Segmental thoracolumbar spinal (subarachnoid) analgesia in conscious horses

A new technique for producing segmental subarachnoid analgesia in adult horses without causing complete loss of locomotor control is described. A 17-gauge Huber point (Tuohy) needle was used to place a catheter with a stylet into the subarachnoid space at the lumbosacral intervertebral space in 13 adult horses (weighing 500 +/- 60 kg, representing both sexes) and to advance the catheter craniad to the thoracolumbar area. The position of the catheter was confirmed radiographically. A 2% mepivacaine hydrochloride solution (1.5 ml, 30 mg) was injected through the catheter at a rate of 0.5 ml/60 s. Segmental analgesia, as determined by the horses' responses to superficial and deep muscular pinpricks, occurred within (onset) 5 to 10 minutes of injection and lasted (duration) 42.4 +/- 15 minutes (min-max, 25 to 68 minutes). Maximal thoracolumbar analgesia extended from spinal cord segments T12 to L3 on both sides of the spinal column. Subarachnoid injection of mepivacaine hydrochloride caused significant (P less than 0.05) increases in heart rate and subcutaneous temperature and decreases in respiratory rate and rectal temperature. Significant changes (P less than 0.05) were not observed in a control group of 7 horses after subarachnoidal injection of sterile water (1.5 ml). The surgical application of segmental subarachnoid analgesia was demonstrated in 1 horse. The benefits and potential complications of segmental subarachnoid analgesia in horses are discussed.