Pain Management in The Intensive Care Unit

1 Do critically ill patients require analgesia?

Intensive care unit (ICU) patients experience pain from underlying medical disease, immobility, surgery, and trauma. Insertion and maintenance of monitoring and therapeutic devices (venous and arterial catheters, chest tubes, drains, and endotracheal tubes) and routine bedside care (dressing changes, mobilization, physical therapy, and airway suctioning) may also cause pain and discomfort in ICU patients.

2 Is pain relief generally adequate in ICU patients?

The degree of analgesia in critically ill patients is often inadequate. Level of pain is harder to assess in ICU patients because patients are often confused, unable to communicate, or even paralyzed. Inadequate use of analgesic agents, together with routine use of sedatives, may result in oversedation and even delirium, and impaired ability of the patient to cooperate with pain assessment and management.

3 How can pain be assessed in critically ill patients?

Pain should be assessed and documented at regular intervals. Pain is a subjective experience and is most reliably measured by using a subjective scale such as the numeric rating scale (Fig. 72-1), which can be employed in patients as young as 5 years. Younger patients’ pain is assessed by using the faces scale (Fig. 72-1), which is a modified visual analog scale. In patients unable to communicate, reliance on objective measures (physiologic or behavioral) becomes necessary. Whereas use of physiologic measures (blood pressure, heart rate, tearing, diaphoresis, mydriasis) can cause underreporting and overreporting of pain, use of behavioral measures such as observation of facial expression has been demonstrated to correlate with subjective reporting.

Figure 72-1 Visual analog and numeric rating scales.

4 Is pain harmful?

Through activation of the stress response, pain exacerbates the negative effects of the sympathoadrenal response to critical illness at a multisystem level. Pain can contribute to the development of delirium, which is increasingly recognized as a contributor to poor outcomes and even long-term cognitive dysfunction. Myocardial ischemia is exacerbated through both increased myocardial oxygen demand and decreased supply. Pain after thoracic or upper abdominal surgery induces an acute restrictive respiratory defect (reflex muscle rigidity, splinting, loss of functional residual capacity) exacerbated by a reflex reduction in phrenic nerve activity with decrease in diaphragmatic contractility. The result is impaired cough, atelectasis, and pneumonia. Sympathoadrenal activation prolongs the adynamic ileus that follows abdominal surgery and that is recognized as a contributor to prolongation of hospital stay. Neuroendocrine components of the stress response interact with the cytokine system to cause a catabolic state with muscle wasting and impaired immunity and to promote a hypercoagulable state with the attendant risks of myocardial infarction and venous thrombosis.

5 What are the treatment options for a critically ill patient in pain?

6 What is the role of opioids in the ICU, and how do they act?

Opioids are the mainstay of analgesic therapy in the ICU. Long-standing familiarity has fostered relative safety in their use. They are mainly effective against visceral pain of a static nature but relatively ineffective against somatic and dynamic pain; analgesic efficacy tends to dissipate once movement including out-of-bed mobilization and respiratory therapy maneuvers are instituted.

The term opioid refers to any agent with activity at an opioid receptor. There are at least four discrete opioid receptors in the central and peripheral nervous system; the analgesic effects of opioids are mediated mostly via mu (μ) or kappa (κ) receptors. These are G protein–coupled receptors that mediate inhibition of adenyl cyclase.

7 Which opioids are recommended for routine administration in ICU patients?

Morphine, fentanyl, hydromorphone, and sufentanil are the analgesic agents most commonly used and recommended in the ICU (Table 72-1).

8 How do you decide which opioid to use?

Before an opioid is ordered, the goal of analgesia is determined and a therapeutic plan developed. The patient’s cardiovascular status and subsystem organ function are assessed.

9 What is the place of meperidine in the ICU?

Meperidine is useful in the treatment of postoperative shivering and has been demonstrated to be superior to morphine and fentanyl for this indication. A small intravenous (IV) dose of 10 to 25 mg is usually sufficient. Postoperative shivering can increase oxygen consumption and may be detrimental if it occurs in patients with ischemic heart disease. However, meperidine is not recommended for repetitive use in ICU patients because of its active metabolite (normeperidine), which may cause central nervous system excitation and its interactions with antidepressants (contraindicated with monoamine oxidase inhibitors, best avoided with selective serotonin reuptake inhibitors).

10 Which other opioids should be avoided in the ICU for routine analgesia?

11 How should opioids be administered for acute pain management in the ICU?

The IV route is considered superior because regional hypoperfusion due to shock or edema may render the absorption of opioids less reliable via the subcutaneous and intramuscular routes. IV opioids can be delivered in three different modes:

12 Explain the concept of PCA

PCA is managed with microprocessor-based pumps that allow the patient to self-administer analgesics according to a predetermined limit set by the physician. The goal of PCA is to produce a relatively stable and effective level of analgesia by allowing the patient to receive multiple small boluses after an initial loading dose. Patients prefer this pain relief technique because they control analgesia administration themselves. The following basic parameters can be set on a PCA pump: bolus (dose administered when the patient pushes the button), lockout interval (minimal length of time between two doses), maximal dose to be given, background infusion (continuous infusion in addition to the boluses). Morphine is commonly prescribed because of its relatively rapid clearance; typical adult doses are 1 to 2 mg bolus, 10-minute lockout interval, and 24 to 30 mg 4-hour limit. Background infusion is probably better avoided in opioid-naïve patients because of the risk of respiratory depression.

13 Why should you avoid routinely prescribing background infusions by PCA?

PCA continuous infusions (background infusion) bypass the intrinsic safety feature of standard, bolus-only PCA by allowing opioids to be continuously delivered even if sedation is excessive. Studies have demonstrated that patients after surgery treated with PCA plus continuous infusion have no improvement in analgesia but have a significantly greater number of side effects compared with patients after surgery receiving standard PCA.

14 What are the side effects of opioids? (See Table 72-2.)

Although seldom a problem in the euvolemic patient, opioids can cause hypotension through reduction in sympathetic tone or histamine release. The latter is more frequent with the traditional phenanthrene derivatives (morphine), and the newer semisynthetic phenylpiperidines (fentanyl, sufentanil) offer greater hemodynamic stability.

^{Table 72-2} Side effects of opioid analgesics

Opioids are respiratory depressants. Classically described as central respiratory depressants that blunt the ventilatory response to elevated arterial carbon dioxide tension, these analgesics also appear to produce an obstructive sleep apnea state. Weakened and delayed phasic inspiratory contraction of pharyngeal dilator muscles may be a major cause of opioid-induced hypoxemia, particularly in patients with an underlying diagnosis of obstructive sleep apnea. Opioids are relatively ineffective at reversing the dynamic pain associated with deep breathing and coughing maneuvers after thoracic and upper abdominal surgery that are responsible for much of the associated postoperative atelectasis, impaired gas exchange, and pneumonia.

Pain-induced delayed gastrointestinal transit time is exacerbated by opioids, which cause sphincter contraction and increased gastrointestinal resting tone through μ-receptor stimulation. Resumption of oral intake is further exacerbated by these agents’ tendency to cause nausea. Judicious use of prokinetic and antiemetic agents is recommended.

15 What is the role of nonopioid analgesics in the ICU, and what are their characteristics?

Acetaminophen and NSAIDs have been shown to decrease the need for opioids and are particularly effective in reducing muscular and skeletal pain. They are often more effective than opioids in reducing pain from pleural or pericardial rubs, a pain that responds poorly to opioids. Basic pharmacology is summarized in Table 72-3.

16 What are the side effects of nonopioid analgesics?

17 What is dexmedetomidine?

Dexmedetomidine is a highly selective α₂-agonist with both sedative and analgesic properties but without respiratory depression. It appears useful in discontinuing mechanical ventilation in patients after surgery. Its opioid-sparing effect may be useful in patients with obstructive sleep apnea and pulmonary disease. Recent research demonstrates a reduction in ventilator days and delirium with its use. The maintenance infusion dose of dexmedetomidine is 0.2 to 0.7 mcg/kg/hour. Although an initial loading dose can be prescribed, caution is advised given its tendency to produce bradycardia.

18 Does ketamine have a role as an analgesic agent in the ICU?

Ketamine is a phencyclidine derivative with unique properties. Originally used in 1964, it has had long-standing use for its intensely analgesic effect, hemodynamic stability, and relative lack of respiratory depression. IV boluses of ketamine (0.5-2 mg/kg) have been used in ICU patients to enhance tolerance of limited but painful procedures, such as dressing changes for burns. More recently, its value as an infusion (0.1 mg/kg/hour) has been recognized because of its bronchodilator and anticonvulsant properties. Although traditionally used with caution in acute brain injury owing to a cerebral vasodilator effect, it appears safe if hypercarbia is avoided, and the N-methyl-D-aspartate antagonist properties of ketamine may be neuroprotective. Furthermore, small doses of ketamine may be a valuable adjunct to routine analgesia to decrease opioid requirements.

19 Does epidural analgesia have a role in ICU patients?

Epidural analgesia with use of a local anesthetic agent offers several theoretic advantages in patients admitted to the ICU after surgery including reduction in pneumonia, myocardial infarction, and venous thrombosis. Improved outcomes should particularly result from the use of thoracic epidural technique after thoracic or upper abdominal surgery in patients with underlying cardiac or pulmonary disease. However, studies demonstrate varying results. There does appear to be improved analgesia, particularly from pain on mobilization, and earlier return of gut function. The latter factors may allow earlier rehabilitation and hospital discharge as part of a well-coordinated recovery program. Other potential advantages of epidural analgesia may result from reduction in opioid administration. As a general practice, physicians insert epidural catheters most commonly at mid- or low-thoracic level for patients undergoing thoracic or upper abdominal surgery, respectively. The physicians use a solution of bupivacaine 0.1% with hydromorphone 10 mcg/mL, which is run at 4 to 6 mL/hour.

20 Is epidural analgesia safe in the setting of deep vein thromboprophylaxis?

Regional anesthesia may be performed in this setting, although practitioners are advised to follow the guidelines of the American Society of Regional Anesthesia and Pain Medicine (www.asra.com) and be vigilant for the development of epidural hematoma. If low-dose unfractionated heparin is being used, needle placement and/or catheter removal should be done ≥ 2 hours after discontinuing heparin, and reheparinization may be started ≥ 1 hour after an uncomplicated epidural insertion. If fractionated low-molecular-weight heparin (LMWH) is being used in prophylactic doses, a waiting period of ≥ 12 hours for any neuraxial technique should be applied after the last dose of LMWH, and the next LMWH dose should be given ≥ 2 hours after an uncomplicated procedure.