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Volume 51, Issue 7
Practical tips for improving your pain protocols and increasing veterinary patient comfort by extending local anesthetic blocks.
Local and regional anesthesia is the technique of applying to the skin or infiltrating tissues with a sodium channel–blocking agent. Numerous articles, videos, and textbooks detail the techniques used to administer local and regional anesthetics. This article focuses on how to extend or enhance local blocks to make your patients comfortable for longer. The most commonly used blocking agents in veterinary medicine, lidocaine and bupivacaine, are the primary local anesthetics discussed in this piece.
Local anesthetics allow an animal in severe pain (e.g., with broken ribs) to feel comfortable within minutes. Large animal practitioners often perform standing surgeries with only local anesthetics on board. Because local anesthetics are so powerful, we are able to lower the overall use of systemic analgesics (and gas anesthetics) that may otherwise cause more concern, particularly in critical patients, by not causing sedation or by causing such adverse effects as excessive panting or hyperthermia. Emerging research has also found some local anesthetics to be anti-inflammatory and even anti-proliferative for certain cancers, suggesting more serious consideration for most, if not all, surgical cases and trauma.1-4
Depending on the type of local anesthetic used and the dose administered, the duration of action can range from 2 to 6 hours. Obviously, most patients will feel pain for longer than this length of time, leaving us to explore methods of extending the local anesthetic if possible. Acute pain, such as from surgery or trauma, and the inflammatory processes that result are at their peak for the first 72 to 96 hours from outset. This does not mean analgesics should not be used past this 96-hour mark; rather, it should trigger us to focus on this acute pain period for better outcomes down the road. Doing so may even prevent chronic or neuropathic pain states from poorly controlled periods of pain.
With topical or surface anesthesia, sodium channel–blocking agents in creams or solutions are administered on the skin or mucous membrane to provide some relief. Unfortunately, many of the agents used in veterinary medicine are not readily absorbed through the skin and must be left on for quite sometime to achieve their effect. The use of lidocaine patches over wounds or incisions has also been described5,6 but has not been found to alleviate the need for other analgesic medications. I have found the effects of lidocaine patches on myself and on patients to be lackluster. Splash blocks, which are local anesthetics sprayed on incised or exposed fleshy tissue, are short-lived or have no meaningful effect. It is important to note this is different than instilling the blocking agent into a body cavity or on mucous membranes.7
Local infiltration is a less precise, yet still effective, means of administering a blocking agent. Basically, a local blocking agent is infiltrated into the area where the surgeon plans to incise the skin and manipulate tissues. There are many positive studies on the efficacy of using this technique for surgical patients.7-9 It is now included in the pain management guidelines provided by the American Animal Hospital Association and American Association of Feline Practitioners.
Regional or nerve blocking techniques are a bit more precise and require thorough knowledge of the nervous system anatomy. To administer a regional block, anatomic landmarks, palpation, or devices are used to infiltrate the blocking solution within millimeters of a nerve. It is important not to pierce or infiltrate the nerve itself, as is commonly done during leg amputations. Recent research has shown that infiltrating the nerve stretches the nerve fibers or can create a creeping nerve toxicity that can sensitize the remaining nerve component adversely.10-12 A nerve stimulator and ultrasound are typically used for these techniques.
A few techniques in veterinary practice can reduce the sting of blocking agents, but is it necessary? Adding sodium bicarbonate to the blocking agent alkalinizes the agent for a less dramatic sting in awake patients, but doing so decreases the duration of the block.13 In addition, adding too much sodium bicarbonate can cause precipitation and decrease efficacy further.13 A solution consisting of one part sodium bicarbonate to three parts blocking agent is safe and allegedly still effective.14,15
This technique should not be used in anesthetized patients. If a greater volume of blocking agent is needed, reach for regular saline instead. Mixing the local anesthetic with saline in a 1:1 ratio will increase volume without compromising efficacy. In addition, there is some suggestion that adding saline to the mixture will better facilitate tissue distribution of the blocking agents.
The longstanding practice of mixing two local blocking agents, one with a quicker onset (lidocaine) and one with a longer onset but longer duration (bupivacaine), has been largely disproven, with a growing body of evidence showing that this mixture can be inferior to bupivacaine alone. There are also a handful of veterinary studies and multiple human studies showing no added benefit to this technique.16-20 The bupivacaine may be washed out of the system based merely on changing concentration gradients and the manipulation of pH levels of each drug when mixed. Interestingly, bupivacaine was found in a 2015 study21 to have some effect on the N-methyl-D-aspartate (NMDA) receptor by its ability to access the sodium ion channel ubiquitously and suppress transmission currents to the dorsal horn. This receptor is critical in the complex physiology of chronic pain.
Because local anesthetics are vasodilators, some practitioners add epinephrine to their anesthetic cocktail to produce local vasoconstriction and thus allow the block to last longer. This is contraindicated in animals with cardiovascular disease,22 however, and not a technique I recommend. Instead, micro doses of dexmedetomidine can be added for this same effect, a longer duration of action, and potentially better analgesia. In a 2018 human meta-analysis study,23 the authors noted no more than a 60-minute prolongation of the local anesthetic when epinephrine was added, again suggesting the superiority of using dexmedetomidine instead.
Dosing recommendations for lidocaine and bupivacaine vary. I use 1 to 2 mg/kg of either drug, doses that fall well within safe dosing ranges for cats and dogs. If a larger volume is needed, I mix the higher-end dose (2 mg/kg) with saline in a 1:1 ratio. My preferred maximum total doses for patients that still may require larger volumes even with volume expansion or multiple site infiltrations are 4 mg/kg total for lidocaine in dogs and 2 mg/kg in cats, and a maximum dose of 2 mg/kg of bupivacaine for both species. (Doses can be increased further if necessary and referenced appropriately).
If there are concerns about toxicity, the addition of an adjunctive medication allows for a decrease in local anesthetic dose while still prolonging the block (Table).23-36 Use of a preservative-free solution is not necessary for peripheral nerve blocks. These blocking cocktails can be re-administered after the initial block has worn off.
With the release of Nocita (bupivacaine liposome injectable suspension — Elanco) in 2016, being a creative mixologist became a lot less … experimental. Nocita is labeled for specific canine and feline surgeries but, like many drugs in veterinary medicine, has been used extensively off label. With a duration of 72 hours (and, in one animal study, a bit longer37,38), it is truly one of my favorite anesthetics to use. However, proper technique is critical for full success with this product.
Limiting factors for Nocita appear to be its cost and shelf-life after initial puncture of the 10- or 20-ml bottle. New research published this year has shown product stability even 4 days after the initial puncture. In light of the fewer number of pain medications required when Nocita is on board, elimination of the need for opioid constant-rate infusions, and getting the animal out of the hospital sooner, use of Nocita may save on costs.
Stephen is a surgical research anesthetist through the Academy of Surgical Research and is a VTS in research anesthesia through the Academy of Laboratory Animal Veterinary Technicians and Nurses. He became one of the first veterinary cannabis counselors and a certified veterinary pain practitioner through the International Veterinary Academy of Pain Management. He is often found on the Veterinary Anesthesia Nerds and Veterinary Cannabinoid Academy Facebook groups when not lecturing or writing.