Postoperative Pain Management

CHAPTER 25 POSTOPERATIVE PAIN MANAGEMENT

Michael Hanania, MD, Charles E. Argoff, MD

1. What is the pathophysiology of acute postoperative pain?

Postoperative pain is mainly nociceptive (see Chapter 2, Classification of Pain), but central sensitization also occurs. At the periphery, inflammatory mediators (prostaglandins, histamine, serotonin, bradykinin, and substance P) increase the sensitivity of nociceptors. Central sensitization is a result of functional reorganization in the dorsal horn of the spinal cord. Both of these processes result in an exaggerated response to noxious stimuli, spread of hyperresponsiveness to noninjured tissue, and a reduced threshold for producing pain.

2. Describe the deleterious physiologic effects of postoperative pain

Muscle splinting secondary to pain in the abdomen or chest results in a decreased vital capacity and, ultimately, decreased alveolar ventilation. Atelectasis is therefore a common postoperative complication. If coughing is very painful and performed with minimal effort or infrequently, retention of secretions and subsequent pneumonia may result. Release of stress hormones and catecholamines secondary to pain may cause persistent tachycardia and hypertension, resulting in increased cardiac work and myocardial oxygen consumption. Increased sympathetic activity decreases intestinal motility and prolongs recovery.

3. What are the principles of postoperative pain management?

Pain is a normal accompaniment of surgical intervention. The severity and duration of postoperative pain can be predicted from a knowledge of the surgical procedure involved. Therefore provision of analgesia for the postoperative period should be planned and should reflect the expected severity and duration of pain. Timed administration of analgesia should be the norm, rather than reliance on an “as required” basis. Logical combinations of analgesics should be used (e.g., opioid and acetaminophen; opioid and nonsteroidal antiinflammatory drugs [NSAIDs]; opioid, acetaminophen and NSAIDs) and side effects predicted (e.g., nausea, vomiting, constipation) with prophylactic measures instituted (e.g., antiemetics, laxatives).

4. What is preemptive analgesia?

Preemptive analgesia provides pain relief prior to surgery and throughout the perioperative period. Acute postoperative pain is associated with alterations in synaptic function and nociceptive processing within the spinal cord dorsal horn, neuroendocrine responses, and sympathoadrenal activation. Theoretically, preemptive analgesia minimizes these responses and prevents the spinal cord “wind-up phenomenon” (central sensitization), which is more resistant to treatment and is associated with chronic pain conditions. Although the concept of preemptive analgesia is well proven in animal pain models, human studies have as yet failed to provide conclusive evidence that analgesia provided before the nociceptive stimulus is inflicted alters the extent or duration of subsequent pain.

5. What evidence is available that preemptive analgesia may work?

According to a study by Brodner and colleagues, patients having more intense pain preoperatively used more patient-controlled analgesia (morphine) following surgery. Patients who received epidural bupivacaine and fentanyl (local anesthetic and opioid) before the surgical incision for a prostatectomy had less pain and were more active postoperatively while hospitalized and 9 weeks later. Preemptive thoracic epidural infusion of an opioid and local anesthetic in patients undergoing abdominothoracic esophagectomy resulted in faster extubation and a shorter intensive care unit (ICU) stay.

6. What is intravenous patient-controlled analgesia (PCA)?

Intravenous patient-controlled analgesia (PCA) is a system of opioid delivery that consists of an infusion pump interfaced with a timing device. It allows the patient to titrate the analgesic dose required for optimal control of pain. The patient presses a button, and a preset dose of analgesic is delivered. A programmed “lockout” period (usually 6 to 15 minutes) prevents inadvertent overdoses and excessive sedation. This system may be used on top of a baseline continuous infusion. The parameters that can be set therefore include the presence, or absence, of a background continuous infusion, the bolus dose of opioid administered, and the “lockout” period (during which further opioid cannot be delivered).

7. What are the advantages of a PCA system over nurse-administered intramuscular (IM) opioids?

There are a number of problems with the traditional nurse-administered intramuscular (IM) opioids, including the following:

Lack of knowledge regarding analgesic pharmacodynamics and overconcern about respiratory depression and addiction liability

A long lag period between the onset of pain and the administration of opioid, because of the process involved in calling for a nurse, obtaining and recording narcotics, and administration of the drug. This lag period is extended by the time required for absorption of an IM dose and further complicated by the pain of IM administration. Intravenous PCA eliminates these factors.

The dose of opioid is titrated with PCA use (the patient will continue to seek a dose until pain relief is achieved). With nurse-administered opioid, a fixed dose is given, and because it is given IM, the lag time to effect is long.

8. Is a continuous background infusion necessary with intravenous PCA?

A continuous background infusion does not improve pain scores and is even associated with more side effects, such as sedation and respiratory depression, when used after less-painful abdominal surgeries such as cesarean section. A continuous background infusion is usually not necessary with other abdominal procedures, but probably serves a useful role in extensive abdominal and thoracic operations; definitive data are not available. Certainly, if a patient has been taking opioids preoperatively, the daily equivalent dose should be administered as a continuous infusion in addition to the PCA bolus.

9. Which opioids are commonly used for intravenous PCA?

Morphine is most commonly used because it is relatively inexpensive and has an intermediate duration of action. The typical adult bolus dose is 1 mg with a 5- to 10-minute lockout (see Question 6). Other opioids used include meperidine (10-mg bolus dose), hydromorphone, and fentanyl. Ultralong-acting opioids, such as methadone, require very long lockout intervals; ultrashort-acting opioids, such as alfentanil, require a very short lockout with a basal infusion, making them suboptimal choices for PCA. An agonist-antagonist such as butorphanol may be used when pain is not severe, because a ceiling effect for analgesia is characteristic of this drug. However, mixed agonist-antagonists cannot be used with pure agonists.