Rebelling against pain transmission
SAN ANTONIO, TX – Blocking the transmission of painful signals via nerve fibres is one of the most effective ways of managing pain. Local anesthetics inhibit generation and transmission of nerve impulses by blocking sodium channels in the neuron’s cell membrane. This slows the rate of depolarization of the neuron cell membrane and prevents the threshold potential from being reached, explained Nancy Shaffran, CVT, VTS (ECC), speaking at the Veterinary Emergency and Critical Care Society Symposium.
Benefits of local anesthetics
Local anesthetics produce true analgesia resulting in the complete absence of pain for the duration of the block. As well, long-term pain states may be diminished or eliminated. Another benefit is that the techniques used to administer these drugs are relatively easy to perform.
Local anesthetics can be used very effectively in a number of procedures, including thoracotomy, elbow surgery, maxillomandibular procedures, local incisions, feline declawing, regional blockades, rear limb procedures, and stifle surgery. In addition to direct regional blockades, using lidocaine as a systemic blocking agent by constant rate infusion is becoming increasingly common.
When administered at an appropriate dose, local anesthetics have relatively few, if any, adverse side effects, explained Ms. Shaffran. Their potential systemic side effects involve the central nervous system and cardiovascular system. Other potential side effects include development of methemoglobinemia, nerve and skeletal muscle toxicities, and allergic reactions, including hypersensitivity or anaphylactic responses.
The 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 3-5 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 15-20 minutes to take effect, 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
The duration of blocking agents can be extended by combination with a 1:200,000 dilution of epinephrine. “Washing” the syringe with epinephrine before drawing up local anesthesia provides sufficient vasoconstriction to extend the block and reduce bleeding in the area. Ms. Shaffran cautioned that epinephrine should never be used in circumferential limb block, such as feline declaw. 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 anesthetics
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. Several commercially prepared topical anesthetic creams and jellies exist that can be applied as a thick paste, however 20 to 30 minutes of direct contact time is required to ensure effective analgesia.
Local infiltration
Injecting lidocaine or bupivacaine into local tissue can reduce pain associated with various painful procedures, such as 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, followed by an appropriate waiting period to ensure adequate desensitization of the area as described above.
Circumferential ring block
This block is especially effective for use in cat declaws and involves subcutaneous injections of bupivacaine or bupivacaine/lidocaine combination. Injections are made just above the carpal bend on the top of the paw and just above the accessory carpal pad on the underside. The skin is tented horizontally and the needle is fed under the skin. As the needle is withdrawn, the 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 eight hours post-surgery. Sterile saline can be added to achieve sufficient coverage for smaller cats.
Dental (oral) nerve block
Mandibular and maxillary nerve blocks provide excellent analgesia for pain anywhere in the muzzle, and work by blocking the infraorbital and mandibular foramen. Tooth extraction is the obvious indication for these blocks but they are also quite effective for gingival surgery, mandibulectomy, maxilectomy, jaw fracture repairs, nasal surgery, or biopsy. Small amounts of bupivacaine are injected near or into the infraorbital and mandibular foramina anesthetizing the main nerve branches. This technique is relatively easy to perform by a skilled veterinary nurse and has minimal associated risks.
The addition of epinephrine to dental blocks causes local vasoconstriction. The anesthetic is held in place longer increasing its duration of action and local bleeding is controlled. Epinephrine can be added to the syringe by simply “washing” with epinephrine prior to drawing up local anesthetic.
Intra-articular (joint space)
Effective analgesia in pre- and post-operative orthopedic cases has been achieved by injecting local anesthetics directly into the joint space, such as in cruciate ligament repair. Intra-articular morphine has also been shown to effectively reduce joint pain. The effectiveness of this technique when used pre-operatively is evident in the smooth plane of anesthesia maintained when the joint capsule is incised. This is in sharp contrast to the spike in heart rate and “lightness” that is observed when the capsule is entered without anesthetic.
Pleural space
Interpleural bupivacaine infusion following thoracotomy surgery may have some analgesic benefit. Bupivacaine is injected via an indwelling chest tube into the pleural space. For maximum coverage, patients are held in sternal recumbency for 5-10 minutes post injection and gently rolled from side to side. The addition of sodium bicarbonate may reduce the stinging sensation in patients that are awake.
Epidural nerve blocks
Epidural analgesia can be very effective for managing pain associated with procedures such as cesarean sections, thoracotomies, pelvic or pelvic limb fractures, amputations, orthopedic procedures, and surgery of the tail or perineum. Local anesthetic epidurals provide excellent muscle relaxation and short-term analgesia, and are inexpensive. The disadvantages include the potential for overdose, hypotension, excessive muscle relaxation, temporary loss of motor function, and injection site discomfort. Injection is generally made at the lumbosacral junction just caudal to the termination of the spinal cord. Epidural catheters can be inserted to allow long-term analgesic administration.
Transdermal
Recently the Lidocaine transdermal patch has gained widespread acceptance in human medicine for management of neuropathic pain associated with back injury or surgery. Work is underway to investigate the use of these patches in veterinary medicine for specific conditions and procedures.
Intravenous
IV administration of lidocaine by constant rate infusion (CRI) is an effective technique for managing a variety of pain states. Because it is safe for use in patients with GI disturbances, lidocaine is a good choice for analgesia in patients with gastric dilatation volvulus or other similar disorders. Lidocaine seems to have benefit in patients undergoing procedures with excessive nerve trauma such as complicated back surgeries or limb amputations. IV lidocaine is extremely short acting and can be discontinued without residual effect almost immediately. Lidocaine CRI should be discontinued if the patient shows signs of toxicity including muscle tremors, seizures, nausea, or vomiting. Ms. Shaffran noted that lidocaine CRI dosages for cats exist, but lidocaine is typically not recommended for use in cats due to potential for severe cardiotoxic effects. CVT
