McKenna's Pharmacology for Nursing, 2e

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P A R T 4  Drugs acting on the central and peripheral nervous systems

Clinically important drug–drug interactions When opioid agonists are given with barbiturate general anaesthetics or with some phenothiazines and MAO inhibitors, the likelihood of respiratory depres- sion, hypotension and sedation or coma is increased. If these drug combinations cannot be avoided, individuals should be monitored closely and appropriate supportive measures taken. Prototype summary: Morphine Indications: Relief of moderate to severe acute or chronic pain; preoperative medication; component of combination therapy for severe chronic pain; intraspinal to reduce intractable pain.

opioids. These drugs have three functions: (1) relief of moderate to severe pain; (2) adjuncts to general anaes- thesia; and (3) relief of pain during labour and delivery. See Table 26.1 for usual indications for each opioid agonist-antagonist agent. Pharmacokinetics Opioid agonists–antagonists are readily absorbed after IM administration and reach peak levels rapidly when given IV. They are metabolised in the liver and excreted in urine or faeces. They are known to cross the placenta and enter breast milk. Buprenorphine is available for use in IM and IV forms. Contraindications and cautions Opioid agonists–antagonists are contraindicated in the presence of any known allergy to any opioid agonist- antagonist to avoid hypersensitivity reactions. Caution should be used in cases of physical depend- ence on an opioid because a withdrawal syndrome may be precipitated ; the opioid antagonistic properties can block the analgesic effect and intensify the pain. Opioid agonists–antagonists may be desirable for relieving chronic pain in people who are susceptible to opioid dependence, but extreme care must be used if they are switched directly from an opioid agonist to one of these drugs. Caution should also be exercised in the following conditions: chronic obstructive pulmonary disease or other respiratory dysfunction, which could be exac- erbated by respiratory depression ; acute myocardial infarction (MI), documented coronary artery disease (CAD) or hypertension, which could be exacerbated by the cardiac stimulatory effects of these drugs ; and renal or hepatic dysfunction, which could interfere with the metabolism and excretion of the drug. There are no adequate studies regarding their effects during pregnancy. They should be used during preg- nancy only if the benefit to the mother clearly outweighs the risk to the fetus because of potential adverse effects on the neonate, including respiratory depression. They can be used to relieve pain during labour and delivery, which provides a short-term exposure to the fetus. They are known to enter breast milk and should be used with caution during breastfeeding because of the potential for adverse effects on the baby . Adverse effects The most frequently seen adverse effects associated with opioid agonists–antagonists relate to their effects on various opioid receptors. Respiratory depression with apnoea and suppression of the cough reflex is associated with the respiratory centre depression. Nausea, vomiting,

Actions: Acts as an agonist at specific opioid receptors in the CNS to produce analgesia, euphoria and sedation. Pharmacokinetics: Route Onset Peak

Duration 5–7 hours

Oral

Varies

60 mins

SC Rapid IM Rapid

50–90 mins 5–7 hours 30–60 mins 5–6 hours

IV Immediate 20 mins

5–6 hours

T 1/2 : 1.5 to 2 hours; metabolised in the liver, excreted in the urine and bile. Adverse effects: Light-headedness, dizziness, sedation, nausea, vomiting, dry mouth, constipation, ureteral spasm, respiratory depression, apnoea, circulatory depression, respiratory arrest, shock, cardiac arrest.

O pioid agonists – antagonists The opioid agonists–antagonists (Table 26.1) stimulate certain opioid receptors but block other such receptors. These drugs, which have less abuse potential than the pure opioid agonists, exert a similar analgesic effect as morphine. Like morphine, they may cause sedation, res- piratory depression and constipation. They have also been associated with more psychotic-like reactions, and they may even induce a withdrawal syndrome in people who have been taking opioids for a long period. The available opioid agonist-antagonist is buprenor- phine ( Norspan , Norgesic ). Therapeutic actions and indications The opioid agonists–antagonists act at specific opioid receptor sites in the CNS to produce analgesia, sedation, euphoria and hallucinations. In addition, they block opioid receptors that may be stimulated by other