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. 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. 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. 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. 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.” 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. 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. 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. 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. 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. 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. 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. 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.

PERIPHERAL NERVE BLOCKADE

Although these blocks have been traditionally underutilized, advances in needles, catheters, and nerve stimulation technology have facilitated the localization of neural structures and have improved the success rate. They minimize exposure to opioids and are ideally suited for patients sensitive to opioid-induced ileus and respiratory depression. The advantages include effective postoperative analgesia, lower opioid consumption, improved rehabilitation, lower complications, and higher patient satisfaction.

Peripheral neural blockades may be provided by infiltration techniques, intra-articular blocks, isolated nerve blocks, or plexus block. Infiltration techniques use injections of local anesthetic at the site of surgery and provide several hours of postoperative analgesia. Preincisional infiltration may prevent the nociceptive input from altering the excitability of the central nervous system (pre-emptively blocking the N -methyl- d -aspartate–induced “wind up” phenomena and release of inflammatory mediators). A number of single-dose and continuous infiltration techniques have been described. Intra-articular injections into the joint capsule and muscles also help prevent peripheral sensitization. Several drugs have been used in combination and consist of a local anesthetic along with a corticosteroid, morphine, clonidine, epinephrine, or ketorolac.

Femoral or sciatic nerve block alone is not capable of providing adequate analgesia for hip surgery but can be very useful adjunctive procedures, allowing less systemic narcotic use. However, the lumbar plexus block in combination with parasacral block has demonstrated satisfactory results. The lumbar plexus covers the skin dermatomes and femoral components, and the parasacral block covers the nervous innervation of the acetabulum. The lumbar plexus may be blocked by three distinct approaches. Block of the full lumbar plexus (femoral, lateral cutaneous, and obturator nerves) is accomplished with the psoas block. Also, it is preferable for hip surgery because it is the most proximal lumbar plexus technique. In comparison, the fascia iliaca and femoral approaches will reliably block the femoral but not the lateral cutaneous and obturator nerves. Complete unilateral lower extremity blockade is achieved by combining a lumbar plexus technique with a proximal sciatic block.

These blocks are usually safe and their unilateral nature makes them ideal for the patient because the contralateral limb is immediately available to assist with early ambulation. However, neurologic dysfunction and intravascular injection are the primary concerns. Most of the neurologic complications have been reported as transient. In addition, because of the proximity of the neuraxis with the psoas approach, epidural spread and intrathecal injection have been reported. Thus, attention to total dose of local anesthetic and incremental aspiration are of paramount importance during injection to diagnose malpositioned needles and catheters. Other theoretical concerns include patient injury related to insensate limb, catheter migration, potential anesthetic toxicity, and masking of surgical-related nerve injury and compartment syndrome.

The duration of effect from a single injection technique is usually not sufficient to result in major improvement in analgesia or outcome. Although continuous nerve blocks have been successful in the inpatient setting, discharging patients to their homes or a rehabilitation facility with an insensate extremity and relying on the patient to administer local anesthetic remains controversial but is subject to controlled clinical investigation. A health care provider network should be accessible to the patient for the use of continuous regional analgesia in the outpatient setting. The standard for success is to provide the same quality of care and level of safety as in the inpatient setting. Initial experience has shown efficacy in terms of pain relief, low incidence of postoperative nausea and vomiting, low opioid requirement, and high patient satisfaction and safety. However, this requires a protocol that includes patient selection, extensive patient education, home health nurse education, and close follow-up.

Peripheral nerve blocks will alter motor and proprioceptive function, sometimes interfering with early ambulation. Newer peripheral nerve and plexus techniques involve the measured use of local anesthetics in a single shot or in combination with the use of longer-acting drugs. Such sustained-release drug strategies eventually obviate the need of indwelling catheters. This may also serve to limit the unwanted side effects of motor and proprioceptive deficits while preserving the desired analgesic blockade. The availability of newer local anesthetics that are alleged to be associated with less toxicity and greater selectivity with respect to sensory and motor blockade (e.g., ropivacaine and levobupivacaine) may further enhance the benefits of local anesthetic supplementation.

Based on the Procedure Specific Postoperative Pain Management (PROSPECT) study, femoral nerve blocks are recommended based on their analgesic efficacy in hip fracture surgery. They are recommended over neuraxial techniques and parenteral opioids based on a decreased risk of side effects. Continuous infusion, patient-controlled, or “on demand” femoral nerve analgesia via a catheter is recommended over a single-shot approach because it provides extended proximal spread of effect and a greater duration of analgesia. Supplementary obturator and lateral cutaneous nerve of the thigh block may be required. Posterior lumbar plexus blocks (psoas sheath blocks) have a greater efficacy than femoral nerve blocks. However, they are associated with more complications and the risk-benefit balance should be determined for individual patients.