Veterinary Anaesthesia Services, International/University of Zurich, Center for Applied Biotechnology and Molecular Medicine, Winterthur, Switzerland
Local anesthetics do represent a unique opportunity in perioperative analgesia, as they have the potential to not only reduce painful sensations, but to abolish them completely. The effective block of nociceptor activity and of nerve conduction not only is reducing or preventing pain sensation, it will also help prevent the buildup of later possible chronic pain development. Particularly when needing to provide perioperative analgesia without the use of opioids, local anesthetics become useful. However, employing local anesthetic blocks in the perioperative period allows for a strong reduction of analgesic requirements anyway, and will produce less painful patients while circumventing side effects that may represent a concern with systemically applied analgesics at higher doses.
The use of local anesthetics is severely underrepresented in small animal clinical practice. Traditionally, small animal veterinarians provided perioperative analgesia using systemic analgesics, such as opioids, alpha-2-agonists, and NSAIDs probably because we could—at least in many countries. Systemic application is easy to perform and much information about dosing schedules is available, allowing for some acceptable clinical safety. In North America, the use of opioids has become challenging, as they are under much discussion. In many other countries of the world, they have never been broadly available. This development and pain pathophysiology issues (hypersensitivity with long-term administration, dependence) call for a broader use of local anesthetics in chronic pain situations, but particularly in the acute pain situation of a perioperative period.
One of the main issues in preventing their broad use is the need for some special technology (ultrasound-guided and electrostimulator-guided nerve blocks) and, maybe more important, the mastering of the application techniques. This is a training issue and a question of mindset.
Local anesthetics can be applied in a multitude of routes:
- The peripheral infiltration and drip-on techniques (splash-blocks), designed to desensitize proprioceptors.
- There are mucosal, corneal and transcutaneous delivery options to block sensations to superficial nociception.
- Intravenous application of lidocaine is of particular importance for dogs and horses to provide systemic analgesia of short duration, or visceral analgesia and pro-peristaltic and anti-endotoxemic effects during and after visceral operations, but is not recommended for cats.
- Peripheral nerve block techniques. There are such blocks that can be easily learned and do not necessarily require special apparatus, such as the dental nerve blocks. Most limb nerve blocks are best performed using an electrical nerve stimulator. Some peripheral blocks (such as the transversus abdominis block, TAP) and some blocks that desensitize a whole plexus (such as the brachial plexus) are probably best performed using the guidance of an ultrasound imaging.
- Neuraxial anesthesia (epidural or subdural) is not a difficult technique that can be used to block sensations, particularly in the hind limbs and the caudal half of the body (abdomen, perineum).
- Intra- and periarticular applications offer perioperative solutions to abolish pain sensation. This very common application technique has been subject to reviving discussions of the last years.
- Wound instillation techniques commonly employ (surgical) placement of so-called wound perfusion catheters, specifically designed perfusion devices that allow instillation of a local anesthetic over time (up to 3–5 days); this is particularly useful for larger wound areas (mastectomies), or areas to which systemically applied analgesics are hard to get to (total ear canal ablation).
- Perineural catheter applications: specifically designed catheters allow for an instillation of local anesthetic over time around a particular large nerve. This is a method of choice to provide analgesia after limb amputations.
- Intravenous regional anesthesia replaces blood in a distal part of a limb with lidocaine and in this way not only desensitizes the limb part, but also makes for a bleeding-free operation site.
- Intratesticular and intra-ovarian application.
No other group of analgesics, or drugs in general, presents with that many options for applications, providing analgesia to the whole body or parts thereof.
Misconceptions and Controversies
There are some points that are discussed.
- As with any drug, a local anesthetic may produce unwanted side effects. Most of these side effects are related to the dose given. Side effects that are commonly listed may include central nervous system depression, cardiac arrhythmias, and seizures. At clinically used doses (particularly after neuraxial application), the most common side effects, however, may be hypotension and hypothermia. As a general line of thought, such side effects do occur at lower doses in cats when compared to dogs. It is a misconception, however, to deduce from this that cats should not receive local anesthetics. Maximum recommended doses for single local anesthetics in dogs and cats have been published in the Guidelines of Recognition, Assessment, and Treatment of Pain by the Global Pain Council.
- Local anesthetics exhibit chondrotoxicity when administered intra-articularly. This has been particularly shown for lidocaine and bupivacaine—less so for ropivacaine—and is a more important side effect when pumps (continuous administration) are used. The use of intra-articular local anesthetic application has never reached the clinical importance it clearly had in human anesthesia. However, currently it seems that ropivacaine demonstrates less chondrotoxicity, and a single administration is associated with a less profound and a shorter duration of metabolic disruption to chondrocytes.
- Side effects due to local anesthetic overdose can be treated using symptomatic crystalloid infusion therapy. However, this may depend on the side effect to counteract. Hypotension, to a degree, may be treated using crystalloid infusions, but CNS depression or seizures may require lipid emulsion infusions.
- Adjuvants to local anesthetics. It is a not infrequent thought to include other drugs in a local anesthetic solution to be administered. These may include epinephrine (adrenaline), hyaluronidase, and bicarbonates with different ideas, effects, and side effects. Their addition to the administered solution requires careful thought for each single-application technique, and a number of application techniques preclude their use.
- Combining a long-acting and a fast-acting local anesthetic provides fast and long-duration analgesia. This is mostly discussed for the combination of lidocaine and bupivacaine. While a faster onset of a peripheral-nerve block is highly likely with such combination, it is as likely to present with a reduced duration of effect. Furthermore, regarding side effects, it may be important to notice that toxicity of local anesthetics is additive. However, a faster onset than with the slow-onset drug alone, and a longer duration than with the fast-acting drug alone, are highly likely.
- That all local anesthetics act the same on all nerve fibers is a common misconception. Local anesthetics may present (for example, mepivacaine, ropivacaine) with a certain preference for sensitivity over motor nerve fibers. Such a differential block may affect the choice of local anesthetic.
During the presentation, these and more of such questions are addressed, revealing some as misconceptions, some as rather correct assumptions, and some as not yet clearly answered in a scientific way.