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.
