Expanding your use of loco-regional anesthetics (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) and to reduce general anesthetic and concurrent analgesic (especially systemic opioid) requirements. 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.

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. Other local beneficial effects include: 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. 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, though anecdotally, higher doses are in usage. The duration of activity can reportedly be doubled with small amounts of an opioid, either morphine or buprenorphine.

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 cavity blocks

Savvas et al 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. These authors used a dose of 2 mg/kg of 0.25% bupivacaine. Carpenter et al 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.

Wound diffusion catheters

A new strategy to extend the duration of local anesthesia may markedly help patients with moderate to severe surgical injury. Implantation of a catheter into the surgical wound site prior to closure allows repeated or continuous infusion of local anesthetics into the affected area. In humans, relatively costly FDA approved cathetersa are used. For veterinary use, two moderately priced types are commercially available. The basic form is a soft pliable catheter with tiny holes along the end that is implanted; functioning somewhat like a garden "soaker hose". Delivery of local anesthetic by continuous infusion of lidocaine, or intermittent bolus dose of bupivacaine can be used. An elastomeric balloon pump is a modestly priced option for ambulatory patients. A recent review of wound incision catheters for surgery in humans, concluded that beneficial outcomes included reduction of pain scores at rest and with activity, reduction of daily consumption of opioids, and trends towards better patient satisfaction and length of hospital stay. Veterinary clinical studies have been reported involving the use of continuous wound infusion of local anesthetics. The most studied surgical indication was total ear canal ablation, but use for extensive soft tissue resection in cats (fibrosarcoma resection) is also reported. Studies reported to date have used either bupivacaine or lidocaine infusion, and, as with human studies, pain was generally perceived to be adequately managed with the infusion, and complication rates low and not perceived to be problematic. Currently, anecdotal reports are that practitioners are using such wound infusion catheters for limb amputation, ear canal ablation, intercostal and sternal thoracotomy, celiotomy, and major soft tissue tumor excision, with excellent results and few complications.

Topical & 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.

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.

Summary

Local anesthetics have a wide variety of applications of in the peri-operative setting. They are an inexpensive and effective addition to an integrated pain management protocol.

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