minutes and lasted for 218 ± 44 minutes (mean ± SD). Subarachnoid ropivacaine significantly reduced respiratory rates and did not change heart rate, rectal temperature, arterial blood pressure, PCV, arterial gas tensions (PaO2 and PaCO2), pH, and arterial standard bicarbonate and base excess from baseline. Conclusion and clinical relevanceRopivacaine (0.2% solution, 5 mL 500 kg−1) can be administered subarachnoidally at midsacral (S2–S3) vertebrae to produce prolonged (>3 hours) bilateral perineal analgesia with minimal changes of behavior, and circulatory and respiratory disturbances in standing mares.