Using local anesthetics for surgical analgesia: why, where, what and when (Proceedings)

Local anesthetics were once a mainstay of pain management in veterinary medicine, and may now be one of the most under-utilized modalities. Administered locally or regionally, they are the only modality that renders complete anesthesia to a site, i.e. no transmission of nociceptive impulses as long as the drug exerts its effect. . Initially used as a means of desensitizing tissues in order to "invade" tissues with scalpels; local anesthetics are enjoying a rebirth as powerful tools to prevent or reduce perioperative pain (as well as procedural and even chronic pain). There is no longer a reason to hold an "either-or" position; "for surgery either I use local anesthetics or I use general anesthesia", in fact, there are many reasons to combine general and local anesthetic for surgical pain relief. First, local blockade permits reduction of general anesthetic requirements, which is inherently safer for patients. Secondly, local anesthesia to a surgical site permits comfortable awakening from anesthesia, with little potential for unwanted systemic effects such as sedation or respiratory depression. Lastly, local anesthetics are recognized to have many beneficial effects beyond blocking nerve conduction; broad anti-inflammatory effects (reduced production of eicosanoids, thromboxane, leukotriene, histamine, and inflammatory cytokines; and scavenging of oxygen free radicals) and even antimicrobial, antifungal and antiviral effects.

Mechanisms

Local anesthetics exert their action by binding to a hydrophilic site within sodium channels, thereby blocking it and disallowing the Na+ influx; thus neurons may not depolarize and thus the effect can be complete anesthesia to a site rather than mere analgesia. Various local anesthetics will have variable onsets and duration of action, and they may be combined for a rapid and extended effect.

Toxicity

Overdose of local anesthetics can be fatal, and so observation of reported dose rates and careful calculation should allow their safe use. Recall that motor and autonomic nerves are also blocked by local anesthetics, and so motor weakness and vasodilation may occur with certain techniques. Blockade of essential nerve function, like that of phrenic nerve, or high epidural blocks, should be avoided. Motor weakness or paralysis of limbs, from spinal or major nerve trunk blockade is transient and as long as the patient is protected from injury and undue stress, should not be of consequence. A commonly held misconception is that local anesthetics impair wound healing – although they can powerfully inhibit the inflammatory component of cellular tissue influx, there is no evidence to support impaired wound healing. Both bupivacaine and ropivacaine have been implicated in myotoxicity, although it appears that this has not been listed as a complication in most human studies where these drugs were infused for 24 – 36 hours postoperatively into a wound bed. With proper technique and avoidance of needle induced trauma, local anesthetics can be used without the fear of negative effects on healing. Safe doses for dogs has been reported as 4 mg/kg and for cats, 0.5 mg/kg.

Locoregional Applications

The locality of administration is often limited only by the clinician's ability to learn various utilities and anatomic landmarks; few are outside the scope of any clinician to master. They include, but are not limited to local line or paraincisional blocks, regional blocks such as carpal ring, dental nerve, and intercostal blocks, subcutaneous diffusion blocks, testicular blocks, intra-articular blocks, and epidurals. Facet blocks are commonly used in humans though not yet described in veterinary medicine, although recently a paravertebral block was described for dogs.

Abdominal Incision and Line blocks

Savvas et al (2008) reported that a subcutaneous incisional midline block prior to celiotomy provided superior pain relief, compared with postoperative bupivcacaine or saline infiltration in dogs having a variety of abdominal surgeries. Subcutaenous infiltration in the surgical area of wound repairs, lumpectomies, and so on are well-established utilities for limiting the depth of sedation or anesthesia required to complete the procedure.

Cavity Blocks

Carpenter et al (2004) compared the effects of intraperitoneal bupivacaine with that of saline and lidocaine in dogs having ovariohysterectomy and found that the bupvacaine treated dogs received less supplemental analgesia and had improved pain scores. These authors used a higher than commonly recommended dose of bupivacaine (4.4 mg kg-1 0.75% bupivacaine diluted with saline to a volume of 0.88 ml/kg), in the cranial intraperitoneal space with an additional 2 ml of 0.75% bupivacaine on the incision prior to closure. Tobias et al 2006 did not find a benefit to a preoperative subcutaneous infiltration of 1.1 mg/kg bupivacaine in cats having ovariohysterectomy, but this dose is low compared to other studies. In another study in cats, lidocaine was infused subcutaneously, dripped on the ovarian pedicles and on the linea prior to closure, and the requirement for supplemental anesthetic doses of ketamine were significantly reduced, compared to cats treated with equivalent volumes of saline. The total dose of lidocaine used was 5 mg/kg. Results of veterinary studies suggest that incisional and intraperitoneal use of local anesthetics can spare opioid requirements and improve pain scores, and this is supported by multiple studies of similar techniques in humans having laparotomy. Infusion of local anesthesia into the pleural cavity has been described to improve comfort in patients with thoracotomy or chest tubes.

Topical and Transdermal

Commercial transdermal products are extremely useful in facilitating catheter placement and for minor procedures involving the dermis and epidermis.

A lidocaine/prilocaine ointment formulation (EMLA®, also comes as a generic) is placed on a shaved area and covered with a non-porous wrap (foil or cellophane). In humans it is recommended to have the product in place for 45 minutes to achieve full affect, but in the author's experience 15-20 minutes appears sufficient in dogs and cats. Penetration depth of analgesia has been reported to be time dependent and from 2-6 mm.

Commercial 5% lidocaine patches (Lidoderm®) provides post-operative wound paraincisional analgesia. However, Lidoderm® patches in fact are manufactured and labeled for post-herpetic neuralgia (Shingles), a very common form of chronic, neuropathic pain in humans. The pharmacokinetics of this product has been investigated in dogs and cats, with minimal systemic absorption noted.

The adhesive patches can be cut formed to the desired size and shape, for example on either side of an incision. One cautionary note is that an entire patch contains 700 mg of lidocaine, obviously a dose that would be toxic if ingested; therefore adequate precautions need to be taken to ensure the patient is unable to access the patch.

Studies in humans with moderate-severe stifle osteoarthritis reveal significant reduction in pain intensity after 2-week use of Lidoderm® patches, and pain relief similar to that achieved by oral NSAID. Their potential for use in animals for chronic pain conditions (e.g. osteoarthritis, osteomyelitis, osteosarcoma) remains plausible but no applications are described in the veterinary literature.

Intra-testicular Block

First described for horses and livestock, this technique allows for the local anesthetic to ascend and anesthetize the spermatic cord and cord-related structures. A 25-gauge needle is inserted into the testicle, aspirated (patient!), and enough local anesthetic injected enough to feel the testicle become turgid.

Dental blocks

A standard in humans, the use of regional nerve blocks in veterinary patients is anesthetic-sparing and permits a more comfortable recovery post-operatively.

Regional IV (Bier) Block

This technique involves the use of a tourniquet on a limb enough to occlude venous but not arterial blood flow, then injecting lidocaine 0.5 ml/kg of 1 or 2% IV distal to the tourniquest (not with epinephrine and not bupivacaine! due to cardiotoxicity). Onset is 10 minutes with a duration of 20 minutes past the removal of the tourniquet. Maximum length of tourniquet application is recommended at 90 minutes. This procedure has also been demonstrated in cats with no adverse hemodynamic effects.

RUM (Radial, Ulnar, Median block)

This block is administered in the vicinity of the carpus to render anesthesia to the region distally, often, for example, for feline declaws but may also used for surgery on any digit. If performed as a "ring block" dilution of the local anesthetic with saline to achieve sufficient volume may be necessary.

Intercostal block

This block is performed when entering a chest for thoracotomy or chest tube; the block is made on the caudal border at the proximal aspect of two ribs cranial and caudal to the chosen IC approach. This procedure has been found in dogs to be of equal efficacy to epidural morphine.

Brachial plexus block:

Intra-articular block:

Epidural Block

Epidurals have been in widespread use for procedures of the hindquarters, and if volume is sufficient to reach far enough cranially, for abdominal procedures as well. The procedure for dogs and cats is well-described in the literature.

Peripheral nerve blocks (perineural)

These are widely used for perioperative analgesia in humans. The duration of "single shot" blocks with bupivacaine is approximately 4 - 8 hours, duration could be extended with use of perineural catheter delivery. In humans, a meta-analysis of evidence for superiority of local anesthetic block over traditional opioid –only methods includes improved pain scores, patient satisfaction and reduced side effects. Furthermore, bupivacaine blocks applied in humans can have hypoalgesic effects long surpassing the expected duration of action of the drug, through uncertain mechanisms. Use of nerve locator devices is becoming more common in veterinary patients, this enhances precision of delivery and reduces the doses needed to block specific nerves. Descriptions of the techniques are available.

Loco-regional anesthesic blocaks are valuable and wide-ranging techniques that can improve patient comfort while sparing the use of systemic analgesics. Clinicians should consider their use in most surgical settings.