CHAPTER 28 Pain Control

Aditya Vikram Maheshwari, William T. Long, Lawrence D. Dorr

The long-term goals of total hip arthroplasty (THA)—to relieve pain, increase function, provide stability, and obtain durability—are accomplished in the vast majority of cases.¹ Recently, the focus has shifted to aggressive perioperative protocols that aim to speed up recovery, reduce morbidity and complications, and create a program of efficiency while maintaining the highest level of patient care. Concurrently, the bone and joint decade (2001-2010) has been characterized by exciting innovations in THA.² Implicit in the move toward rapid recovery after THA is the most effective management of perioperative pain. However, changes in pain management have probably had the greatest impact on patients because they are more comfortable and cause less nausea and less lethargy. This improved metabolism promotes participation in postoperative physical therapy and earlier discharge with the desire of patients to go to their home and not to a rehabilitation unit. The efficacy and side effects of analgesic therapy are major determinants of patient satisfaction.²

Optimal pain management can be one of the factors that shorten length of stay and allow quicker rehabilitation; however, adequate pain management is not always achieved. The consequences of severe postoperative pain are prolonged hospital stay, increased hospital readmission, increased use of opioids with subsequent increase in postoperative complications including nausea and vomiting, and overall low patient satisfaction.² The American Pain Society defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”³ Adequate pain management has become a priority in the minds of the public and the Joint Commission on Accreditation of Healthcare Organizations (JCAHO).⁴ Pain, which has become the “fifth vital sign” in the view of the JCAHO, demands consideration in the care of the patient, including taking it into account in the discharge decision as well as in the entire inpatient and outpatient course. Patients with pain demand treatment, and failure to provide adequate treatment can result in litigation or sanction.⁴

PRINCIPLES OF ACUTE PAIN PHYSIOLOGY

Operative procedures produce an initial afferent barrage of pain signals and generate a secondary inflammatory response, both of which contribute substantially to postoperative pain. The signals have the capacity to initiate prolonged changes in both the peripheral and central nervous system that can lead to amplification and prolongation of postoperative pain.³ Peripheral sensitization, a reduction in the threshold of nociceptor afferent peripheral terminals, is a result of inflammation at the site of surgical trauma. Central sensitization, an activity-dependent increase in the excitability of spinal neurons, is a result of persistent exposure to nociceptive afferent input from the peripheral neurons. Taken together, these two processes contribute to the postoperative hypersensitivity state (spinal wind-up) that is responsible for a decrease in the pain threshold, both at the site of injury (primary hyperalgesia) and in the surrounding uninjured tissue (secondary hyperalgesia).

Patient reaction to poorly controlled postoperative pain can include a wide range of physical and emotional responses.³ Physiologically, pain perception reflects the activation of nociceptors after injury to tissue, afferent transmission to the spinal cord, and relay via the dorsal horn to higher cortical centers. Pain perception has two major components: the sensory discriminating component, which describes the location and quality of the stimulus, and the affective-motivational component, which underlies the emotional effects of the pain and is responsible for learned avoidance and other behavioral responses.³ In addition to ethical and humanitarian reasons for minimizing pain, lack of its control can result in anxiety, sleeplessness, and release of catecholamines (neuroendocrine responses and activation of sympathoadrenal system). All of these can have deleterious effects on postoperative outcome, particularly in the elderly or critically ill. Humoral and neurologic alterations in and around the injury may be responsible for increased postoperative discomfort and disability. Continued activation of nociceptors may initiate reflex motor responses that lead to spasm and myofascial pain. Alteration in blood flow and efferent outflow may be responsible for sympathetically maintained pain and the persistent pain syndromes (chronic pain) that can lead to prolonged disability and impaired rehabilitation.

Over the past decade, a greater understanding of these pain mechanisms has led to the concept of pre-emptive analgesia. Pre-emptive analgesia involves the administration of analgesics before painful stimuli to prevent the establishment of central sensitization and thus the amplification of postoperative pain. It starts before surgery and covers both the period of surgery and the initial postoperative period. Pre-emptive analgesia prevents (or reduces) pathologic pain that is different from physiologic pain in several aspects; it is excessive (in intensity and spread) and can be activated by low-intensity stimuli (allodynia, hyperalgesia) and hyperpathia. The interventions, therefore, must produce a dense blockade of appropriate duration to block the transmission of noxious afferent information from the peripheral nervous system to the spinal cord and the brain. The concept of pre-emptive analgesia is based on animal studies, and the results of human clinical studies confirm the beneficial effect of pre-emptive analgesia.

Multimodal analgesia is a multidisciplinary approach to pain management with a goal to maximize the analgesic effect and minimize the side effects of the medications.^3,⁵ It takes advantage of the additive or synergistic effects of various analgesics, permitting the use of smaller doses with a concomitant reduction in side effects. Because many of the negative effects of analgesic therapy are related to parenteral opioids, limiting its use is a major objective of multimodal analgesia.

Another new concept is the evolution of the “pain services.”³ Acute pain management services include caregivers trained to formulate and provide safe and effective therapy. The pain service generally is multidisciplinary and multidepartmental and consists not only of surgeons and the anesthesiologists but also of nurses, pharmacists, physicians, and nursing assistants. The team is involved in implementation of standardized protocols that are known to be effective in addressing pain and minimizing complications associated with administration of analgesics.

TECHNIQUES OF PAIN MANAGEMENT

Optimal pain control is an individualized prescription that considers the following factors: the physiologic and psychological states of the patient, the pathophysiologic alteration that results from the surgery, and the technical and economic resources available during recovery. Anesthesia and analgesia for THA present a challenge, because patients are most often elderly and may have significant comorbid medical conditions. All of these can adversely affect patient management in the perioperative period. It is therefore important to choose an effective analgesic regimen with minimal side effects to allow timely mobility and optimal functional recovery while decreasing postoperative morbidity and mortality. Although several treatment options involving various combinations of systemic analgesics and/or regional analgesia, with or without opioids, are available for postoperative pain, a gold standard has not been established. Recently, both individual and combined uses of a number of treatment options have been evaluated with respect to post-THA pain control and include opioid patient-controlled analgesia (PCA), epidural opioids with and without local analgesia, intra-articular local anesthetics, peripheral nerve blocks, plexus blocks, and daily nonsteroidal anti-inflammatory drugs (NSAIDs).

Traditionally, postoperative pain control after THA was provided by either PCA or epidural analgesia.⁵ Combining several of these analgesic modalities after surgery, thus taking a multimodal approach, has been advocated and is known to produce better analgesia with reduced adverse effects. However, each technique has distinct advantages and disadvantages. Opioids do not consistently provide adequate pain relief and often cause sedation, ileus, nausea and vomiting, pruritus, respiratory depression, cognitive changes, urinary retention, bradycardia, and hypotension.⁶^–¹⁴ Epidural infusions containing local anesthetics (with or without opioids) provide superior analgesia but are also associated with hypotension, urinary retention, motor block that limits ambulation, unrecognized compartment syndromes, and spinal hematoma secondary to anticoagulation.²^–⁵ ⁷ Thus the shift is toward a multimodal protocol that minimizes all these side effects and provides adequate analgesia after THA.

GENERAL VERSUS REGIONAL ANESTHESIA

A positive trend has been shown for regional anesthesia compared with general anesthesia, especially in terms of lower incidence of complications.² Apart from lowering the blood loss and preventing deep vein thrombosis, postoperative epidural analgesia provides excellent pain relief after THA and allows early painless range of motion and weight bearing, enhancing overall patient satisfaction.

PARENTERAL OPIOID ANALGESICS

Although adequate analgesia may be attained with parenteral opioids, significant dose-related side effects are common and include sedation, ileus, nausea and vomiting, pruritus, respiratory depression, cognitive changes, urinary retention, bradycardia, and hypotension.⁶^–¹⁴ Nonetheless, they are still widely used for post-THA pain relief. These can be administered by intravenous, intramuscular, or intrathecal routes.

Currently, the most common regimen consists of intravenous PCA for 24 to 48 hours postoperatively, with subsequent conversion to oral agents.⁵ The key to successful initiation of PCA is the administration of an opioid loading dose, which provides a baseline plasma concentration of analgesic that can be then augmented by patient-controlled boluses. The PCA device may be programmed for several variables, including bolus dose, lock out interval, and background infusion. The optimal bolus dose is determined by the relative potency of the opioids; insufficient dosing results in inadequate analgesia, whereas excessive dosing increases the potential side effects. The lockout interval is based on the onset of analgesic effects; too short a lockout may allow the patient to self-administer additional medication before achieving the full analgesic effect, thus resulting in possible overdose or accumulation of the opioids. Conversely, a prolonged locking interval will not provide adequate analgesia. Although most PCA devices allow the addition of a background infusion, routine use in an opioid-naive patient is not recommended. There may be a role for background opioid infusion in opioid-tolerant patients, however. Because of the variation in patient tolerance, PCA dosing regimens may require adjustment to balance the analgesic effects against the adverse effects. Despite the ease of administration and titratability, parenteral opioids typically do not provide adequate analgesia for patients having total joint repair by themselves, particularly during movements with ambulation. As the trend shifts toward early rehabilitation and mobilization, protocols are trying to avoid the routine use of parenteral opioids. In a systemic review, Wheeler and associates reported respiratory side effects in 1.8%, pruritus in 14.7%, gastrointestinal side effects (including nausea, vomiting, and ileus) in 37.1%, urinary retention in 16.4%, and cognitive side effects (somnolence, hallucination, dizziness) in 33.9% of patients receiving PCA opioid analgesia.⁶ The unique analgesic efficacy of opioids analgesics mandates that they be used when indicated for moderate to severe pain. Responsible use of opioids mandates that they not be used casually but when the potential benefits outweigh the potential risk. To make this judgment, clinicians must be aware of the possible risks and complications associated with opioid use.

NEURAXIAL ANALGESIA

A variety of single-dose and continuous infusion neuraxial (epidural or spinal) techniques may be applied to provide pain control after THA.

Single-Dose Spinal and Epidural Opioids

Neuraxial opioids provide superior analgesia compared with systemic opioids but may also be associated with more adverse effects. The onset and duration is determined by lipophilicity of the drug.⁵ Lipophilic opioids, such as fentanyl, provide a rapid onset of analgesia, limited spread within the cerebrospinal fluid (and less respiratory depression), and rapid clearance and resolution. Conversely, hydrophilic opioids, such as morphine and hydromorphone, have a longer duration of action but are associated with greater frequency of side effects as well as delayed respiratory depression. A sustained-release formulation of epidural morphine has recently become available.¹⁴ The analgesic effect is present for approximately 48 hours. Unfortunately, it is not to be administered in the presence of local anesthetics (an epidural anesthetic may not be converted to provide epidural analgesia). Because of greater side effects, patients who exhibit sensitivity to an opioid when it is administered systemically should not receive that agent neuraxially.

Epidural Analgesia

Epidural analgesia may consist of a local anesthetic, an opioid, or a combination of both. A pure opioid epidural infusion may not provide adequate analgesia, and a pure local anesthetic may provide dense sensory and motor blockade, such that the patient may not be able to walk or void. Thus, a combination of an opioid and a local anesthetic creates a synergistic analgesic effect that allows lower concentration of each component in the solution.⁵

Continuous low dose infusion has been advocated as a method to control postoperative pain.² Continuous infusion permits analgesia to be more precisely titrated to the level of pain stimulus and rapidly terminated if problems occur. The technique avoids peak concentrations that follows intermittent boluses and reduces the risk of rostral cerebrospinal fluid spread and delayed respiratory depression. Other benefits, in comparison with intermittent dosing techniques, include decreased time spent administrating agents and assessing effects and a reduced risk of contamination and medication errors. Continuous infusion techniques also provide greater therapeutic versatility and a reduced side-effect profile.

Patient-controlled epidural analgesia offers higher analgesic efficacy and lower dose requirements than intravenous PCA and provides greater control and patient satisfaction than do either single-dose or continuous infusions. However, despite better pain control, patients still prefer intravenous PCA because of fewer technical problems and side effects and more uniform and sustained analgesia with more autonomy.³ The latency-to-peak effect and the risk of delayed-onset respiratory depression of morphine represent undesirable characteristics for patient-controlled epidural analgesia; therefore, hydromorphone and more lipophilic opioids like fentanyl, which offer greater titratability, have become the agents of choice in this setting.

Epidural infusions provide superior analgesia but are also associated with hypotension, hemodynamic instability, headache, urinary retention, motor block that limits ambulation, unrecognized compartment syndromes, and spinal hematoma secondary to anticoagulation.²^–⁵ ⁷ To maximize effective pain relief at rest and with movement, and to reduce unacceptable side effects, the epidural tip should be placed at the dermatomal level of the surgery (e.g., L2-3 for hip surgery).³