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Regional analgesia for oral surgery in dogs and cats (Proceedings)

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Aug 01, 2010

Agents

This author commonly uses lidocaine and bupivicaine combined in the same syringe for regional oral nerve blocks. Lidocaine is not desirable as a sole agent due to its limited effect post administration (1-2 hours). Bupivicaine requires administration much earlier in the anesthetic event than lidocaine in order to be effective prior to surgical tissue manipulation. These limitations can be minimized or eliminated by using them in combination. The quick onset of lidocaine coupled with Bupivicaine's extended duration of effect (up to 8 hours5) make them a good choice for combined use in regional nerve blocks. The agents may be used with or without epinephrine.

Although rare, complications are possible with inadvertent systemic administration or systemic uptake of locally delivered local anesthetic agents. Central nervous system excitement including seizures followed by depression, apnea, severe cardiovascular compromise, dysrhythmias and hypotension are all possible. Direct toxicity to skeletal muscle and anaphylactic reactions are possible with local administration. The author has never experienced any of these complications at the doses recommended in this article.


Table 1 Recommended infusion volumes per site of lidocaine:bupivicaine
A maximum dose of 1.0 mg/kg of each agent in combination in the same syringe is recommended. Care should be exercised, especially in smaller patients and cats, so that maximum doses are not exceeded. The ratio of lidocaine:bupivicaine is 1:4 when using concentrations of 2% and 0.5 % respectively. The total volume used depends upon the size of the patient and the number of sites that require analgesia. The maximum number of sites would be four if the patient requires surgical manipulation in all four quadrants. Table 1 shows recommended infusion volumes per site based upon patient size. Please note that these volumes represent both agents mixed together. Table 2 shows a sample calculation for a 4 kg dog.


Table 2 Sample calculation for a regional nerve block for oral surgery for a 40 kg dog.
Physiologic parameters will determine if the block is effective. If blood pressure, heart rate and respiration rate increase with surgical manipulation the block either was not correctly placed or hasn't had the proper amount of time for onset. (5 minutes) If enough time has elapsed to be comfortable that onset is not an issue the block may be repeated as long as the maximum total dose is not exceeded. It should be noted that the low ph of inflamed tissue will render local anesthetics less effective.

Opiates

Based upon human research the use of local analgesics and opiates in combination provides extended duration of action. The addition of morphine or buprenorphine to a regional brachial plexus block for limb amputation demonstrated close to a twofold increase in duration of effect over patients receiving only bupivicaine in one study. A similar model was used in another study where researchers demonstrated that the addition of buprenorphine to the regional block provided significant increases in duration compared to giving the buprenorphine intramuscularly.

The author currently ulitizes opiates in the lidocaine:bupivicaine mixture for procedures where increased duration of effect are desirable. Patients that are particularly difficult to medicate, either in the hospital or at home, may benefit from this combination. Also patients where chronic pain states exist, as seen in canine or feline stomatitis, may make postoperative pain management challenging. Morphine may be utlilized as a portion of the premedication protocol at 0.5 mg/kg IM 20 minutes prior to induction. One fourth of that same volume is added to the local mixture per site to be blocked. Hydromorphone is administered at 0.1 mg/kg as a portion of the premedication protocol. One fourth of that same volume is also added to the local mixture per site to be blocked. Buprenorphine may also be used as a premedication at 0.015 mg/kg following the same volume recommendations for local infiltration as with the examples above. Morphine and hydromporphone are pure mu agonists and are preferred for management of severe pain. Buprenorphine is a partial mu agonist is a good choice for mild to moderate pain. All three agents may be used in either species, however pure mu agonists, in particular hydromorphone, are known to cause hyperthermia in cats. If pure mu agonists are used either either locally or systemically in cats, temperature should be monitored during and up to 5 hours post procedure.