Chapter 26

Acute pain

Stephan A. Schug, Deborah Watson and Esther M. Pogatzki-Zahn

OVERVIEW

Acute pain is seen in a multitude of clinical situations: in the post-operative and post-trauma setting, following burns, in acute medical diseases (e.g. pancreatitis, myocardial infarction) and as obstetric pain. Post-operative pain is the most common form of acute pain (and of major relevance for therapists), therefore this chapter will direct its focus towards management of post-operative patients.

There is widespread agreement in the literature on the inadequacy of acute pain management. On the other hand, there is also a wide body of evidence which suggests that relief of acute pain is not only an integral part of humane health care, but can also have profound effects on patient outcomes. Only over the last decade has a considerable amount of scientific and clinical effort been invested in providing the patient in acute pain with the best quality analgesia, at the same time ensuring safety from potentially life-threatening adverse events for each modality of analgesia (Macintyre et al 2010).

This chapter focuses on the importance of effective management of acute pain. It incorporates the most important pharmacological and non-pharmacological modalities of analgesia in the treatment of acute pain (for definitions see Box 26.1); reference will be made to comparison of the efficacy, benefits and adverse effects of these modalities. Some of the newer techniques used have accumulated substantial evidence demonstrating an improvement not only in quality of analgesia and patient satisfaction, but also in short- and long-term morbidity, and, potentially, even mortality. In addition, there is now increasing evidence of a relative association between the experience of acute pain and the development of chronic pain. Prevention of such progression is another important goal of adequate acute pain management.

Box 26.1 Key terms defined

Systemic analgesia: The provision of pain relief by administration of medications systemically, usually by the parenteral, oral, transdermal or rectal route.

Regional analgesia: Techniques which administer local anaesthetics or other adjuvants in such a way that they act locally or regionally, e.g. by administration close to a peripheral nerve or the spinal cord.

Patient-controlled analgesia (PCA): A technique using a programmable infusion device that can be activated by the patient to deliver small bolus doses of analgesics on demand.

Epidural analgesia: A technique whereby drugs are administered into the epidural space, close to the spinal cord but outside the spinal meninges.

Intrathecal analgesia: A technique whereby drugs are administered into the intrathecal space, close to the spinal cord and inside the meninges.

Pre-emptive analgesia: Techniques administered where it has been shown that preoperative treatment is more effective than the identical treatment being administered after incision or surgery.

Preventive analgesia: An analgesic intervention wherein the effect lasts longer than the expected duration of action of the intervention.

However, despite the theoretical availability of a wide range of appropriate agents and techniques, the cause of poor acute pain management is often their insufficient, inappropriate or unsupervised application. Therefore, the chapter would be incomplete without a discussion of appropriate organizational structures to provide acute pain relief, that is, the concept and the role of the acute pain service.

PRINCIPLES OF ACUTE PAIN MANAGEMENT

Post-operatively, up to 80% of patients experience pain, and it is moderate, severe or extreme in 86% of these cases (Apfelbaum et al 2003). The Apfelbaum et al study also found pain to be the most common concern of patients after surgery (59%). Psychological factors such as pre-operative anxiety, depression, catastrophizing, expectation of pain, fear of death and associated sleep deprivation influence post-operative pain control, and attention must therefore be given to individual patient differences, which may lead to an improved outcome.

Traditionally, post-operative pain has been managed using fixed doses of intramuscularly (IM) administered opioids on an as-needed basis; this treatment approach has led to unrelieved pain in more than 50% of post-operative patients (Oden 1989). The major problem with this approach is explained by the huge interindividual variation in dose requirements, which can vary more than ten-fold for patients of similar age and weight having the same operation. Furthermore, opioid concentrations following IM bolus doses exhibit a pronounced peak and trough pattern, with periods of inadequate analgesia, but also the associated risk of delayed overdose (Cashman & Dolin 2004).

A more appropriate approach to acute pain management should therefore consider a much wider array of techniques and their combination. These include the following pharmacological options:

• administration of systemic opioids (intravenously [IV], subcutaneously [SC], orally [PO], transmucosally or transdermally) on a regular and/or as-required basis or via patient-controlled analgesia (PCA)

• administration of non-opioid analgesics, such as paracetamol, non-steroidal anti-inflammatory drugs (NSAIDs) and cyclo-oxygenase-2 (COX-2) inhibitors

• administration of other systemic agents that have uses in particular settings, such as nitrous oxide (Entonox), ketamine, adrenergic drugs, antidepressants and anticonvulsants

• neuraxial analgesia (epidural or intrathecal administration of opioid and/or local anaesthetic drugs)

• intermittent or continuous peripheral neural blockade with local anaesthetic drugs.

Besides these, there are many non-pharmacological options, which are often under-utilized, despite their simplicity:

• explanation, reassurance and discussion of analgesic options

• cognitive–behavioural interventions such as relaxation, distraction and imagery, which can be taught preoperatively

• various physical interventions such as splints, massage, application of heat or cold, acupuncture and transcutaneous electrical nerve stimulation (TENS).

The majority of this chapter focuses on the pharmacological options, discussing their efficacy, benefits, side-effect profiles and combination therapy possibilities with other analgesic agents.

Systemic Pharmocological Modalities

The current recommendation and standard practice is to avoid this route if at all possible (Macintyre et al 2010). If, for organizational, political or training (better: lack of training) reasons, IM injections are the only parenteral route of administration permitted or – inappropriately – deemed safe in a certain environment, then the dose used should be based on age and medical condition, and the administration interval should be shortened to 2 hourly PRN, to increase flexibility (Macintyre & Schug 2007).

Subcutaneous opioids

Opioids can be given intermittently or as a low-volume continuous infusion via the SC route. The absorption profile is similar to that of IM administration (Semple et al 1997), and both routes have similar analgesic and side-effect profiles. However, patients prefer the SC route, particularly if used via an indwelling SC cannula, for obvious reasons (Cooper 1996). The approach has been shown to be beneficial as a continuous infusion (volumes < 1–2 ml/h) in severe cancer pain and in post-operative patients in whom IV access is not, or not easily, available. Morphine and hydromorphone are used preferentially as they are low-irritants to the SC tissue; treatment algorithms in this area have been published (Macintyre & Schug 2007). For patients with an indwelling IV line (i.e. most early post-operative patients), there are no advantages, but some disadvantages (delayed onset of analgesia, second access) of the SC route in comparison to the IV route.

Intravenous opioids

Opioids can be given as boluses (e.g. 0.5–4 mg morphine every 3–5 minutes as directed by a formal IV protocol; Aubrun et al 2001), as a continuous infusion or via PCA devices through the IV route. The IV route is the route of choice following major surgery, but there is a risk of respiratory depression with inappropriate dosing, and close monitoring and safety precautions are therefore required.

Intermittent IV boluses

Intermittent IV boluses provide a rapid, predictable and observable response compared to other parenteral routes. This is the rationale behind use of IV PCA. The IV route is particularly useful for:

• obtaining initial and rapid pain relief such as in the immediate post-operative period and in acute trauma (Aubrun et al 2001)

• patients who are hypovolaemic and/or hypotensive, and will absorb IM/SC opioids in a delayed and unpredictable fashion

• treating so-called ‘incident pain’ caused by events such as dressing changes, mobilization and physiotherapy.

Intermittent boluses are also an ideal path to titrated pain relief in the recovery room, and bridge times of severe pain until medical review and/or more appropriate analgesic methods become accessible. Most commonly, nurse-administered bolus doses, prescribed according to a protocol or algorithm, are used. Such protocols specify (or permit some flexibility with regard to) bolus size, assessments and ‘lock-out’ time (Macintyre & Schug 2007).

Continuous IV infusion

This form of infusion avoids the peaks and troughs in blood concentrations associated with intermittent administration, but it has proven difficult to predict the required individual blood concentration for optimal analgesia. A continuous infusion requires reliable infusion devices and frequent assessment and monitoring by staff who are trained to monitor patients with an emphasis on level of sedation, and who are authorized to adjust the infusion rate and give bolus doses. To provide adequate analgesic blood concentrations can take up to 20 hours (five half-lives); consequently, if analgesia is inadequate, a bolus is given as well as the rate of infusion being increased.

The risk of respiratory depression using a continuous morphine infusion (up to 1.65%) is the highest of all parenteral routes (Schug & Torrie 1993). This needs to be considered carefully, as fatal outcomes are reported, in particular in sleeping or sedated patients (Macintyre & Schug 2007).

Patient-controlled analgesia

PCA was introduced to overcome the variability in individual morphine dose requirements and the problems associated with insufficient analgesia and potentially serious adverse outcomes. This is primarily a concept granting a patient control of his/her pain relief, and can be utilized by various routes of systemic and regional drug administration, but is commonly associated with IV drug administration.

A PCA device is a sophisticated, programmable infusion instrument that can be activated by the patient to self-administer small bolus doses of IV opioid on demand, separated by a lock-out period, during which the device does not respond to further activation. As such, the PCA concept overcomes the interindividual variation in opioid requirements, and allows the patient to adjust the level of analgesia to their own desired level of comfort, balanced to an individually acceptable severity of side effects. Intravenous opioids administered by PCA improve analgesia and patient satisfaction (Hudcova et al 2006). It has been demonstrated that, for morphine, a bolus dose of 1 mg with a 5-minute lock-out period is ideal for most patients; other programmes are associated with either inadequate analgesia or sedation and increased respiratory compromise (Owen et al 1989). However, some patients might need different programmes, depending on age, co-morbidity, pain intensity and previous opioid exposure, therefore regular review of all patients using PCA devices by experienced personnel is mandatory for a good outcome. Other opioids such as fentanyl, hydromorphone or tramadol can also be used.

Following surgery, the average patient will require PCA for 2–4 days. Drug consumption is maximal within the first 24 hours and thereafter rapidly declines. Use after abdominal surgery tends to be increased and relatively prolonged (Sidebotham et al 1997), which reflects the major physiological insult and the additional pain associated with mobilization and physiotherapy. Women use 20–30% more morphine early after surgery than men (Aubrun et al 2005), but less morphine thereafter (Sidebotham et al 1997). Age is the best predictor of post-operative opioid requirements (Macintyre & Jarvis 1996), but there is little correlation between patient weight and levels of consumption (Burns et al 1989).

The technique provides effective, steady analgesia and is popular with patients. However, analgesia at rest and on movement is not perfect. About 40% of patients using PCA have a pain score > 3/10 at rest on day one post-operatively. The occurrence of unpleasant side effects from increased opioid usage may be responsible for some of this inadequate analgesia, which prevents 20% of patients from complying with physiotherapy on day one post-operatively (Schug & Fry 1994; Sidebotham et al 1997). It requires special infusion pumps and staff education. In addition, patients require instructions preoperatively to be able to understand the principles underlying the PCA technique and how/when to activate it (Chumbley et al 2002).

Although PCA is the safest method of administering systemic opioids, there still remains a small risk of respiratory depression (incidence in the range 0.1–0.8%) (Macintyre 2001). This risk is much smaller than that associated with continuous IV infusion or intermittent IM injection. This advantage with regard to safety is due to the fact that acute pain causes stimulation of respiratory centres in the brain and, consequently, respiratory depression does not occur simultaneously with acute pain. As patients use the PCA device by titrating opioids to effect, there is less likelihood of respiratory depression. This is even more the case as the sedated patient will stop using the device. In the rare cases of respiratory depression, the causes are commonly:

• operator error (e.g. inappropriate prescription, incorrect programming of PCA device, incorrect dilution of medication)

• equipment failure (e.g. cracked syringes with gravity siphoning of opioid solution [rare]) (Macintyre 2001).

Other side effects associated with PCA administration of opioids are nausea and vomiting in 35% of cases (Sidebotham et al 1997), occurring mainly on the first post-operative day, as well as sedation in 18% and confusion in 12% of cases (Schug & Fry 1994). These problems occur with similar incidence to other methods of opioid administration and are not reduced by the PCA approach (Hudcova et al 2006).

Continuous low-dose IV infusion, when given together with PCA, has been shown to increase the risk of side effects (Schug & Torrie 1993; Sidebotham et al 1997) without significantly improving analgesia (Dal et al 2003, Parker et al 1992); the incidence of respiratory depression is five to eight times higher than in the case of PCA alone, as the inherent safety concept of PCA is violated. Hence, the only patients who should be prescribed a background opioid infusion are those already receiving opioids. These patients already have some degree of opioid tolerance as well as increased requirements (e.g. chronic pain, recreational abuse, methadone substitution) (Mitra & Sinatra 2004).

All routes of opioid administration, especially parenteral routes, need to be carefully monitored for side effects, notably respiratory depression. Specific protocols are written for each route of administration so that patients receive optimal analgesia whilst always being safeguarded against respiratory depression and monitored for other side effects of opioids. Safe and appropriate use of the PCA method requires frequent and informed monitoring by nurses who have undergone relevant education and accreditation in the management of these devices. Standard orders and drug dilutions are suggested to maximize the effectiveness of the PCA and minimize complications.

The risk of opioid addiction is often cited as a reason for provision of inadequate analgesia. However, it has been demonstrated that addiction to opioids is rare when used in the treatment of acute pain. Patients choose not to fully relieve their pain, despite free access to drugs, and demands tend to be conservative, with patients opting to remain alert and in a small amount of discomfort (Macintyre 2001). There is at present no evidence that opioid use in the management of acute pain leads to opioid dependence or addiction (Chapman & Hill 1989; Schug & Torrie 1993).