By Stephanie Krein, DVM, DACVAA
Locoregional anesthesia has been widely used in human medicine for decades in a wide array of procedures including orthopedic surgeries, oncologic surgeries, abdominal procedures, and even laparoscopic procedures. Regional anesthesia has been shown to reduce risks associated with general anesthesia by decreasing the amount of anesthetic needed, to improve intraoperative analgesia, and to improve post-operative comfort and recovery.1 Recently, there has been an increase in interest in locoregional anesthesia in veterinary medicine and a surge in the number of articles in veterinary literature. The focus of this article will be to review the basics of local anesthetics used in clinical practice, discuss the techniques used to perform regional anesthesia, and to give examples of some of the more commonly used techniques in veterinary medicine. The specific steps to perform each type of block are beyond the scope of this article and can be found in both textbooks and the literature.
Local anesthesia results from the blockade of nerve impulses to abolish sensation. All currently available local anesthetics are either aminoesters or aminoamides. Local anesthetics work by preventing electrical impulse conduction by membranes of nerves and muscle. They can be applied topically, injected around peripheral nerves or major nerve trunks, or administered within the epidural or subarachnoid spaces. Local anesthetics act mainly on voltage-gated sodium channels blocking inward movement of sodium thereby impeding membrane depolarization and nerve excitation and conduction. Local anesthetics express what is called a differential pattern of sensory and motor blockage: first vasodilation occurs, followed by loss of sensation of temperature, sharp pain, light touch, and lastly loss of motor activity.2 In addition to blocking sodium channels, local anesthetics also inhibit other ion channels such as potassium or calcium channels at the level of the spinal cord which most likely contributes to their nociceptive effects. Local anesthetics commonly used today are for the most part amides and include lidocaine, bupivacaine, ropivacaine, and mepivacaine. Lidocaine is one of the most commonly used local anesthetics and has a rapid onset of action and a duration of action of one to two hours.3 Lidocaine can be used for peripheral nerve blocks, epidurals, and systemically. The toxic dose of lidocaine in dogs is 6-10 mg/kg and in cats is 3-5 mg/kg2. Bupivacaine is also very commonly used in small animal medicine and has a slower onset of action of about 20 minutes and a longer duration of action of 8 to 12 hours.3 Unlike lidocaine, bupivacaine cannot be given intravascularly and is reserved for local blocks and epidurals only. Ropivacaine and mepivacaine are more commonly used in large animal medicine and surgery.
Figure 1. The top picture shows the proper subcutaneous administration of a local anesthetic at midline spanning from umbilicus along the anticipated incision length. The bottom picture shows the administration of local anesthetic into the body of the testicle and away from the epididymis. Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, ed. Veterinary Anesthesia and Analgesia: The fifth Edition of Lumb and Jones; Wiley Blackwell. 2015
Many factors affect the onset and duration of local anesthetics including pKa, protein binding, liver and renal function, age of patient, vascularity of tissue site, pregnancy status, and presence of additives. It is important that the veterinarian is familiar with the comorbidities of the patient in which a local anesthetic will be used. Both renal and hepatic dysfunction can lead to accumulation of local anesthetics and an increased risk of toxicity because of reduced metabolism and excretion (of the parent drug and active metabolites).2 Signs of toxicity include CNS signs and cardiovascular collapse. CNS signs follow a progression and include anxiety, mild sedation, ataxia, muscle twitching and seizures. These signs are thought to precede the signs of cardiovascular toxicity. Cardiovascular collapse is due to the binding of cardiac sodium channels and signs include arrhythmias, hypotension, bradycardia, reduced myocardial contractility, and asystole. The treatment of local anesthetic toxicity includes supportive care such as oxygenation and ventilation and possibly lipid treatment.
Figure 2. Ultrasound appearance of the femoral nerve located at the femoral triangle. This is the target nerve for perineural administration of local anesthetic when providing anesthesia/analgesia to the stifle and distal. Grimm KA, Lamont LA, Tranquilli
Many different types of locoregional anesthesia can be employed in everyday practice, ranging from simple blocks such as line blocks to more complicated blocks such as those requiring an ultrasound or a nerve stimulator. Rarely is there a surgery or procedure that the patient will not benefit from the addition of a local block as they are often fast and easy to perform, and also inexpensive. Simple blocks that can be performed in general practice on a daily basis include line blocks for ovariohysterectomies, testicular blocks for castrations, wound infiltration, and ring blocks for procedures distal to the carpus or tarsus. These blocks can be performed by both technicians and doctors (Figure 1) and provide excellent patient comfort and reduced postoperative use of opioids.4 Drug choice for these simple blocks depends on time to initial incision, intended or needed duration of analgesia, and the invasive nature of the procedure. Lidocaine is often a good choice if the procedure will occur only a few minutes after the block such as with wound infiltration. Bupivacaine is commonly used for testicular blocks and incisional blocks as it provides a longer duration of analgesia than lidocaine, which is warranted in painful procedures. When performing any local block it is of utmost importance to always avoid intravascular and intraneural injections. The addition of an intraperitoneal block to any spay or abdominal procedure has also been shown to reduce post-operative pain and opioid consumption in both human and veterinary patients.5 Intraperitoneal blocks are easily performed by splashing 1.5 to 2 mg/kg bupivacaine onto the peritoneal surfaces where vast numbers of nociceptors are present.
In addition to simple blocks, more complicated blocks are performed by anesthesiologists using ultrasound guidance or nerve stimulator localization. Special needles are used when performing both ultrasound-guided and nerve stimulator-guided nerve blocks. When performing ultrasound-guided nerve blocks, echogenic needles are used to allow for easier visualization of the needle and more accurate delivery of the local anesthetic around the nerve (Figure 2). When performing blocks using the nerve stimulator, needles insulated (most often with Teflon) over their entire length are used to allow the electricity to exit only from the tip of the needles allowing precise nerve localization. (Figure 3). Locoregional anesthesia using the ultrasound or nerve stimulator or a combination of both allow anesthesia and analgesia to be provided to almost any part of the body. Commonly performed blocks at our hospital are performed using the nerve stimulator and include: femoral/sciatic nerve blocks (Figure 4), providing anesthesia and analgesia to the stifle and anything distal to it; brachial plexus blocks, providing anesthesia/analgesia to the distal humerus and anything beyond it; RUMM blocks providing anesthesia/analgesia to the distal radius/ulna and anything beyond it. Lumbosacral epidurals are commonly used to provide anesthesia/analgesia for abdominal procedures, pelvic fractures, TPLOs, any painful procedure involving the hind limbs, and amputations. Caudal epidurals can be performed with or without a nerve stimulator at S3-Cd1 or Cd1-Cd2 and provide perineal, rectal, vaginal/vulvar anesthesia and analgesia.
Locoregional anesthesia is an underutilized mode of providing excellent analgesia and improved patient experience overall. Anesthesiology and pain medicine in humans is trending heavily toward performing as many procedures as possible under sedation and locoregional anesthesia and hopefully veterinary medicine will follow in their footsteps.
Figure 3. Location of insulated needle insertion during performance of femoral lock. Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, ed. Veterinary Anesthesia and Analgesia: The fifth Edition of Lumb and Jones; Wiley Blackwell. 2015
Figure 4. Anatomic dissection of the femoral nerve. Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, ed. Veterinary Anesthesia and Analgesia: The fifth Edition of Lumb and Jones; Wiley Blackwell. 2015
- In: Miller, RD, ed. Miller’s Anestheisa, 7th edition: Local anesthetics. Philadelphia: Churchill, Livingstone, Elsevier; 2010: 913-940.
- In: Grimm KA, Lamont LA, Tranquilli WJ, Greene SA, Robertson SA, ed. Veterinary Anesthesia and Analgesia: The fifth Edition of Lumb and Jones; Local anesthetics. Iowa: Wiley Blackwell. 2015:332-354.
- In: Campoy LC, Read MR, ed. Small Animal Regional Anesthesia and Analgesia. Iowa: Wiley Blackwell. 2013.
- Savvas, Papazoglou LG, Anagnostou T, Tsioli V, Raptopoulos D. Incisional block with bupivacaine for analgesia after celiotomy in dogs. J Am Anim Hosp Assoc. 2008 Mar-Apr; 44(2):60-6.
- Campagnol D, Teixeria-Neto FJ, Monteiro ER, Restitutti F, Minto BW. Effect of intraperitoneal or incisional bupivacaine on pain and the analgesic requirement after ovariohysterectomy in dogs. Vet Anaesth Analg. 2012 Jul; 39(4): 426-30.