Analgesic drugs and sedatives (Proceedings)

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Analgesic drugs and sedatives (Proceedings)

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Apr 01, 2010

Pain management in veterinary medicine was practically unheard of twenty years ago, and it has advanced dramatically over the past decade. Not only is the physiology of pain and its effects becoming better understood, pain management is considered a vital part of most treatment plans. The approach towards pain has changed from questioning whether animals feel pain to widespread acceptance that not only do animals feel pain, but pain will inhibit recovery and cause undue suffering.

The International Veterinary Academy of Pain Management ( http://www.IVAPM.org/) was formed in 2003 to promote and advance pain management, and currently has a credentialing process in it. All veterinary personnel can benefit from knowledge of analgesics and understanding approaches to this topic.

It is important to recognize that analgesics cannot be treated as a "one size fits all" approach to pain management for every drug will affect every animal differently. Since the goal is to maximize a drug's benefits while minimizing side effects, it is common to use a multi-modal approach to pain management. Multi-modal pain management uses combinations of drugs to calm the patient and keep them pain free. There is a synergistic effect by using combinations of drugs meaning the drugs are more effective at lower doses, thus limiting their side effects.

Understanding the physiology of pain allows one to treat it more effectively. A simple definition of pain is it is an electrical impulse that travels from a nerve ending, via a nerve tract, and is perceived in the brain. Free nerve endings called nociceptors are the pain receptors, and there are different types of receptors that respond to different types of pain. The receptors associated with opioids are mu, kappa, and sigma. Mu and kappa receptors are responsible for sedation, analgesia, and respiratory depression, while sigma receptors are involved with opioid side effects such as dysphoria and hallucinations. The drug's duration of action must be considered because nociceptors can continue to send pain signals even after the stimuli stops and can cause ongoing discomfort or tissue damage. Repeated doses are sometimes needed.

Opioids

Opioids are very effective analgesic drugs so they are used most often. There are three categories:
     • Pure agonists stimulate the entire receptor. They are the most potent and effective with acute, traumatic, and intense pain. These drugs provide the most analgesia while the side effects such as hypotension, respiratory depression, hypothermia, vomiting, and ileus are more likely and possibly more profound. These drugs include Morphine (0.5-2 mg/kg in dogs and 0.1-0.3 mg/kg in cats), Hydromorphone (0.05-0.2 mg/kg), Oxymorphone (0.05-0.2 mg/kg), and Fentanyl. Fentanyl's duration is very short so it must be administered intravenously as a continuous rate infusion (CRI) to maintain analgesia. There are also Fentanyl transdermal patches, which are applied to the skin and provide continuous pain relief for up to 72 hours, but absorption, efficacy, and duration can vary greatly depending on proper placement and the animal.
     • Partial agonists stimulate only part of the receptor. These drugs provide less analgesia, and the side effects are the same as pure agonists, but with less intensity. They are commonly used in routine procedures such as a minor laceration or abscess repair, and castrations. Drugs include Buprenorphine (0.01-0.02 mg/kg), and Butorphanol (0.1-0.4 mg/kg). Buprenorphine also can be given sublingually to cats making it a good medication for continuing pain relief at home. A synthetic opioid such as Tramadol (2-4 mg/kg up to QID), an oral tablet, is also a good medication to send home with owners. Butorphanol's analgesic effects are limited, but it is a good sedative. It is often used in conjunction with other medications to provide sedation and analgesia

It should be noted that partial agonists should not be used with pure agonists because they counteract each other's effects. Each one is trying to stimulate the receptor, and they end up "pushing" one another off.

Partial agonists also have a "ceiling effect" with regards to cardiovascular effects, while pure agonists do not. Overdosing pure agonists can cause respiratory or cardiovascular arrest, while this is not likely with an overdose of a partial agonist.
     • Antagonists block the receptors and are used to reverse opioid effects if the patient is not responding well. The most common antagonist is Naloxone (0.002-0.2 mg/kg for a duration of one hour).

It should also be noted that opioid administration can induce an excitability phase before taking its full effect, but adding tranquilizers minimizes the possibility. Often low doses of tranquilizers are beneficial to minimize the effect of stress. Sleeping is a healing process, and recovery is inhibited for hospitalized patients that lack rest or sleep.