INTRODUCTION
Opioids are commonly administered to dogs and cats in the perioperative period for a number of reasons. They provide analgesia and sedation, and they reduce the subsequent quantity of injectable and inhalational agent required to induce and maintain anesthesia. They can also be administered to induce anesthesia and to supplement analgesia during inhalational anesthesia.
ANALGESIA
Some of the opioids are potent analgesic agents, others are much less effective. The route of administration can also affect the efficacy of analgesia provided by the opioids. Oxymorphone, morphine, methadone, and fentanyl provide dose-dependent, moderate to excellent analgesia, whereas butorphanol and meperidine are rather ineffective analgesic agents at most doses. Buprenorphine can provide effective analgesia if the dosage is sufficient.
Oxymorphone, morphine, and methadone can provide analgesia of moderate duration when given by the parenteral route, whereas fentanyl has too short an action to be a useful analgesic when given this way. Fentanyl patches have been developed which allow for transdermal penetration over a long period to provide long-term (several days) relief for painful patients. Buprenorphine provides analgesia for about 8–12 hours when given parenterally; meperidine or butorphanol are effective for less than two hours.
Side effects are not uncommon with opioid administration and these can include sedation, vomition, defaecation, bradycardia, and hypoventilation. These effects are variable between agents, dosage, route of administration, and severity of pain present at the time of administration.
Sedation is most common with oxymorphone and morphine and can be profound in some patients at high doses. This effect is much less common with methadone, buprenorphine, and pethidine.
Vomiting is commonly seen after morphine administration in dogs when given pre-operatively but is rarely seen when administered post-operatively. About 90% of dogs will vomit after morphine administration but few cats will do so. Vomiting occurs despite the concurrent administration of acepromazine, a moderately effective anti-emetic agent. The other opioid agents are less likely to induce vomiting in either species. Like vomiting, defecation is most commonly seen after morphine administration in dogs.
Bradycardia is commonly encountered with the potent µ agonist opioids such as oxymorphone, morphine, methadone and fentanyl with the latter agent having the potential to induce particularly profound bradycardia. Despite also being µ agonists, meperidine and buprenorphine infrequently cause bradycardia. Atropine or glycopyrrolate are usually administered with the potent µ agonists to prevent or counteract bradycardia.
Hypoventilation can be seen if high doses of potent µ agonists are administered. It is less likely to be seen in the animal that is in pain compared to the non-painful animal that is receiving an opioid for sedation. Hypoventilation is also common in animals that are concurrently receiving inhalational anesthetic agents; intermittent positive pressure ventilation (IPPV) may be required to maintain PaCO2 or end tidal CO2 below 45–50 mmHg. This is especially so with parenterally administered fentanyl.
Parenteral administration of potent opioids is more likely to result in the aforementioned side effects. Transdermal fentanyl produces fewer side effects, as does epidural administration of opioids.
SEDATION
The opioids are commonly administered either alone or concurrently with other sedative agents to quiet the patient during induction and recovery. They can be safely administered to most patients, because other than bradycardia, the opioids produce few cardiovascular side effects. The depth and duration of sedation depend upon the specific agent and dosage administered and the concurrent use of other drugs. Oxymorphone and morphine can provide profound sedation when given alone whereas the other agents generally do not. Sedation is enhanced by the provision of acepromazine, but since this agent is not recommended in some patients with cardiovascular disease, these opioids can be given with midazolam to improve induction conditions. Methadone alone or with acepromazine provides moderate sedation when given by the subcutaneous or intramuscular route, whereas sedation is profound and rapid (within five minutes) if the drugs are given slowly intravenously. Butorphanol in combination with acepromazine also produces good sedation, whereas acepromazine in combination with either meperidine or buprenorphine, provides only mild to moderate sedation.
EFFECT ON INJECTABLE AND INHALATIONAL AGENT REQUIREMENTS
The provision of an opioid in the pre-operative period can reduce the requirements for injectable and inhalational agents by a small to large amount, depending upon agent, dosage, and route of administration. Because the opioids can produce profound analgesia, it is not always necessary to maintain the patient at an inhalational concentration that provides good muscle relaxation as the “end-point” for adequate anesthesia. At times, it is possible to carry out surgery at a plane of anesthesia in which the patient has quite tight jaw tone when opioid analgesia is present. It is possible to provide additional analgesia during surgery by administering intra-operative opioids to patients that would not tolerate an increase in inhalational agent concentration. Boluses of fentanyl, morphine or oxymorphone can be given, or, alternatively fentanyl or morphine infusions can be administered, throughout the surgery. The latter is my preferred technique in most patients requiring intra-operative analgesia. The disadvantage to providing intraoperative opioids (especially a longer acting agent such as morphine) is that extubation takes longer than usual. However, the patients generally awaken free from pain and thus recovery is smooth. A small proportion of patients may be dysphoric in recovery as a result of opioid presence and a small dose of intravenous acepromazine may be required to settle them.
EPIDURAL OPIOIDS
This topic will be covered in greater detail later in the day, but this is a technique that provides excellent post-operative analgesia and can markedly reduce the required concentration of maintenance agent.