Assessment of the epidural distribution of sequential boluses of iohexol injected at the sacrococcygeal space in anesthetized cats.

OBJECTIVE

To determine the distribution of injectate (iohexol) at the feline sacrococcygeal space using CT and thereby identify the volume necessary to reach the 10th thoracic intervertebral foramina (T10-T11), where nociceptive neurons from the ovaries enter the spinal cord.

METHODS

7 healthy, purpose-bred, 1- to 2-year-old intact female domestic shorthair cats were anesthetized and positioned in sternal recumbency. After baseline vertebral column CT, a stimulating needle connected to a nerve stimulator (current, 0.2 mA; frequency, 2 Hz; pulse width, 0.1 milliseconds) was inserted at the sacrococcygeal space. After distal tail twitching was observed, a syringe driver injected iohexol (44 mg/mL; 0.25 mL/kg over 30 seconds). Computed tomography was performed 30 seconds after the injection ended. This was repeated 3 times, with 4 minutes separating the start of each injection. A linear mixed-effects model was constructed to determine if the number of vertebrae traversed as the iohexol distributed cranially could be predicted as a function of injectate volume, side of vertebral column, and their interaction (cat as a random effect).

RESULTS

The association between injectate volume and number of vertebrae traversed was significant. The volume required to reach the T10-T11 intervertebral foramen bilaterally was 0.75 mL/kg in 3 cats and 1.0 mL/kg in 3 cats; in 1 cat, 1.0 mL/kg did not reach this level.

CONCLUSIONS

Sacrococcygeal epidural injection of 0.75 to 1.0 mL/kg or more of local anesthetic may be required for segmental analgesia for feline ovariohysterectomy.

CLINICAL RELEVANCE: The volume required is greater than the volume injected epidurally in previous studies and could increase the risk of producing hemodynamic side effects.