A. Pain is an unpleasant sensory and emotional experience that is associated with actual or potential tissue damage or described in terms of such damage (The International Association for the Study of Pain). Different categories of pain can be defined according to the duration, etiology, or perception of the painful experience.

1. Acute pain is pain that is secondary to physical injury to body tissues and usually resolves as the wound heals. Improved control of acute pain following surgery has become a focus for practice improvement measures leading to a more timely and diversified approach to pain therapy. Studies demonstrate that techniques that effectively reduce acute pain are associated with a lower incidence of subsequent chronic pain.

2. Chronic pain is pain that continues for months or longer and is often defined as pain persisting beyond 3 to 6 months. Common chronic pain conditions include low back pain, complex regional pain syndrome, postherpetic neuralgia, cancer pain, and myofascial pain.

3. Neuropathic pain results from pathologic function of the somatosensory system, either in peripheral elements (receptor or peripheral nerves) or in the central nervous system. The abnormal somatosensory function of neuropathic pain is the direct result of injury to the nervous system, and this type of pain persists even after tissue healing appears to be complete. It is most frequently described as burning, radiating, lancinating, or shooting in nature. Neuropathic pain can result in allodynia, which is the perception of pain from a normally innocuous stimulus, for example, light touch perceived as pain.

4. Nociceptive pain results from injury that activates peripheral nociceptors, which can be somatic or visceral in origin, and is the well-localized pain associated with most acute injuries. Visceral pain arises from distention or injury to the viscera and is usually less well localized than somatic pain, owing to the less dense innervation of the organs as compared to other tissues.

5. Inflammatory pain categorized as nociceptive pain in the presence of acute inflammation, but chronic inflammatory states can play mechanistic roles in neuropathic pain states (Loeser & Treede, Kyoto). Inflammation resulting from tissue damage can lead to hyperalgesia, which is the exaggerated painful perception of a known noxious stimulus.

1. Nonsteroidal anti-inflammatory drugs (NSAIDs, Table 39.1) can effectively treat mild-to-moderate pain, particularly pain associated with inflammatory conditions. Drugs classified as NSAIDs have diverse chemical structures, but all share the ability to inhibit the enzyme cyclooxygenase (COX) and thereby inhibit the formation of prostaglandins from arachidonic acid.

a. Mechanism of action and COX selectivity. The apparent mechanism for analgesia produced by the NSAIDs is the prevention of neuronal
sensitization by diminishing prostaglandin production. Type I cyclooxygenase (COX-1) is a constitutively expressed enzyme that is present in varying amounts in most cells at a fairly constant level. COX-1 serves a key role in cellular homeostasis and is the primary form of the enzyme present in platelets, the kidney, stomach, and vascular smooth muscle. COX-2-selective inhibitors were developed with the goal to reduce side effects such as gastrointestinal (GI) bleeding associated with NSAIDs, and they were successful in doing so, but they have also been associated with a slight increase in the risk of adverse cardiovascular effects (e.g., myocardial infarction and stroke). COX-2 inhibitors should be used with caution when cardiovascular risk factors are present and are contraindicated during coronary artery bypass graft surgery. Celecoxib is now the only available COX-2 inhibitor in the United States. For an overview of the NSAIDs according to their inhibition of COX and their selectivity for the COX-2 isoenzyme, see Table 39.1.

b. Toxicity from NSAIDs impacts primarily the GI, renal, hematologic, and hepatic systems.

1. GI. Dyspepsia is the most common side effect, and nonselective NSAIDs lead to asymptomatic ulcers in 20% to 25% of users within 1 week of administration. Complicated ulcers, including perforated ulcers, upper GI bleeding, and obstruction occur in a significant number of long-term NSAID users. Factors that increase the risk of NSAID-induced GI toxicity are shown in Table 39.2.

2. Renal impairment occurs in some patients taking NSAIDs and results from reduction in renal perfusion due to inhibition of prostaglandin synthesis. In patients with contraction of their intravascular volume (e.g., congestive heart failure, acute blood loss, and hepatic cirrhosis), renal perfusion is maintained through the vasodilatory effects of prostaglandins. Renal toxicity may manifest as acute interstitial nephritis or nephrotic syndrome. Acute renal failure occurs in as many as 5% of patients using NSAIDs; renal impairment typically resolves with the discontinuation of
NSAID therapy but, rarely, progresses to end-stage renal disease. Factors that increase the risk of NSAID-induced renal toxicity are shown in Table 39.3.

3. Hematologic toxicity associated with NSAIDs takes the form of inhibition of normal platelet function. Platelet activation is blocked by the inhibitory effects of NSAIDs on cyclooxygenase and the secondary decrease of prostaglandin conversion to thromboxane A₂ (a platelet activator). Aspirin irreversibly acetylates cyclooxygenase, and thus, the platelet inhibition resulting from aspirin use persists for the 7 to 10 days required for new platelet formation. Nonaspirin NSAIDs induce reversible platelet inhibition that resolves when most of the drug has been eliminated.

4. Hepatic toxicity may also result from NSAID use. Minor elevations in hepatic enzyme levels appear in 1% to 3% of patients. The mechanism appears to be immunologic or metabolic-mediated direct hepatocellular injury, with dose-related toxicity occurring with both acetaminophen and aspirin. Periodic assessment of liver function is recommended in those on long-term NSAID therapy.

5. Inhibition of normal bone formation has been reported in both clinical and animal models. The clinical relevance to NSAID use in the immediate postorthopedic surgery period and following acute fractures requires further study; despite the frequent use of NSAIDs to provide analgesia after orthopedic surgery and injury, there is little evidence that they dramatically affect healing.

c. Clinical uses. NSAIDs are used most widely to treat the pain and inflammation associated with rheumatic and degenerative arthritides. They also serve as a useful adjunct to opioids for providing control of acute pain. Addition of an NSAID can often reduce opioid requirements and related side effects in the postoperative period. Numerous agents are available for oral administration, and several are available without prescription. Thus, they are among the most common first-line analgesics.

d. Available formulations. Ketorolac and diclofenac are currently the only parenteral NSAIDs approved for clinical use in the United States. Both are potent analgesics and antipyretics, and several studies have demonstrated its usefulness in treating moderate post-operative pain. Ketorolac and diclofenac are nonselective NSAIDs, and despite a parenteral form, intravenous administration is still associated with GI toxicity similar to other orally administered NSAIDs. Familiarity with the dosing and administration of several oral NSAIDs as well as the parenteral formulations is an important tool for those treating acute pain. For a summary of comparative efficacy and dosages of commonly used nonopioid analgesics, see Table 39.4. Combination therapy with the addition of opioids to

NSAID therapy during the perioperative period can often provide synergistic analgesia and reduce opioid-related side effects. While it is important to avoid NSAIDs in patient populations at significant risk for toxicity, many patients having surgery can benefit from their addition.

2. Acetaminophen is a para-aminophenol derivative with analgesic and antipyretic properties similar to the NSAIDs. Acetaminophen does not produce any significant peripheral inhibition of prostaglandin production. Acetaminophen causes no significant GI toxicity or platelet dysfunction, and there are few side effects within the normal dose range. Acetaminophen is entirely metabolized by the liver, and minor metabolites are responsible for the hepatotoxicity associated with overdose. The most common oral analgesics used to treat moderate-to-severe pain incorporate acetaminophen in combination with one of the opioids. Standing per os or rectum dosing of 1 g of acetaminophen every 6 hours can be a very useful adjunct in the postoperative setting and can significantly improve pain and reduce opioid requirement. An intravenous formulation of acetaminophen was recently approved in the United States for treating mild-to-moderate pain.

3. Ketamine is an atypical anesthetic and potent analgesic that is an NMDA receptor antagonist. In contrast to opioids, spontaneous respiration and airway reflexes are relatively well maintained. Hypersalivation is a common side effect that can be eased by coadministration of an antisi-alagogue such as glycopyrrolate. Ketamine causes indirect stimulation of the sympathetic nervous system by inducing a catecholamine release. In high doses, ketamine causes a “dissociative” state and is associated with unpleasant side effects such as nightmares, which may be attenuated by concomitant administration of benzodiazepines. A low-dose ketamine infusion (5 to 10 µg/kg/min) can be used as an intraoperative anesthetic adjunct. A Cochrane review of perioperative ketamine demonstrated both reduced pain and opioid consumption, increased time to first analgesic, and decreased postoperative nausea and vomiting, at the consequence of increased dysphoric side effects (hallucinations, unpleasant dreams, nystagmus). Ketamine bolus can also be used in the immediate postoperative period as a rescue analgesic, especially after opioid rescue has failed. Patients should be premedicated with a benzodiazepine to mitigate dysphoria and be monitored on telemetry (bolus 10 to 30 mg). A Cochrane review reported that 27 of 37 studies demonstrated a significant reduction in postoperative pain with the use of ketamine. Use of ketamine as an adjuvant anesthetic has been shown to result in decreased opioid requirements in the immediate postoperative period in the majority of studies without significant increase in adverse outcomes. Ketamine is especially useful in the management of perioperative pain in patients on chronic opioid therapy.

4. Opiates and opioids. Opiates are among the most universally effective agents available for treating acute pain. Morphine, the prototypical opiate, is derived from the milk of the scored seed pod of the Oriental poppy, Papaver somniferum. Several other compounds can be derived directly through the chemical modification of morphine. Those drugs derived directly from morphine are termed the opiates. Other synthetic compounds have been produced that act via opiate receptors—all compounds that act via opiate receptors are termed the opioids. While opioids form the cornerstone of effective acute pain management, they have significant side effects, and their long-term effectiveness is limited
by tolerance, physical dependence, and the possibility of addiction. Common prescribing practices in the United States have led to an epidemic of prescription opioid misuse and abuse. In 2013, more overdose deaths in the United States were attributed to prescription opioids than heroin and cocaine overdoses combined. Significant reform in physicians’ prescribing patterns likely represent the first step in addressing this public health issue. Opioids are extremely useful for treating acute pain; although they are in widespread clinical use, their long-term effectiveness for treating chronic, noncancer pain is less clear.

a. Metabolism. Following injection, morphine rapidly undergoes hepatic conjugation with glucuronic acid; morphine remains largely in the ionized form at physiologic pH and is highly protein bound. The plasma concentration attained after an identical dose of morphine increases progressively with increasing age of patients (Fig. 39.1). Plasma concentration of morphine correlates poorly with its pharmacologic effect. Analgesia and depressed ventilation correlate more closely with cerebrospinal fluid (CSF) concentration. After intravenous injection of morphine, a metabolite, morphine glucuronide can be detected within 1 minute. While morphine-6-glucuronide (M-6-G) is produced in smaller amounts to morphine-3-glucuronide (1:9), M-6-G is pharmacologically active producing both analgesia and respiratory depression via interaction with µ-opioid receptors. M-6-G elimination is significantly impaired in patients with renal failure (Fig. 39.2) and can lead to prolonged depression of ventilation. Histamine release follows IV morphine administration but not fentanyl (Rosow et al., 1982), resulting decrease in SVR and BP following morphine administration (Fig. 39.3).