Xylazine Indications: Xylazine is a sedative with analgesic and muscle relaxant effects. It can also be used to induce vomiting in cats. However, xylazine has been largely replaced by medetomidine in small animals because of its side effects. Adverse effects: ● Muscle tremors ● Seizures ● Slows heart rate with partial heart block and slow breathing rate ● Cardiovascular depression can be profound ● Causes vomiting and retching and may result in aspiration pneumonia. Contraindications: ● Renal or hepatic impairment Benzodiazepines Primarily used as sedative, pre-anesthetics, and anesthesia induction but are never used alone to provide or maintain anesthesia. (CU Denver Veterinary Formulary, n.d.) They do not exert an analgesic effect. Effects: ● Rapid onset of action ● Skeletal muscle relaxation ● Anticonvulsant properties ● Very safe to use on sick and geriatric patients ● Anti-anxiety or calming effect; in rare cases, excitation can be caused rather than tranquilization Opioids Opioids are primarily used for their reliable and consistent analgesic properties. These drugs have a sparing effect: by giving opioids, you can decrease the dosage of other anesthetic drugs. Opioids induce minimal cardiovascular depression (Lee, 2024). ● Respiratory dysfunction ● Cardiac abnormalities ● Hypotension and/or shock ● Diabetes ● Known seizure disorder ● Animal is severely debilitated Opioid analgesics remain the cornerstone of acute and perioperative pain management in companion animals due to their efficacy, ease of administration, and relative safety. Topical therapy to protect the cornea is currently recommended following opioid administration in both dogs and cats (Kerr et al., 2023).
● Should not be used in the last trimester of pregnancy because it can induce premature labor. ● Cats with a blocked urethra because xylazine may cause increased urination in cats. ● Do not use in animals with esophageal obstruction and torsion of the stomach as the muscle relaxant properties of the drug appear to accentuate the effects of the obstruction. ● Brachycephalic dogs with air way disease or malfunction may develop life-threatening dyspnea. ● Puppies and kittens younger than 6 weeks. Precautions: ● Xylazine may interact with other medications such as epinephrine, certain narcotics, barbiturates, prochlorperazine, and acepromazine. ● Xylazine inhibits normal motility of the intestines, thus promoting the accumulation of gas in the stomach. This can make interpretation of gastrointestinal radiographs less certain. A stomach tube may be required in certain situations to relieve excess air in the stomach. Reversal agents : Yohimbine and Tolazoline Diazepam and midazolam are the two most commonly used benzodiazepines in United States today. Diazepam is an irritant to the tissue, so it is recommended not to be administered IM. Combination of benzodiazepines with butorphanol (or buprenorphine) is a useful alternative to the combination with pure opioids to reduce the risk of respiratory depression in brachycephalics. Reversal agent : Flumazenil Note : Flumazenil has a very short duration of action, so it may be necessary to administer multiple dosages to fully reverse benzodiazepines. In general, mu agonists such as morphine and hydromorphone provide better analgesia than partial mu agonists (buprenorphine). Mu antagonist-kappa agonists (butorphanol) are indicated for moderate-to-severe pain associated with thoracic, intra-abdominal, or orthopedic surgery in companion animals (Kerr et al., 2023). Reversal agent : Naloxone
Table 1: Commonly Used Opioids for Small Animals Pre-Anesthetic Medication Opioid Type Advantages/Benefits/Indications Effects Morphine mu (µ) opioid agonist • Safe, effective analgesic • Effective for mild to severe pain
• Respiratory depression, bradycardia, vomiting, defecation, dysphoria, and histamine related vasodilation and hypotension *generally dose and route of administration dependent • Morphine is less well metabolized in cats than in dogs due to deficient 12 glucuronyl metabolic pathways; at higher doses, it is more likely to get overdosed and increases the likelihood of opioid induced excitement
• Longer duration of action and is less expensive than other commonly used µ opioid agonists; lasts 2 to 3 hours in dogs and 4 to 6 hours in cats • Commonly used in combination with acepromazine or alpha 2 agonists
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