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Staying local – safe and effective local and regional anesthetic blocks for pain control in animals (Proceedings)

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Aug 01, 2009

Blocking the transmission of painful signals via nerve fibers is one of the most effective ways of managing pain. Many techniques can be easily and safely performed in veterinary medicine including dental, declaw and epidural blocks. In addition to direct regional blockades, using lidocaine as a systemic blocking agent by constant rate infusion is becoming increasingly popular.

There has been a great deal of work recently reviving the use of local or regional analgesia. Applying analgesia directly to the affected nerve endings can provide excellent pain control while reducing the overall need for systemic drugs. Local anesthetics work by totally disrupting neural transmission of information by axons at the treatment site and provide true analgesia. This is extremely effective in managing pain associated with procedures where specific nerves can be identified such as oral surgery.

There are several blocking agents available. Choice of blocking agent is typically made based on onset of action, duration of action and route of administration:

  • Lidocaine, the most widely used local anesthetic, takes effect in 3 to 5 minutes and is effective for 60-90 minutes. Lidocaine can be used topically, regionally or as an IV infusion in dogs.
  • Mepivacaine (Carbocaine) has a medium duration of action 2-3 hours and fairly rapid onset of about 10 minutes.
  • Bupivacaine (Marcaine) takes longer to take effect (15 to 20 minutes), but its anesthetic and analgesic effects last 6 or more hours. Bupivacaine is not effective as a topical analgesic, but is an excellent choice for local infiltration.

All local anesthetics cause vasodilation that decreases their duration of action. The duration of blocking agents can be extended by combination with a 1:200,000 dilution of epinephrine. Epi "washing" the syringe prior to drawing up local anesthesia provides sufficient vasoconstriction to extend the block and reduce bleeding in the area. Epinephrine should NEVER be used in circumferential limb block such as feline declaw.

Local anesthetics are safe if correctly administered. Most cases of toxicity in small animals occur as a result of accidental overdose or inadvertent intravenous administration. Signs of toxicity include seizures, coma, neurotoxicity, and cardiovascular collapse.

Topical

Application of topical analgesia to the surface skin or mucosa can reduce pain associated with minor procedures such as wound suturing, venipuncture, arterial puncture, nasal cannulization and urinary catheterization. Solutions of lidocaine or tetracaine with or without epinephrine can be used alone or in various combinations to provide desensitization at the application site. Gauze pads soaked with solutions can be applied directly to the site. Alternately, there are several commercially prepared topical anesthetic creams and jellies that can be applied as a thick paste however 20 to 30 minutes of direct contact time is required to insure effective analgesia.

Local infiltration

Injection of lidocaine or bupivacaine into local tissue can reduce pain associated with various painful procedures. This technique is useful for small mass removal, digit amputation, arterial catheter placement, thoracocentesis, abdominocentesis, bone marrow sampling, etc. The entry area is infiltrated with small amounts of anesthetic prior to tissue penetration. An appropriate waiting time must be observed to insure adequate desensitization of the area as described above.

Circumferential ring block

This block is especially effective for use in cat declaws and involves SQ injections of bupivacaine or bupivacaine/lidocaine combination. Injections are made just above the carpal bend on the top side of the paw and just above the accessory carpal pad on the under side. The skin is tented horizontally and the needle in fed under the skin. Then, as then needle is withdrawn, drug is injected slowly to leave behind a "line". When this is done on both surfaces the lines will connect creating a bracelet or ring block around the limb. This 4 injection technique provides regional nerve block sufficient to eliminate pain for up to 8 hour post surgery. The dosage is 1 cc of 0.5% bupivacaine per 10 lbs of body weight divided among the injection sites. Sterile saline can be added to achieve sufficient coverage for smaller cats.