Blocking the transmission of painful signals via nerve fibers is one of the most effective ways of managing pain. Local anesthetics are inexpensive to use and quite effective in blocking the transmission of nociceptive signals (nerve impulses) at the source. Disrupting neural transmission of pain information results in diminished signaling to the spinal cord with a likely reduction in further neuropathic 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.
Use of local anesthetics offers a number of benefits. First, local anesthetics produce true analgesia resulting in the complete absence of pain for the duration of the block. Second, these drugs are non-scheduled agents, so there's no cumbersome paperwork or special license required. Thirdly, long-term pain states may be diminished or eliminated. Finally, the techniques used to administer these drugs are relatively easy to perform.
When administered at an appropriate dose, local anesthetics have relatively few, if any, adverse side effects. The potential systemic side effects of local anesthetics 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.
Use of local anesthetics has become more common in small animal practice in recent years. 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 popular.
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 of 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 it 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.
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.
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 ensure adequate desensitization of the area as described above.
Dental (Oral) Nerve Block
The entire muzzle can be anesthetized by blocking the infraorbital and mandibular foramen. Mandibular and maxillary nerve blocks provide excellent analgesia for pain anywhere in the muzzle. Tooth extraction is the obvious indication for these blocks, but they are also quite effective for gingival surgery, mandibulectomy, maxillectomy, jaw fracture repairs, nasal surgery or biopsy. Small amounts of bupivacaine (not to exceed 1 cc of 0.5% bupivacaine per 10 lbs of body weight) 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 benefits are twofold. First, the anesthetic is held in place longer, increasing its duration of action; second, local bleeding is controlled. Epinephrine can be added to the syringe by simply "washing" with epi prior to drawing up local anesthetic.
Intraarticular (Joint Space)
Effective analgesia in pre- and postoperative orthopedic cases has been achieved by injection of local anesthetics directly into the joint space, such as in cruciate ligament repair. Intraarticular morphine has also been shown to effectively reduce joint pain. The effectiveness of this technique when used preoperatively 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. No doubt these responses are due to pain.
Interpleural bupivacaine infusion following thoracotomy surgery may have some analgesic benefit. Bupivacaine (1.5–2 mg/kg) is injected via an indwelling chest tube into the pleural space. Analgesia is thought to occur by direct blocking of the intercostal nerves. For maximum coverage, patients are held in sternal recumbency for 5–10 minutes postinjection and gently rolled from side to side. Drug absorption through the pleural tissue should be considered. The addition of 0.1 cc/10 cc of Na bicarbonate may reduce the stinging sensation in awake patients.
Epidural Nerve Blocks
Injection of local anesthetics and/or opioids directly into the epidural space is a fairly simple and safe technique to provide long-duration analgesia to the caudal half of the body while minimizing systemic side effects. 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. Successful epidurals may reduce gas inhalant as well as post-op pain medication requirements. Local anesthetic epidurals provide excellent muscle relaxation and short-term analgesia. They are inexpensive and do not require the use of scheduled drugs. 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.
Most recently, the lidocaine transdermal patch (Lidoderm®) 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 transdermal lidocaine patches in veterinary medicine for specific conditions and procedures.
IV administration of lidocaine by constant-rate infusion (CRI) is an effective technique for managing a variety of pain states. At the cardiac dose of 30–80 micrograms per kg per minute, lidocaine provides excellent analgesia for visceral pain (e.g., pancreatitis, parvovirus) as well as in procedures with extensive nerve involvement, such as limb amputation. Because it is safe for use in patients with GI disturbances, lidocaine is a good choice for analgesia in patients with gastric dilatation volvulus (GDV) 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.
The CRI dose for lidocaine is: Dog: 1–2 mg/kg IV followed by 30–50 µg/kg/min.
Note: There are reported lidocaine CRI dosages for cats, but typically lidocaine is not recommended for use in cats due to potential for severe cardiotoxic effects.
References are available upon request.