Today's Veterinary Practice

SEP-OCT 2018

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FEATURES todaysveterinarypractice.com SEPTEMBER/OCTOBER 2018 23 Descending bulbospinal noradrenergic inhibition is activated in the locus coeruleus that enhances noradrenergic release at the spinal level. 28,29 A side effect of gabapentin can be mild sedation, which can be desirable in certain conditions. For cats at home, even those without neuropathic pain, gabapentin can be a sedative option. Gabapentin can also be used when clients are taking cats to the veterinary hospital; 50 to 100 mg PO administered 2 to 3 hours beforehand will produce mild to heavy sedation and possible ataxia. Gabapentin produces no known cardiovascular effects, direct or indirect, in cats. Injectable Anesthetics Ketamine Ketamine is an NMDA receptor antagonist. It is thought to produce anesthesia by overstimulating selective sites in the central nervous system (CNS) and inducing a cataleptic state. It depresses the thalamoneocortical system and activates the limbic system, thereby producing a dissociative state. 30 Ketamine produces both direct and indirect effects on the cardiovascular system. Stimulation of the CNS produces a sympathomimetic effect that allows ketamine to indirectly increase cardiovascular parameters (cardiac output, heart rate, mean aortic pressure, and pulmonary artery pressure) via norepinephrine release. This increased myocardial work and oxygen consumption is most profound at anesthetic doses (5 to 10 mg/kg IV) and can be detrimental to cats with cardiovascular disease, especially hypertrophic heart disease, potentially causing ventricular arrhythmias. Ketamine produces direct myocardial depressant and vasodilatory effects that occur in the absence of sympathomimetic effects; the overall cardiovascular effect is probably a product of the patient's underlying sympathetic tone. 31,32 Cats anesthetized with ketamine retain protective reflexes (coughing, swallowing) and other reflexes (corneal, pedal). This drug is also considered a potent noncompetitive antagonist at NMDA receptors in the spinal cord, which are thought to be responsible for wind-up pain. Ketamine is unique in that it provides analgesia and with increasing doses; its effects range from sedation to general anesthesia. Low doses (1 to 3 mg/kg IM) of ketamine coadministered with an opioid and sedative or muscle relaxant can produce profound sedation with minimal cardiovascular changes. 24 When cats with unknown cardiovascular status need to be sedated or chemically restrained because they are difficult to handle (e.g., aggressive cats), a low-dose ketamine-based sedation protocol (≤2 mg/kg IM or IV) may produce the least harmful side effects compared with alternative options (alpha-2 agonists, volatile inhalant chamber induction). For cats with confirmed hypertrophic cardiomyopathy, alternative anesthetics that do not have such a profound stimulant effect on the cardiovascular system (e.g., alfaxalone) may be used. Alfaxalone Alfaxalone is a synthetic water-soluble neurosteroidal anesthetic that binds GABA type A receptors and potentiates the effect of endogenous GABA to create a hypnotic anesthetic state. Increasing doses of alfaxalone in cats will produce a dose-dependent decrease in cardiac output and arterial blood pressure; however, at clinical doses, heart rate is preserved and decreases in systemic vascular resistance are minimal. 33,34 Alfaxalone presents an advantage over other injectable anesthetics in its ability to be injected intramuscularly to facilitate sedation and physical restraint, which could be advantageous for echocardiographic analysis in difficult-to-handle cats. Recumbency and sedation after intramuscular injection is generally observed within 2 to 15 minutes and lasts approximately 30 minutes. The only other injectable anesthetic labeled for intramuscular use is ketamine. Combinations of alfaxalone with butorphanol administered intramuscularly to healthy cats produced significant decreases in fractional shortening and ejection fraction but no effect on heart rate or left ventricular internal diameter at systole or diastole. 35 Propofol Propofol is a short-acting sedative hypnotic that produces anesthesia by binding GABA A receptors and potentiating GABA-induced chloride conductance. Although its mechanism of action is similar to that of increasing doses of alfaxalone, propofol differs from alfaxalone in its cardiovascular effects. Propofol produces dose-dependent reductions in systemic blood pressure, myocardial contractility (negative inotropy), and cardiac output; these effects are more pronounced in the hypovolemic animal. 36 In addition to arteriolar dilation, which serves to decrease afterload, venodilation also occurs and results in reduced preload and cardiac output. 37 In general, because of propofol's vasodilatory effects, its use in cats with hypertrophic cardiomyopathy should be avoided if other sedative or anesthetic options are available.

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