Veterinary Medicine - January 2009 - (Page 29)

including direct effects on opioid receptors in the periphery, anti-in ammatory effects, systemic absorption, and action at the spinal cord due to slow retrograde axonal transport along the nerve.4-6 Directly applying opioids to a peripheral nerve at high concentrations can produce analgesia that is not naloxonereversible and is thought to be a nonspeci c local anesthetic-like action.4 In addition, opioid mu and kappa receptors are present on the distant peripheral terminals of C bers, and these receptors increase in number and become activated when there is in ammation at the site.4-6 Opioids attenuate the excitability of peripheral nociceptive terminals of primary afferent neurons during in ammation by inhibiting high-voltage activated calcium channels, which decreases the propagation of action potentials and the release of C ber excitatory transmitters, such as substance P.4-6 Opioids may also mediate local analgesia through anti-in ammatory effects. Opioid receptors on immune cells and endogenous opioid peptides expressed in resident immune cells within peripheral in amed tissue mediate lymphocyte function suppression and cytokine synthesis and release. Both mu and kappa opioid agonists have shown potent antiin ammatory activity.5,6 Morphine is often used in intraarticular and epidural blocks, along with local anesthetics, and seems to provide additional analgesia in people and dogs.7-9 Buprenorphine has been shown to extend the duration of analgesia with brachial plexus and epidural blocks in people, compared with local anesthetic administration alone.10-13 Epidural morphine and buprenorphine have induced thermal nociception in cats, but morphine antinociception lasted longer and was more intense.14 The mechanism for the increased efcacy and duration of analgesia with peripherally administered opioids appears to be related to the lipophilicity of the opioid since the highly lipophilic buprenorphine provided a longer dura- tion of satisfactory analgesia in people after a brachial plexus block than did the less lipophilic sufentanil.13 The antiin ammatory effects of opioids likely also play a role in prolonging analgesia.5 Minimal adverse affects are associated with the peripheral administration of opioids because doses administered peripherally result in low plasma concentrations in most species.2,5 Alpha2 agonists Alpha2 agonists, such as medetomidine, can be added to local anesthetic blocks to extend the duration and increase the ef cacy of the block.15-18 Alpha2 agonists likely exert these effects through several mechanisms, including vasoconstriction, resulting in decreased systemic absorption of local anesthetics; facilitation of C ber blockade by the local anesthetic; and action at the spinal cord due to slow retrograde axonal transport along the nerve.17 Norepinephrine is a principal neurotransmitter in the sympathetic nervous system that is also involved in nociception through alpha2-adrenergic receptors expressed on peripheral sensory neurons. In pathological pain states, such as neuropathic pain and in ammation, nociceptors develop increased sensitivity to norepinephrine, and peripheral antinociception is likely mediated through these peripheral dogs, and cats and extended the duration of radial nerve blockade with mepivacaine in dogs.15,19-22 Dexmedetomidine (Dexdomitor—P zer Animal Health) has potentiated and prolonged the effects of intrathecal and epidural local anesthetics in rats, sheep, guinea pigs, and people.23 Spinal administration of dexmedetomidine provided a powerful antinociceptive effect in dogs,24 and epidural dexmedetomidine was found to decrease the minimum alveolar concentration (MAC) of iso urane in dogs.25 The addition of dexmedetomidine to the local anesthetic solution in intravenous regional anesthesia in people improved the quality of anesthesia and decreased analgesic requirements but had no effect on the sensory and motor blockade onset or duration.26 Sedation, bradycardia, and decreased cardiac output can occur from systemic absorption, so alpha2 agonists should not be used in hypovolemic patients or patients with impaired cardiovascular function.15,17,22,25 LOCAL AND REGIONAL BLOCKS Strict adherence to sterile technique will prevent complications from infection. Always surgically prepare the skin, wear sterile gloves, and use only sterile needles, syringes, and catheters. Sterile skin preparation is not required for The anti-in ammatory effects of opioids likely also play a role in prolonging analgesia. alpha2-adrenergic receptors.17,18 Clonidine has been used for intraarticular, epidural, intrathecal, intravenous regional, and peripheral nerve blocks in people.17,18 Medetomidine provided effective analgesia when administered epidurally in goats, cows, digital nerve blocks, but sterile needles and syringes should be used. To avoid complications from intravenous or intraarterial injection of local anesthetics, always aspirate before injection. In addition, always calculate the maximum total dose for each patient, reducing the VETERINARY MEDICINE January 2009 29

Table of Contents Feed for the Digital Edition of Veterinary Medicine - January 2009

Veterinary Medicine - January 2009 Contents Leading Off Letters Research Updates Idea Exchange A Modified Subconjunctival Enucleation Technique in Dogs and Cats Local and Regional Anesthesia Techniques Author Guidelines Product Preview CE Form/Advertiser Index Marketplace/Classifieds Mind Over Miller

Veterinary Medicine - January 2009

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