Anesthesia drugs for the small animal patient (Proceedings) - Veterinary Healthcare


Anesthesia drugs for the small animal patient (Proceedings)



These drugs have provided the most consistent and effective analgesia for many years and are arguably still the best drugs available for pain control in small animals. The use of opioid drugs in practice is sometimes deterred due to the fact that these drugs are controlled substances. Their use requires a DEA license and additional paperwork as well as security, due to their abuse potential. The extra work is well justified by the analgesic benefit to the patient. There are multiple types of opiate receptors in the CNS and they each have different roles to play in the activity of the nervous system. The two most important receptors with respect to pain are the mu and the kappa receptors. The drugs we use have different affinities for these receptors and this explains the differences in duration and efficacy between the available drugs. None of the opioids produce a loss of consciousness in veterinary patients, therefore opioids are not used solely to induce general anesthesia. They are used in conjunction with other drugs such as benzodiazepines and inhaled agents to produce general anesthesia. There is wide variation in the dosage of these drugs. Each patient's response to these drugs should be assessed and the dose adjusted according to the patient's response. Opioids effects are reversible with pure opioid antagonists such as naloxone. The beneficial effects of the opioids include 1) excellent analgesia with mild cardiopulmonary effects that are dose dependent, 2) reduce the quantity of other anesthetic drugs needed to produce general anesthesia, 3) dose dependent CNS depression. Sedation produced by these drugs can outlast the analgesic effects.

Opioids Agonist-Antagonist

These drugs in general act as an antagonist at the mu opioid receptor and act as an agonist at the kappa opioid receptors or provide a partial agonist response from the mu receptor. They may be used to reverse the effects of the pure mu agonists. These drugs are not as effective analgesics as the pure mu agonists, but are thought to cause less dysphoria and vomiting than mu agonists.

Neuroleptanalgesic Combinations

Tranquilizers (a phenothiazine, benzodiazepine or butyrophenone) can be combined with an opioid to provide both sedation and analgesia. The effect of these drug combinations tend to be synergistic, therefore lower doses of each drug are required to produce the desired effect. When these drug combinations are used in an anesthesia plan they provide preemptive analgesia and sedation to facilitate catheter placement and further reduce the quantity of other drugs needed to induce and maintain general anesthesia.

Alpha2 Adrenergic-Agonists

The alpha-adrenergic family of receptors is subclassified into alpha1 and alpha2. Alpha2 adrenergic receptors located within the CNS induce both sedation and analgesia not unlike that produced by opioid receptor activation. Alpha2 and opioid receptors are found in similar regions of the CNS, even on some of the same neurons. The functional CNS effects are quite similar between alpha2 and opioid receptors. The primary advantages of these drugs are that they provide analgesia and sedation while producing minimal respiratory depression and their effects are reversible with the use of alpha-adrenergic antagonists. Typical patient response to the IV administration of these drugs alone is CNS depression, analgesia, profound hemodynamic changes (increased blood pressure initially, due to vasoconstriction, followed by hypotension and bradycardia as sympathetic tone decreases and vagal tone increases), and respiratory rates may decrease (tidal volume increase). Perhaps the best advice with respect to these drugs would be to use as low a dose as possible and proceed cautiously with their administration. Great caution is advised if using alpha-adrenergic agonists in the compromised patient and they should not be used in patients with a reduced cardiopulmonary reserve.

Alpha2 Adrenergic Antagonists

A significant advance in veterinary medicine during the last decade has been the utilization of antagonists for the reversal of some injectable anesthetic regimes. The ability to reverse the effects of alpha2 adrenergic agonists has helped to promote their use in veterinary anesthesia. Yohimbine and Atipamezole may be used to reverse the effects of the alpha2 adrenergic agonists.


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