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.
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.
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.