Morphine remains the standard by which other analgesic drugs are compared. The predominant effects of morphine are at the ^-opioid receptor, although it interacts with other opioid receptors as well. Morphine is indicated for the treatment of moderate to severe and chronic pain. It is useful preoperatively for sedation, anxiolytic effects, and to reduce the dose of anesthetics. Morphine is the drug of choice for the treatment of myocardial infarction because of its bradycardiac and vasodilatory effects. In addition, morphine is the most commonly used drug for the treatment of dyspnea-associated pulmonary edema. It is thought that morphine reduces the anxiety associated with shortness of breath in these patients along with the cardiac preload and afterload.
The use of morphine via the oral route has drawbacks because of its first-pass effect; however, oral morphine has been recommended for use in cancer patients for its ease of administration. In particular, the long-acting preparations of morphine, such as MS-Contin and Ora-Morph, are described as the cornerstone of pain treatment in cancer patients, either alone or in combination with nonopioids.
Morphine is the most commonly used analgesic drug administered via the epidural route because it is potent, efficacious, and hydrophilic. The more hydrophilic the drug, the slower the onset and the longer the duration of action following epidural administration. Single-dose or continuous infusion of morphine is used to provide pain relief in thoracic and abdominal surgical patients and in cancer patients at high risk for developing side effects associated with systemic opioids. Since morphine does not produce anesthesia via the epidural route, the patient is able to move about normally; motor function is preserved. The drawback to epidural use of morphine is that certain types of pain are relatively unresponsive, such as that associated with visceral stimuli, as in pancreatitis, and neuropathic pain from nerve deafferenta-tion. In addition, patients can develop respiratory depression and nausea from the rostral flow of the drug to medullary centers, although the effects are much less severe than those observed following the systemic administration of the drug, and can be alleviated by elevation of the head of the patient at a 30-degree angle. Patients may also itch because of histamine release.
Patient-controlled analgesia (PCA) is an alternative method of administration of morphine. The use of an indwelling catheter allows the patient to administer the drug at frequent intervals for pain relief. PCA systems allow patients the freedom to assess the need for their own analgesia and to titrate a dose tailored to their needs. Dependence is rarely observed in patients using PCA for acute pain management.
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