Common Dosages** Fentanyl CRI Loading Dose 5 to 10 mcg/kg CRI 0.3 to 1.0mcg/kg/min (anes)

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1 ADVANCED ANESTHETIC AND ANALGESIC TECHNIQUES Jody Nugent-Deal, RVT, VTS (Anesthesia/Analgesia) (CP-Exotic Companion Animal) University of California Davis, William R. Pritchard Veterinary Medical Teaching Hospital Small Animal Anesthesia Department Advanced pain management techniques such as local and regional blocks, analgesic constant rate infusions and epidural anesthesia/analgesia can be incorporated into almost any clinical setting. You do not need to work in a specialty referral hospital or academic institution to utilize and effectively perform advanced pain management techniques. INTRODUCTION Controlling pain in small animals, including exotic small mammals is extremely important. The use of local and regional blocks and constant rate infusions (CRIs) of analgesic drugs can be an extremely effective way to manage pain. The species, type of procedure, and stat of the patient must be taken into consideration prior to administration of any anesthetic protocol. EPIDURAL ANESTHESIA AND ANALGESIA An epidural should be considered for patients requiring painful procedures of the hind limbs, abdomen, thorax, and even the forelimbs. Epidural placement can be performed in as little as five minutes, therefore adding very little additional time to the total time unde anesthesia. The patient is generally positioned in sternal recumbency although lateral recumbency is another positioning option. The wings the ilium should first be palpated. The lumbosacral space can be palpated between vertebral bodies L7 and S1. Prior to placing the epidural needle, the area should be shaved and aseptically prepared as you would for a surgical procedure. Sterile gloves must be worn when administering the epidural. A 25 or 22 gauge spinal needle is generally utilized with a length ranging from 1 to 3 depending on the size of the patient. A regular hypodermic needle can be used, but in the author s opinion, a spinal needle is ideal. The spinal needle contains a stylet that runs through the center of the needle. This stylet helps keep the needle from becoming obstructed with tissue during placement. The spinal needle should be placed perpendicular on midline and slowly inserted through the skin and into the epidural space. A pop will be fe as the needle passes through the ligamentum flavum and enters the epidural space. If the needle touches bone, you have gone too far and wil need to back the needle out slightly. A sterile glass syringe containing a small amount of air (0.5 to 1mL) can be placed on the spinal needle and injected into the space. If the air injects easily, then you are likely in the correct spot. If there is a vacuum on the syringe, you are likely not in the correct space and will need to reposition the needle. This technique is called the loss of resistance technique. A glass syringe is chosen over a plastic syringe because there is less friction during injection; therefore, making it easier to determine if the needle has been correctly placed into the epidural space. You can also use a technique called the hanging drop technique. This technique is often preferred because it doesn t require the u of additional equipment such as a glass syringe. Because you do not have to handle a second syringe, there is a decreased chance of accidentally moving the spinal needle out of the epidural space. Once you have placed the spinal needle under the skin, you can remove the stylet and place a drop of saline into the hub. The needle will then need to be slowly advanced into the epidural space. It is important to monitor the saline drop as the needle is advanced. If the drop is sucked into the hub of the spinal needle, the needle had been placed into the correct area. If the drop of saline is not sucked into the hub of the needle as it is advanced, it doesn t necessarily mean that the needle has bee incorrectly placed. The needle could be sitting against epidural fat. If this is the case, suction of the saline will likely not occur. Correct placement can be confirmed with the use of a glass syringe. The hanging drop technique works well in most medium and large sized canin patients. In the author s experience, this technique is not as consistent in smaller dogs, cats, and exotic small mammals. Having a 1 or 3ml glass syringe available for these patients is generally helpful. The syringe containing the drugs should be attached to the hub once the spinal needle has been correctly seated within the epidural space. It is important to gently pull back on the plunger of the syringe to ensure there is blood or spinal fluid aspirated. If blood is aspirated, you need to remove the needle and start over. If spinal fluid is aspirated, it is suggested that only 1/4 of the initial calculated dose is delivered. Common drugs used for epidural administration include preservative free morphine, buprenorphine, lidocaine and bupivacaine. It is important to note that epidural anesthesia/analgesia should not be administered to patients that are septic, have signs of pyoderma or hav suspected neoplasia around the epidural site. Common Drug Dosages Preservative Free Morphine mg/kg diluted to 0.33 ml/kg with sterile saline, administered into the epidural space (with a maximum volume of 6.0 ml regardless of patient size) Buprenorphine mcg/kg diluted to 0.33 ml/kg with sterile saline administered into the epidural space (with a maximum volume of 6.0 ml regardless of patient size) Lidocaine 0.5 mg/kg to 1.0 mg/kg diluted to 0.33 ml/kg with sterile saline administered into the epidural space (with a maximum volume of 6.0 ml regardless of patient size)

2 Bupivacaine 0.5 mg to 1.0 mg/kg diluted to 0.33 ml/kg with sterile saline administered into the epidural (with a maximum volume 6.0 ml regardless of patient size) Preservative Free Morphine & Bupivacaine 0.1 mg/kg of preservative free morphine mixed with 0.5 to 1.0 mg/kg bupivacaine (with maximum volume 6 ml regardless of patient size) Buprenorphine & Bupivacaine 12.5 mcg/kg buprenorphine mixed with 0.5 to 1.0 mg/kg bupivacaine (with a maximum volume 6.0 ml regardless of patient size) Example: You will be administering a morphine and bupivacaine epidural to a 10 kg dog. Calculation: (wt.) X (0.33mL/kg) = total volume therefore (10 kg) X (0.33mL/kg) = 3.3 ml total epidural volume Preservative Free Morphine ((wt.) X (dose)) / (concentration of drug) = morphine dose in ml Therefore ((10 kg) X (0.1 mg/kg)) / (25 mg/ml) = 0.04 ml Bupivacaine ((10 kg) X (1 mg/kg)) / (5 mg/ml) = 2 ml Now add the volume of morphine to the volume of bupivacaine. The total volume is 2.04 ml. We need to give a total volume of 3.3 ml, so the remaining volume of 1.26 ml will be added to the syringe of drugs. We will use preservative free saline for this remaining volume. It is important to note that the maximum dose of lidocaine and bupivacaine should not exceed 2 mg/kg and 1 mg/kg respectively. You must take into account any lidocaine and/or bupivacaine administered not only in the epidural, but also given in other local blocks, such as, ring blocks, testicular blocks, line blocks, or even small amounts administered onto the tracheal opening to prevent laryngospasms. Note: This is especially true for small exotic mammals and very small kittens, puppies, toy breeds, etc. CONSTANT RATE INFUSIONS Delivering a constant rate infusion(s) during and after general anesthesia is an excellent way to provide additional analgesia to the patient. Common drugs used for analgesic CRIs include ketamine, lidocaine, hydromorphone, morphine and fentanyl. Using one or a combination of these drugs not only helps provide additional analgesia, but depending on the species may also help reduce the percentage of gas anesthesia (MAC) needed to keep the patient in a surgical plane of anesthesia. Reducing the amount of inhalant anesthesia has many benefits including helping reduce hypotension commonly experienced with inhalants, such as, isoflurane and sevoflurane. If an opioid is use as a CRI, the patient should generally be intubated and placed on intermittent positive pressure ventilation (either manual or mechanical) because opioids can cause severe respiratory depression. In many instances, analgesic CRIs require a loading dose given at the onset of CRI delivery. A loading dose will quickly increase th drug plasma concentration levels; enabling the low dose CRI to become effective quickly. Common Dosages** Ketamine CRI Loading Dose 1 to 2 mg/kg CRI - 10 to 20 mcg/kg/min Fentanyl CRI Loading Dose 5 to 10 mcg/kg CRI 0.3 to 1.0mcg/kg/min (anes) CRI 0.05 to 0.3 mcg/kg/min (post-op) Hydromorphone Loading Dose 0.05 mg/kg CRI 0.05 to 1.0 mg/kg/hr (anes) CRI 0.01 to 0.02 mg/kg/hr (post-op) Lidocaine Loading Dose 1 mg/kg CRI mcg/kg/min Morphine Loading Dose 0.5 mg/kg CRI mg/kg/hour (Cats are generally given a fentanyl loading dose of 5 mcg/kg and a CRI of 0.2 to 0.4 mcg/kg/min) It is important to note that post-operative doses are much smaller than anesthetic doses. Since these doses are so much smaller, common side effects as in moderate to severe respiratory depression are not generally an issue. If a syringe pump or pumps are not available, one or more analgesic drugs can be delivered by adding them to a bag of crystalloid fluids. Th

3 author prefers the use of syringe pumps so that drugs can be titrated to effect as needed. If a bag of fluids is used to deliver the CRI, another bag of fluids without any drugs added should be available in the event that a fluid bolus is necessary. Bolusing fluids with drugs added can lead to adverse effects, for instance, bradycardia, respiratory depression or even anesthetic overdose. The following calculation is used for adding a drug to a bag of fluids: X (amount of drug to add in ml) = CRI rate (mg/kg/min) or (mcg/kg/min) x total volume in bag (ml) Fluid rate (ml/kg/hr) Example: You have been asked to prepare a 0.2 mg/kg/hr CRI of morphine. You are anesthetizing a large dog; therefore, a 1000mL bag of fluids was chosen for the dilution. The calculation should be as follows: X = 0.2 mg/kg/hr x 1000mL 5 ml/kg/hr X = 0.04 x 1000 X = 40 mg of morphine To convert mg to ml you must divide by the concentration of the drug. Therefore X = 40 mg of morphine = 2.7 ml 15 mg/ml You must add 2.7 ml of 15 mg/ml morphine to 1000mL of crystalloid fluids in order to administer a 0.2 mg/kg/hr CRI at a fluid rate of 5 ml/kg/hr. This same equation is used when other drugs such as ketamine and/or lidocaine are incorporated into the CRI. If general anesthesia is induced with any of these drugs, the induction dose can be used as the loading dose as long as the CRI is started withi a few minutes of induction. CALCULATING A CONSTANT RATE INFUSION Calculating a CRI is very easy once you understand what formula to use. For example, you are about to anesthetize a patient that requires a fracture repair of the right femur. How would you calculate a CRI of fentanyl? The formula for calculating a CRI is as follows using a syringe pump: [(Patient s weight) X (Dosage of the drug) X (*Time factor)] / Concentration of the drug *The time factor for this equation is 60 minutes/hour Let s say the patient weighs 2.0 kg and the dose of fentanyl that we are going to administer is 0.7 mcg/kg/min. Since the dose is given as mcg/kg/min, you will need to convert this to ml/hr. The concentration of fentanyl is 50 mcg/ml. You now have all the information needed to calculate the CRI. [(2.0 kg) X (0.7 mcg/kg/min) X (60 min/hr)] / 50 mcg/ml = 1.68 ml/hr This standard equation can be used for any CRI that is administered via a syringe pump. LOCAL ANESTHETIC TECHNIQUES Local and regional anesthetic techniques are the only way to provide a complete blockade of peripheral nociceptive input. Therefore they are the most effective way to prevent sensitization of the central nervous system and development of pathological pain. The onset and duration of local anesthetics will vary based on the drug chosen. However, the pre-operative use of local anesthetics will reduce inhalant anesthetic requirements and can often help patients have a smoother and less painful recovery. It is important to note that lidocaine has a qui onset, but a short duration of action while bupivacaine has a longer onset and longer duration of action. Lidocaine will become effective in a little as 5 minutes and will last about 1 to 2+ hours. Bupivacaine will become effective in about 15 to 20 minutes and last about 4 to 6+ hour TOPICAL ANESTHETICS Topical anesthetics such as 2.5% lidocaine mixed with 2.5% prilocaine (EMLA cream) can be applied to skin for minor procedures, for example, intravenous and arterial catheter placement. It is advisable to shave the area of interest, spread on a thin layer of cream, and pla a semi-occlusive dressing over the area of application for at least 10 minutes. Semi-occlusive dressing helps to increase absorption of the loc anesthetic drugs. The author prefers this technique for placing arterial catheters into the auricular arteries of rabbit s ears.

4 SPLASH BLOCKS Local anesthetics can be administered into existing wounds or open surgical sites. This is usually accomplished by splashing (infiltrating) the local anesthetic into the open wound or surgical site. INFILTRATION OF LOCAL ANESTHETICS Local anesthetics are commonly used to provide additional anesthesia and analgesia for procedures, such as, minor laceration repair skin biopsies, and removing small tumors lying just under the skin. A variety of blocks can also be performed for major surgical procedures and act as an adjunct to pain relief. Local anesthetics (lidocaine and bupivacaine) can be injected into the tissue to provide the local blockade Infiltration of local anesthetics is generally quite easy and relatively quick. The area should be shaved and aseptically prepared prio to administering any drugs. Aseptic technique will help prevent accidental contamination of the tissues with skin bacteria when the local anesthetic is injected. Generally, a small 25 to 27-gauge needle attached to a 1 ml, 3 ml or 6 ml syringe is used to prevent tissue damage a allow for more precise administration of the drug. The volume of drug to be administered will vary based on the area of interest and size of patient. If the calculated volume to be delivered is small, it will be necessary to dilute the local anesthetic prior to administration. Diluting th local anesthetic enables better distribution of the drug and an overall more accurate block. Sodium chloride 0.9% is the most common fluid used for dilution. It is important to remember to aspirate the syringe prior to giving the injection. If blood is aspirated, reposition and start over. This is true for any injection of a local anesthetic. **Common local nerve blocks include dental, paravertebral, brachial plexus, intercostal, testicular, and distal limb blocks. The use soaker catheters are also a great way to provide long lasting pain relief over hours. Common dental nerve blocks include the maxillary, infraorbital, inferior alveolar, and mental blocks. Blocking these nerves provide excellent anesthesia for extractions and facial surgery. The maxillary nerve block provides anesthesia for the caudal portion of the maxilla. The infraorbital nerve block provides anesthesia for the rostral portion of the maxilla. The inferior alveolar nerve block provides anesthesia t the caudal portion of the mandible while the mental nerve block provides anesthesia to the rostral portion of the mandible. It is ideal to use a insulin syringe with attached needle for very small patients. A 1 to 3mL syringe with a 25 to 22-gauge needle attached can be used for larger dogs. Suggested Volumes for Local Anesthetic Dental Blocks: Cat and small dog Medium dog Large to extra large dog 0.25mL per site 0.5mL per site 1.0mL per site *Volumes are based on author s experience. Maximum dose should be calculated prior to administering local anesthetic. Blockade of the brachial plexus is generally used to manage perioperative pain of the forelimb. The best technique for successfully blocking the brachial plexus is to block the nerves of the brachial plexus at C6 to T1. It is ideal to block the nerves as close to the interverteb foramina rather than the axillary space. It is important to note that this procedure is not benign. The phrenic nerve can become blocked, anesthetizing a portion of the diaphragm, thus a brachial plexus block should only be performed unilaterally. Other complications include pneumothorax, intravenous, and intrathecal injection. A nerve stimulator and insulated needle should be used to increase the success rate of this block. Intercostal nerve blocks can be used for patients with fractured ribs or for those undergoing a lateral thoracotomy. To provide complete blockade of the affected area, the local anesthetic should be injected not only at the caudal boarder of the rib at the fracture or incisi site, but also two to three ribs on either side of the fracture or incision site. Testicular blocks are a cost effective and easy way to provide MAC reduction and additional pain relief both during and after castration. Blocks can be done in most species including canine, feline, and exotic small mammal patients. Prior to injection, the testicle should be aseptically prepared. This block is generally performed in two steps. The needle is first inserted into the caudal pole of each testic and inserted cranially toward the spermatic cord. In the second step, an incisional block is performed at the anticipated surgical site. Distal limb blocks are effective for procedures involving the lower extremity and digits. Examples include dewclaw removal, cat declaws, mass removals, etc. A small amount of local anesthetic is injected around the nerve subcutaneously and caudal to the surgical site. Common sites include the radial, ulnar, and median nerves. Soaker catheters are used for procedures such a large wound repairs, mass removals, and limb amputations. The catheters are

5 aseptically placed by the surgeon just prior to closure of the surgical site. Soaker catheters are available commercially, but can also be easily made using a rubber feeding tube. To make a soaker catheter, first set up a sterile barrier drape and remove the feeding tube using aseptic technique. A sterile large bore needle is used to make several holes along the length of the tube. Sterile saline is then injected through the tu to ensure the holes are patent. After these steps have been completed, the soaker catheter can be handed to the surgeon. Once the catheter ha been sutured in place, a bolus of local anesthetic can be administered. In some cases, a CRI may be used to infuse small volumes of drug ove a specific time frame. In general, soaker catheters are used for 24 to 48 hours post-surgery. SUGGESTED READING Bryant, Susan Anesthesia for Veterinary Technicians. Wiley and Sons. Seymour, Chris and Tanya Duke-Novakovski BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. 2 nd ed. British Small Animal Veterinary Association. **All drugs listed in this article are common dose ranges used in the Small Animal Anesthesia Department at the UC Davis VMTH. Doses should be approved by a veterinarian prior to administration.