Nerve blocks improve general anesthesia (Proceedings)

Epidural Block

Administration of analgesic agents into the epidural site has been documented to provide analgesia during anesthesia for surgery in dogs (Smith & Yu 2001; Kona-Boun et al. 2006). A retrospective investigation of the effects from epidural morphine compared with systemic analgesic administration provided information that the duration of analgesia from epidural morphine administered in conjunction with general anesthesia was an average of 19.6 hours compared with a significantly shorter duration of effect from systemic opioids (Troncy et al. 2002). Further, calculation of the cost of postoperative analgesia provided figures that indicate that inclusion of epidural morphine decreased the cost of analgesia per dog by an average of 38%. The addition of bupivacaine to morphine for epidural analgesia in dogs for orthopedic surgery has been reported to increase the effectiveness of analgesia in a prospective study (Kona-Boun et al. 2006). Plasma cortisol concentrations were significantly lower in dogs that received morphine and bupivacaine for 2 hours after anesthesia and surgery, and were significantly lower in dogs receiving a morphine epidural compared with a saline epidural.

Bupivacaine (Marcaine, Sensocaine), 0.5 mg/kg, will provide 4-6 hours of analgesia for procedures performed on the hind limbs, tail, and pelvis. Sensory blockade during surgery modifies the potential for central sensitization of nociceptive neurons and minimizes the development of hyperalgesia in recovery.

Morphine (preservative-free Duramorph® 1 mg/ml), 0.1 mg/kg, provides analgesia that is not sufficient alone for surgery but allows a decrease in the concentration of inhalation agent. Analgesia lasts on average 20 hours. Additional systemic analgesia will be need for some patients in the first hours after anesthesia. An advantage of using morphine is that the dose is considerably less than that used for systemic analgesia and that insufficient drug is present to be absorbed and cause sedation.

Oxymorphone (Numorphan), 0.05 mg/kg diluted with 0.1-0.2 ml/kg of saline, provides analgesia for 8-10 hours.

Buprenorphine (Buprenex), 0.005 mg/kg diluted with 0.1-0.2 ml/kg saline, was determined to provide equivalent analgesia postoperatively as morphine (Smith et al 2001). In this study of clinical patients, additional systemic opioid had to be given to 50% of patients receiving either a morphine or buprenorphine epidural.

Combination of bupivicaine with morphine is a common technique.

Total volume to be injected is controversial. Many anesthesiologists prefer to limit the volume in larger dogs; my preference is to use a maximum of 10 ml.

Incidence of complications:

The incidence of reported complications after epidural nerve block is low, but includes urine retention, pruritis, myoclonus, and persistent sensory or motor blockade (Troncy et al. 2002). In this clinical evaluation of epidural analgesia in 242 dogs and 23 cats, complications included a 7% failure rate, urine retention in 7 dogs and 2 cats, and pruritis in 2 dogs. Cardiovascular depression may develop after administration of oxymorphone epidurally due to systemic absorption (Torske et al. 1999), however, the dose of epidural morphine is too low to produce significant systemic effect. Hypotension has been reported after epidural administration of bupivacaine and morphine or bupivacaine and oxymorphone (Torske et al. 1999; Kona-Boun et al. 2006) and it was calculated that periods of hypotension were 8.1 times more likely to occur in dogs given bupivacaine and morphine compared with morphine alone (Kona-Boun et al. 2006). A frequent esthetic complication that many veterinarians have noted is that hair that has been clipped from over the lumbosacral region is slow to grow.

Contraindications to epidural:

Skin infection at the site of needle placement and coagulopathy are contraindications to epidural block.

Location of the lumbosacral space in dogs:

A line between the cranial border of each ilium crosses mid-line between L6 and L7. The prominence caudal to the intersection is the spinous process of L7. The next depression is the lumbosacral junction. Alternatively, a line joining the most dorsally prominent points of the pelvis (dorsal ischiatic spines) intersects midline at the lumbosacral junction. The needle should be inserted exactly in the center of the depression.

Location of the lumbosacral space in cats:

The cat differs from the dog in that the spinous process of the seventh lumbar vertebra is long and sloping so that the spinous process is more cranial when palpated through the skin. To enter the lumbosacral space, the needle must be inserted perpendicularly to the skin just cranial to the sacrum. The pointed surface of the first sacral vertebra is used as a landmark.

Technique:

The dog or cat can be in lateral or prone position. The prone position, with the hind limbs pulled forward under the body, opens the lumbosacral space. Patients with fractures of the hind limbs are better positioned in lateral recumbency. Hair should be clipped over and around the lumbosacral space, and the skin cleaned as for surgery. A 22 ga spinal needle 1.5 or 2.5 inch is used. The needle is introduced perpendicularly to the skin. The needle will initially have resistance to passage through the tissues. At the depth of the spinal cord you should feel an obvious resistance and then a 'pop' as the needle passes through the ligamentum flavum. Remove the stilette and attach an empty syringe with a gentle twisting motion, not pushing, to avoid moving the tip of the needle. The plunger of the syringe is withdrawn to create a vacuum. If blood or cerebrospinal fluid is aspirated, withdraw the needle a few mm and aspirate again. The syringe is then disconnected and 0.25-0.5 ml of air or 0.02 ml/kg sterile 0.9% saline drawn in. Attach the syringe to the spinal needle and gently inject the air or saline. There should be no resistance to injection. If preferred, a 'hanging drop' technique can be used. After determining that the needle is not in CSF or a blood vessel, immediately after the stilette is removed from the needle, a drop of the analgesic solution is placed on the hub. The tip of the needle should be in the epidural space if the drop of fluid is sucked into the needle. The reverse is not necessarily true; a negative hanging drop test does not mean that the needle is not in the epidural space. Next, change syringes (carefully so as not to move the tip of the needle), direct the bevel of the needle cranially, and slowly inject the drug over ≥30 secs. Onset of action depends on drug used but may be as long as 40 min for morphine.

Brachial Plexus Block

Injection of bupivicaine, 2.5 mg/kg, over the brachial plexus will produce motor and sensory block of the forelimb below the elbow. Excellent analgesia is provided for surgery of the radius and ulna, carpus and foot.

The block must be performed as sterile technique. The most likely complication is laceration of a blood vessel. A complication that is unlikely to occur but is a potential hazard is penetration of the chest.

A 22 gauge spinal needle will be required that must extend across the narrow part of the scapula. A 1.5 inch needle is used for small dogs, 2.5 inch for medium dogs and 3.5 inch for big dogs.

The hair must be clipped cranial and dorsal to the point of the shoulder and cleaned as for surgery. Insertion of the needle is easier if one person can hold the leg up and away from the thoracic wall. The bupivicaine should be drawn into a sterile syringe by the operator wearing surgical gloves. The site for insertion of the needle is half-way between the point of the shoulder/ greater tubercle of the humerus and the body of the sixth cervical vertebra. The brachial plexus is three flat bands formed from the ventral branches of the 6th, 7th, and 8th cervical and 1st and 2nd thoracic spinal nerves. Local anesthetic solution must be spread over a wide area to block all the nerves. Bupivacaine 0.5%, 2.5 mg/kg, will be injected as a series of blebs along a line from the first rib to the cranial border of the scapula.

Dental Nerve Blocks

This author's preference is to inject bupivacaine into the infraorbital canal with the dog's nose elevated to allow caudal flow of the solution. This usually provides analgesia of the molars and eliminates the need to attempt block of the maxillary nerve. Aspiration should be done before injection to ensure that the needle is not in a blood vessel.

Extreme care must be taken in blocking the infraorbital nerve of cats as the foramen is open to the orbit and does not form a canal.

References

1. Kona-Boun J-J, Cuvielliez S, Troncy E (2006) Evaluation of epidural administration of morphine or morphine and bupivacaine for postoperative analgesia after premedication with an opioid analgesic and orthopedic surgery in dogs. J Am Vet Med Assoc 229, 1103-1112.

2. Smith LJ, Yu JK-A (2001) A comparison of epidural buprenorphine with epidural morphine for postoperative analgesia following stifle surgery in dogs. Vet Anaesth Analg 28, 87-96.

3. Torske KE, Dyson DH, Conlon PD (1999) Cardiovascular effects of epidurally administered oxymorphone an an oxymorphone-bupivacaine combination in halothane-anesthetized dogs. Am J Vet Res 60, 194-200.

4. Troncy E, Junot S, Keroack S, et al. (2002) Results of preemptive epidural administration of morphine with or without bupivacaine in dogs and cats undergoing surgery: 265 cases (1997-1999). J Am Vet Med Assoc 221, 666-672.