Overview of anesthetic drugs: review, applications, contraindications (Proceedings)
Opioids provide both analgesia and sedation. They are considered the gold standard for analgesia in veterinary patients. While opioids are generally considered very safe in a variety of patients, they can also have some undesirable side effects. Euphoria, dysphoria and excitement are sometimes seen after administration, particularly in feline patients. Simultaneous administration of an anxiolytic drug can help minimize these effects. Opioids work by binding to opioid receptors in the brain, spinal cord, and various other parts of the body. The specific effects of these drugs depend on the type of opioid analgesic used and what receptors it binds to. Opioids can act on several different receptors but the most familiar are the mu (m) and kappa (k) receptors. Drugs can either bind to these receptors and activate them (agonists) or bind to the receptors and block them, preventing activation (antagonists).
Opioid agonist drugs include morphine, hydrophone, oxymorphone and fentanyl. These drugs have a high potential for abuse and are therefore closely regulated and are labeled as DEA schedule II drugs. Their main features include the ability to provide analgesia for moderate to severe pain and the ability to be reversed if necessary. Side effects commonly seen include vomiting, bradycardia, respiratory depression, and excitement (especially in cats).The most familiar agonist-antagonist drug is butorphanol (Torbugesic ®). This drug is an antagonist at the mu opioid receptor and an agonist at the kappa opioid receptor. Because of this feature, butorphanol can be used to partially reverse the effects of full agonist drugs. Butorphanol is a DEA schedule IV drug due to potential for abuse. Butorphanol is generally effective for mild pain relief, sedation, and cough suppression but its short duration of action can be a limiting factor in its use for analgesia.
Buprenorphine is a partial agonist at the mu receptor and an antagonist at the kappa receptor. This drug binds tightly to its receptors which gives it a relatively long duration (4-12 hrs in dogs) and also makes it very difficult to reverse. Generally this drug causes less sedation, excitement, and respiratory depression than full agonist drugs and provides mild to moderate analgesia.
Naloxone is an opioid antagonist than can be used to reverse the effects of other opioids. Naloxone is a non-scheduled drug and it does not provide analgesia or sedation. It has a rapid onset and can be used in emergencies to completely reverse the effects of other opioid drugs (respiratory depression, bradycardia, sedation). Excitement can be seen after administration and re-dosing is sometimes necessary as it has a short duration of action
Phenothiazine tranquilizers (Major tranquilizers)
Acepromazine is the most common phenothiazine used in small animals. This drug is an alpha adrenergic antagonist whose effects include sedation, anxiolysis, and vasodilation (resulting in hypotension and hypothermia). Acepromazine is also known to have anti-arrhythmic, anti-histaminic, and anti-emetic properties. This drug generally provides excellent sedation but provides no analgesia on its own. However, acepromazine can enhance the effects of analgesic drugs when co-administered. Acepromazine should be avoided in hypotensive, hypovolemic and geriatric patients as well as those with liver dysfunction. Acepromazine causes sequestration of red blood cells in the spleen and therefore can lower PCV. Certain breeds are known to be sensitive to the effects of acepromazine (Boxers, Greyhounds) so caution should be used if this drug is to be included in the anesthetic protocol.
Benzodiazepine tranquilizers (Minor tranquilizers)
Diazepam (Valium®) and midazolam (Versed®) are the most commonly used benzodiazepines. Effects of these drugs include anxiolysis, muscle relaxation, increased seizure threshold, decreased inhibitions, and excitement. Benzodiazepines have few cardiovascular effects and are reversible if necessary making them very safe in a variety of patients. They can be excellent as part of premedication in geriatric patients or those with systemic disease. Excitement, dysphoria, and impulse inhibition is often seen when benzodiazepines are administered to young or healthy patients and may not be the best choice as a sedative.
Anticholinergics are used during anesthesia to prevent and treat bradycardia from anesthetic drugs and/or vagal stimulation. Other effects of anticholinergics include decreased salivary secretions and decreased GI motility. Anticholinergic drugs should generally be avoided in patients that are tachycardic, have GI stasis, increased intracranial pressure, or cardiac disease. They are indicated for neonates or pediatric patients (who rely on HR to maintain CO), surgery involving the neck or airway (which can induce a vagal reflex), ocular procedures (which can induce a vagal reflex), brachycephalic patients (who generally have high vagal tone), vagally or opioid induced bradycardia.
There are two main choices of anticholinergics; atropine and glycopyrrolate. Atropine is most commonly used in emergency situations because of its rapid onset. Atropine is less commonly used as part of a standard premedication protocol due to its tendency to cause sinus tachycardia and tachyarrhythmias. Glycopyrrolate is less likely to cause severe tachycardia and is less arrhythmogenic than atropine. Because of this feature, glycopyrrolate may be a safer choice in cardiac patients that cannot tolerate significant increases in heart rate. After administration of an anticholinergic, it is common to see first or second degree AV block occur before the heart rate begins to increase. This is seen more commonly after administration of glycopyrrolate but can also be seen if low doses of atropine are used.