Local and regional anesthetic techniques (Proceedings)

Epidural analgesia/anesthesia

      Technique

Patients are typically sedated or anesthetized and placed in sternal or lateral recumbency. Next, the cranial edge of the wings of the ilia are palpated. Once located, a 10 cm by 10 cm area of hair directly over the lumbosacral junction is clipped and the skin is surgically prepared. Needle insertion is made directly over the depression formed by the lumbosacral junction with the needle initially positioned perpendicular to the skin. It is important the stylet is correctly positioned within the needle to prevent transplantation of skin into the epidural space. Following insertion, the needle is observed for flow of cerebral spinal fluid or blood. Once the tip of the needle is confirmed to be in the epidural space, the syringe is attached to the hub of the epidural needle and a slow injection of the analgesic agent is begun. Signs indicating correct needle placement may include twitching of the tail muscles and a change of respiratory pattern during injection.

      Complications and contraindications

Sterile technique is mandatory.

Inflammation, coagulopathy, or other pathology in the area of the lumbosacral junction may be a contraindication to epidural placement.

Local anesthetics should not be given by epidural route when animals are hypotensive. Sympathetic blockade may worsen hypotension.

Epidural catheter placement

      Indications

Used for procedures where repeated epidural administration of analgesic drugs is anticipated such as pancreatitis or orthopedic surgery of the hind limb or pelvis.

      Complications and contraindications

Remember that application of local anesthetics to the cranial thoracic or cervical spinal cord can cause motor blockade to the respiratory muscles and block sympathetic nerves responsible for regulation of cardiovascular function.

Repeated injection of drugs which contain preservatives may result in damage to the spinal cord and neurological dysfunction.

The same complications and contraindications apply as for epidural drug administration with the addition of possible hind limb weakness and prolonged urinary retention.

It is important to reposition the animals ever 2 to 4 hours since normal sensation to a significant part of the body may be reduced or absent. Prolonged pressure on nerves and muscles may result in temporary or permanent dysfunction.

The risk of serious infection will usually increase over time; therefore the catheter should be removed as soon as it is no longer needed.

Aseptic technique is essential for placement of the catheter and for handling during subsequent injections.

Catheter insertion and removal may be associated with epidural hematoma development. Animals should be carefully monitored for development of this serious complication.

Brachial plexus block

Technique

An area cranial and dorsal to the point of the shoulder is clipped and prepared with surgical scrub. This is often done during the preparation of the leg for surgery. With the neck in a natural position, the cervical transverse processes will form a line that, if continued, will usually traverse the proximal brachial plexus. It is important the needle be guided beneath the scapula, but outside the thorax. Then the syringe is attached and aspiration is attempted to check for accidental puncture of a blood vessel. After confirmation of correct needle placement, 1 or 2 mls of the analgesic solution is injected. Then the needle is withdrawn approximately 1 cm, and the process of aspirating and injecting is repeated until the needle is just ready to exit the skin.

Complications and contraindications

Brachial plexus blocks are relatively safe and easy to perform. Accidental insertion of needle tip into thoracic cavity should be guarded against.

Contraindications would include infection in the nearby tissues or history of sensitivity to the drugs being used.

Radial/ulnar/median nerve block

      Proximal technique

The median and ulnar nerves can be palpated at the point of bifurcation just caudo-medial to the mid humerus. Blockade of the median and ulnar nerve should desensitize the medial and caudal / palmar aspect of the antebrachium and paw. The radial nerve can be palpated on the caudo-lateral aspect of the mid humerus opposite the median and ulnar nerves. The nerve can usually be palpated at this level. Blockade of the radial nerve should desensitize the cranial / dorsal and lateral aspects of the antebrachium and paw.

      Distal technique

The superficial branches of the radial nerve, the median nerve, and the dorsal and palmar branches of the ulnar nerve can be blocked more distally. The median nerve and palmar branch of the ulnar nerve can be blocked medial to the accessory carpal pad. The dorsal branch of the ulnar nerve can be blocked lateral and proximal to the accessory carpal pad. Finally, the superficial branches of the radial nerve can be blocked at the dorso-medial aspect of the proximal carpus.

      Complications and contraindications

Infection at the injection site may be a contraindication for median / ulnar nerve blocks.

The animal should be observed for signs of self-mutilation following recovery.

Intraarticular analgesia

      Technique

This technique is most commonly employed for stifle arthrotomy. The landmarks for deposition into the stifle are the lateral femoral condyle, the lateral aspect of the tibial tuberosity, patellar ligament, and patella. This technique should be performed in a sterile fashion. If injection is done during surgery, direct visualization of the joint capsule will help with location. The needle is inserted and joint fluid is allowed to flow out of the joint. The analgesic/anesthetic solution is then injected until the desired volume is delivered (usually slight distention of the joint capsule corresponds to correct volume).

      Complications and contraindications

Injections of infected joints may be contraindicated.

If sterile technique is not used or contaminated drug solutions are injected there is a risk of developing septic arthritis.

Dental nerve blocks

      Technique

Dental blocks are typically performed while the animal is anesthetized for dentistry.

      Infraorbital nerve

Several branches of the infraorbital nerve supply sensory innervation to the upper dental arcade. The rostral maxillary alveolar nerve supplies innervation to the upper canine tooth and incisors. The infraorbital artery and vein travel with the infraorbital nerve within the canal and should be avoided when analgesic solution injection is made. The infraorbital foramen is palpated through the buccal mucosa dorsal to the upper third premolar. Blockade of the nerve at this point will provide anesthesia to the ipsilateral canine tooth and incisors. The needle is inserted to just inside the infraorbital foramen. Aspiration is attempted before injection to ascertain if the needle has inadvertently entered a vessel. Then 0.25-0.5 ml of local anesthetic is injected.

      Mandibular nerve

The mandibular nerve enters the mandibular foramen on the medial aspect of the mandible just rostral to the angle of the mandible. The foramen is easily palpated from inside the mouth just caudal to the last molar. It is sometimes easier to palpate the nerve under the mucosa and follow it to the mandibular foramen. Blockade at this level provides analgesia to the teeth of the ipsilateral mandible. To perform the block, palpate the mandibular foramen with the index finger of one hand and introduce the syringe with the other. Once the tip of the needle is palpated next to the foramen, aspiration is attempted and the anesthetic can be injected.

      Complications and contraindications

An injection into infected tissues is contraindicated.

Animals may be at risk for self-mutilation if they have sensory loss to the tongue or lips.

Tranquilization may be required in a small percentage of animals to offset anxiety induced by loss of sensation to the mouth.

Intercostal nerve block

      Technique

Intercostal nerve blocks are typically performed intraoperatively, either during or after thoracotomy. A minimum of 2 adjacent intercostal spaces both cranial and caudal to the incision or injury site must be blocked due to overlapping nerve supply. The needle is introduced percutaneously, using aseptic technique, at the caudal border of the rib near the level of the intervertebral foramen. The needle should penetrate skin, subcutaneous tissue, and then intercostal muscles before the appropriate volume of local anesthetic is deposited

Toxic dose                                          Recommended volume of either local anesthetic

Lidocaine (2.0%)- 3.0 - 5.0 mg/kg       0.25 ml/site for small dogs

Bupivicaine (0.5%)- 1.5 - 2.0 mg/kg     0.5 ml/site for medium dogs

                                                            1.0 ml/site for large dogs

      Complications and contraindications

Intrathoracic injection with the possibility of pneumothorax or pulmonary laceration.

Slight risk of systemic local anesthetic toxicity.

Interpleural analgesia/anesthesia

      Technique

The procedure is done with the animal either heavily sedated or anesthetized at the end of a thoracotomy. Typically, chest tubes are placed in most animals that would benefit from interpleural anesthesia following surgery. Prior to injection of local anesthetic, the catheter should be aspirated for air or blood. Then the analgesic solution is injected and the catheter cleared with 1.0 to 2.0 mls of air or saline solution, closed to room air, and secured to the thoracic wall. The animal is then positioned to facilitate fluid movement to the area of tissue damage. Care must be taken to prevent inadvertent pneumothorax during catheter placement or anesthetic injection. After injection of the local anesthetic the chest tube is not aspirated for 60 minutes unless the animal appears to be experiencing respiratory distress associated with pneumo- or hemothorax.

      Complications and contraindications

Pneumothorax, inadvertent pulmonary trauma during catheter placement, and infection.

Systemic local anesthetic toxicity may occur.

Aseptic technique is mandatory.