Local and regional anesthesia techniques, Part 4: Epidural anesthesia and analgesia - Veterinary Medicine
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Local and regional anesthesia techniques, Part 4: Epidural anesthesia and analgesia
Consider this straightforward and economical technique to relieve your patients' pain. And if you haven't already learned the technique, attend a wet lab or visit a local referral practice. You and your patients will benefit.


VETERINARY MEDICINE


Local anesthetics

Lumbosacral epidural anesthesia with local anesthetics provides complete anesthesia to the caudal half of the body by blocking the intradural spinal nerve roots and the peripheral layer of the spinal cord.1 This technique is useful for pelvic and hindlimb orthopedic procedures, perineal and anal surgeries, exploratory laparotomy, and cesarean section. In addition to complete anesthesia, at least some sympathetic and motor blockade is produced with local anesthetics. Lidocaine, mepivacaine, and bupivacaine consistently cause motor blockade, while motor blockade is less intense and of shorter duration with levobupivacaine and ropivacaine.1 The duration of motor blockade is generally shorter than the duration of analgesia, and, depending on the procedure and local anesthetic chosen, motor function usually returns by the time a patient recovers from anesthesia.1-3

Doses and precautions. The type and volume of local anesthetic chosen depends on the desired result. Lidocaine (2%) administered at 1 ml/6 kg completely anesthetizes the pelvic limbs and posterior abdomen, caudal to L1, within 10 to 15 minutes and lasts 60 to 120 minutes. With a comparable volume of 0.5% to 0.75% bupivacaine, the onset is 20 to 30 minutes with a duration of four to six hours. Volumes of 1 ml/7.5 kg are adequate for pelvic, perineal, and hindlimb procedures.2,3

If the total epidural injection volume of local anesthetic and other adjunct analgesics exceeds 1 ml/5 kg, high blockade of the sympathetic nerve roots can occur and Horner's syndrome, vasodilation, and hypotension can result.2 Total drug volume should not exceed 8 ml, and doses of epidural drugs should be calculated by using the ideal lean body weight of the patient. Doses should be further reduced by 25% in pediatric, geriatric, and pregnant animals since the epidural space may be smaller than expected.2,4 Hypotension is more likely in animals with hypovolemia or cardiovascular compromise, so correct hypovolemia before administering local anesthetics epidurally. Hypotension is treated with intravenous crystalloids (20 to 30 ml/kg), colloids such as hetastarch (5 to 20 ml/kg), or vasopressors or inotropes such as ephedrine, vasopressin, or dopamine.1,2

In addition to sympathetic blockade, local anesthetic overdosage can also result in blockade of the motor nerves innervating intercostal muscles and even paralysis of the diaphragm should the local anesthetic reach the level of C5; hypoventilation, hypercapnia, and hypoxia can result.1,2 Ventilatory support, using positive pressure ventilation, may be required. Careful attention to total volume injected will reduce the incidence of these adverse effects.

Controversy exists as to whether a patient should be rotated after epidural injection of local anesthetics, which exert their action by contact with the nerve roots and spinal cord. In people, small injection volumes (< 10 ml), comparable to those used in small animals, spread under the effect of gravity to the dependent side, though other factors, including cardiovascular function and CSF pressure are also important.4 In our practice, we do not rotate the patient after epidural injection of local anesthetics other than to position the patient to prepare the surgery site. Body positioning with the head down and the hindquarters elevated could result in further cranial spread of the local anesthetic and should be avoided.


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Source: VETERINARY MEDICINE,
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