February 2005 (full review 11-11)
Alpha-2 agonists provide sedation, analgesia, muscle relaxation and anxiolysis. A variety of compounds have been developed for use in human and veterinary medicine, including clonidine, romifidine, detomidine, xylazine, medetomidine, and dexmedetomidine. The latter three have been the most widely used drugs in small animal medicine. While xylazine still enjoys popularity in equine and food animal medicine, medetomidine and dexmedetomidine have replaced xylazine in dogs and cats as the alpha-2 of choice, due to their greater alpha-2:alpha-1 affinity (approximately 1620:1 for medetomidine, vs. 160:1 for xylazine). This increased selectivity results in more predictable and effective sedation and analgesia and fewer side effects.
The sedative-hypnotic effects of alpha-2s are a result of inhibition of norepinephrine release from noradrenergic receptors (autoreceptors) in the locus ceruleus section of the brainstem. Analgesic effects are principally due to spinal anti-nociception via binding to non-noradrenergic receptors (heteroreceptors) located on the dorsal horn neurons of the spinal cord. These heteroreceptors are found presynaptically, where they inhibit the release of neurotransmitters and neuropeptides, and postsynaptically, where they decrease ascending spinal nociceptive transmission. There is some evidence of supraspinal analgesic mechanisms as well; suppression of norepinephrine release in the locus ceruleus leads, via disinhibition of certain catecholaminergic nuclei in the pons, to increased release of norepinephrine from dorsal horn terminals, which then activates presynaptic and postsynaptic heteroreceptors.
At low doses, both the sedative and analgesic effects of alpha-2 agonists are dose-dependent. As the dose is increased, there is a ceiling on the degree of analgesia, and further dosing only acts to lengthen the duration of sedation and increase the risk of adverse effects. For this reason, it’s best to administer an alpha-2 in conjunction with other agents (opioids, dissociatives) as part of a balanced regimen. Opioids work particularly well with alpha-2's, as receptors for both compounds occupy similar sites in the brain and on some neurons and produce similar actions (membrane associated G protein activation leading to neuronal hyperpolarization and a reduced response to excitatory input). This results in a synergistic effect, leading to improved quality and duration of analgesia.
Side effects occur frequently with alpha-2 agonists. The most common effect noted is an initial hypertension (due to peripheral postsynaptic adrenoreceptors causing vasoconstriction), which results in a baroreceptor-mediated reflex bradycardia. As the peripheral effects diminish, central alpha-2 actions predominate, leading to decreased blood pressure and cardiac output. Anticholinergics have been advocated to reduce the bradycardic effects, but their use is controversial (elevating heart rate in the presence of high systemic vascular resistance can result in increased cardiac workload and myocardial oxygen consumption). If used, it's probably best to give the anticholinergic before or with the alpha-2 (to minimize the risk of reflex tachycardia occurring at the time of peak hypertension); severe bradycardia occurring after the alpha-2 has been administered should be treated with a reversal agent.
Other side effects can include short term A-V block (most often first or second degree), sinus arrhythmia, sinoatrial block, decreased respiratory rate, peripheral venous desaturation (resulting in a cyanotic appearance to the mucous membranes), vomiting, increased urine output, transient hyperglycemia (due to inhibition of insulin secretion) and increased myometrial tone and intrauterine pressure. Additionally, xylazine has been associated with the development of aerophagia, gastric dilatation, gastric reflux and cholinergic bradycardia.
Traditionally, the use of alpha-2 agonists has been limited to healthy adult dogs and cats with adequate cardiovascular reserve capacity and no evidence of heart disease, liver or kidney failure, shock or severe debilitation. More recently, work from the University of Illinois1 using butorphanol (0.22 mg/kg) with medetomidine (0.0085 mg/kg in dogs averaging 8.9 years of age, 0.0165 mg/kg in cats averaging 10.8 years of age) to create heavy sedation during radiation therapy for cancer patients suggests a broader application of these agents. The dogs averaged 12 sedative events, the cats 15. There were 8191 total events over the eight-year period. No fatalities occurred.
There is consideration for alpha-2 agonist use in feline hypertrophic cardiomyopathy patients with concurrent left ventricular outflow obstruction as 20 µg/kg (0.020 mg/kg) medetomidine has been shown to improve the hemodynamics of these patients.2
Alpha-2 agonists should be avoided in pediatric patients less than 12 weeks of age (cardiac output is heart rate dependent, limited ability to increase contractility) and used with caution in advanced geriatric animals (where cardiovascular reserve may be diminished).
It is important to note that in most instances, combining an alpha-2 with an opioid will enable lower doses to be used while enhancing sedation/analgesia. The following are some suggested applications and dosages for dexmedetomidine (DexDomitor®) in dogs and cats (to convert to medetomidine dosing, simply double the listed ug/kg amount):
Sedation/restraint-- for radiographs, bandage changes, wound care, minor lacerations, control of fractious animals, etc.
Several important factors should be kept in mind when using dexmedetomidine/opioid sedation:
Pre-anesthetic-- in combination with an opioid (and anti-cholinergic depending on user preference and/or patient/procedure) as part of a balanced anesthetic approach. The following are the most common dose ranges for dogs and cats:
Important factors to remember when using dexmedetomidine as a pre-anesthetic include:
IM only anesthetic protocol: for routine feline elective procedures, some practitioners favor what has come to be known as "DKT"(Dexdomitor/Ketamine/Torbugesic). This consists of dexmedetomidine 12.5-15 µg/kg, butorphanol 0.2 mg/kg and ketamine 5 mg/kg given IM. Many short surgical procedures (including OHE, castration, declawing, etc) can be completed without the need for additional induction agents or inhalant agents, although all cats should be intubated and on oxygen support, placed on IV fluids and administered appropriate analgesics postoperatively. Note that these patients are usually ready for intubation within 3 to 5 minutes of the IM injection.
Constant rate infusion-- of dexmedetomidine can be used in severely painful or anxious patients to provide sedation and analgesia. The alpha-2s dexmedetomidine and clonidine are currently being used for human patient management in the ICU setting where it has been shown that controlling the neuroendocrine stress response reduces patient morbidity and shortens the clinical course for many trauma and surgical patients. Alpha-2 agonists have also been shown to influence hormonal patterns to counter protein catabolism and nitrogen losses. Dexmedetomidine can be added to a preexisting analgesic infusion (ketamine, ketamine/morphine, ketamine/morphine/lidocaine -- see the section on "Constant Rate Infusions" for more information). A simple formula involves adding 0.5 cc (250 µg or 0.25 mg) of dexmedetomidine to a 1 liter bag; when delivered at 2 ml/kg/hour, this will provide 0.5 µg/kg/hour; this rate can be doubled if needed. At these low dose rates (0.0005 to 0.001 mg/kg/hr), there are rarely any significant cardiovascular effects3 but, because dexmedetomidine CRI's have the potential to cause severe bradycardia/hypotension, these patients should be monitored very closely. A loading dose of at least 0.5 µg/kg (0.0005 mg/kg) dexmedetomidine IV should precede the initiation of the dexmedetomidine CRI.
Epidural use-- of dexmedetomidine can enhance the analgesic effects of other agents given epidurally. Besides the aforementioned action at heterotropic spinal receptors, dexmedetomidine also produces analgesia by stimulation of cholinergic interneurons when given epidurally. It acts synergistically with opioids, improving the quality and duration of analgesia, and there is some evidence that it prolongs the effects of local anesthetics. This author routinely uses dexmedetomidine 2.5 µg/kg (0.0025 mg/kg) in an epidural cocktail containing an opioid and local anesthetics. It should be noted that, being highly lipophilic, dexmedetomidine is rapidly absorbed from the epidural space, anatomically limiting the site of action and leading to systemic levels of the drug.
Intra-articular use-- of dexmedetomidine is very effective. Both opioid and alpha-2 adrenoreceptors are distributed throughout the peripheral nervous system (on the terminal ends of primary afferent nociceptive fibers). By inhibiting the release of norepinephrine locally, they reduce noxious input and minimize the development of peripheral sensitization. The combination of dexmedetomidine 1-2.5 µg/kg (0.001-0.0025 mg/kg), morphine 0.1 mg/kg and bupivicaine 0.5-1.0 mg/kg can provide long-lasting analgesia when given in the joint after closure of the arthrotomy.
General reading references:
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|1Medetomidine-butorphanol-glycopyrrolate sedation for radiation therapy: an eight year study. Grimm JB, de Lorimier LP, Grimm KA : University of Illinois, Urbana, IL (as yet unpublished)|
|2Doppler echocardiographic effects of medetomidine on dynamic left ventricular outflow tract obstruction in cats. Lamont LA, Bulmer BJ, Sisson DD, Grimm KA, Tranquilli WJ: J Am Vet Med Assoc. 2002 Nov 1;221(9):1276-81|
|3The Cardiovascular effects of dexmedetomidine given by continuous infusion during isoflurane anesthesia in dogs. Pascoe PJ: Proceedings of the Annual ACVA Meeting 2004: 61|
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