Pain is an extraordinarily complex and emotive sensation which is difficult to define and equally difficult to measure in an accurate, objective manner. It has been defined as the sensory appreciation of afferent nociceptive stimulation, which elicits an affective (or autonomic) component; both are subjected to rational interpretation by the patient. It may be represented as a Venn diagram (Fig. 41.1), the shaded area of which represents the quantum of suffering experienced by the patient. The Venn diagram illustrates easily and effectively that the sensation of pain differs among individual patients; the emotional component may vary according to the patient’s current psychological state and pre-existing psychological composition and the rational component varies with the patient’s previous experience, insight and motivation.
Postoperative pain differs from other types of pain in that it is usually, but by no means always, transitory, with progressive improvement over a relatively short time-course. Typically, the affective component tends towards an anxiety state associated with diagnosis of the condition, concern regarding the effectiveness of surgery to provide a cure and/or relief of existing pain, and fear of delay in provision of adequate analgesic therapy by attendants. In contrast, chronic pain is persistent, frequently with fluctuating intensity, may remain without an acceptable diagnosis and the affective component contains a greater depressive element, with the patient needing and potentially actively searching for adequate pain relief. Thus, postoperative pain is more easily amenable to therapy than chronic pain. Despite this, a recent survey showed that most adults still expect to have significant postoperative pain after surgery, and that this is their primary concern before surgery. Such concern may be justified, because traditional management of postoperative pain, using intramuscular opioid administration given on demand, often failed to produce good analgesia. This was highlighted by the Royal College of Surgeons’ report on postoperative pain in 1990, which paved the way to the national development of Acute Pain Services and improved methods of administering pain relief.
The traditional management of postoperative pain comprised the prescription of a standard dose of an opioid, to be given on demand by a nurse when the patient’s pain threshold had been exceeded. This leads to poor control of postoperative pain for the following reasons:
Responsibility for management of pain is delegated to the nursing staff, who err on the side of caution in the administration of opioids. They tend to give too small a dose of drug too infrequently because of exaggerated fears of producing ventilatory depression or addiction.
The roles and availability of nurses have changed in line with an evolving healthcare system. This has reduced the number of nurses available to administer opioid analgesia in an effective and safe manner. The statutory regulations on the prescribing and administration of opioid medication remains unchanged, but with reduced staffing levels, this can impact on the time delay between the request for analgesia and the administration of the medication. This is often most noticeable at night.
Because the administration of drugs is left entirely to the discretion of the nursing staff, the degree of empathy between nurse and patient affects analgesic administration. This may be part of the explanation for the common observation that the mean dose of morphine given for a standard operation varies among hospitals and even among wards in the same hospital.
These inadequacies in the traditional management of postoperative pain have been confirmed by a survey of ethical problems faced by nurses, which showed that they regard pain management to be a significant problem in their clinical practice.
The physiology of acute pain is no longer considered to be a simple ‘hard-wired’ system with a pure ‘stimulus-response’ relationship. Rather, tissue damage or disease sets up a process involving tissue receptors, the peripheral, central and autonomic nervous systems, and higher centres in the brain which produce the perception of pain.
Nociceptors are receptors which require a strong (high threshold) stimulus for activation. Most are polymodal, i.e. respond to a variety of noxious stimuli (heat, mechanical, chemical). The usual stimulus for activation is mechanical distortion of the receptor, followed by local increases in K+ and H+ ions. Inflammation leads to a reduction in the threshold for stimulation, and activation of dormant or ‘silent’ nociceptors. This is termed ‘peripheral sensitization’.
Primary afferent fibres conduct impulses from the nociceptor to the spinal cord, and have their cell bodies in the dorsal root ganglion. There are two types of primary afferent fibres from nociceptors, and they are distinguished mainly by their speed of conduction. Aδ fibres have a high speed of conduction, and are responsible for ‘immediate’, sharp pain and reflex withdrawal. C fibres conduct at a lower speed, and are responsible for persistent pain and central sensitization in the spinal cord (Table 41.1). Most primary afferent fibres terminate by synapsing with dorsal horn neurones (Fig. 41.2).
A cross-section of the spinal cord shows 10 anatomically and physiologically distinct layers called Rexed laminae (Fig. 41.3). Laminae 1–6 and lamina 10 are sites at which sensory afferents synapse with dorsal horn cells. Laminae 7, 8 and 9 represent the motor horn. Aδ and C fibres terminate in several layers, including the outer (marginal) zone and in particular lamina 2 (substantia gelatinosa).
Some dorsal horn cells respond to painful and non-painful stimuli, and are called ‘wide dynamic range’ (WDR) neurones. They exhibit ‘wind up’ in which their output increases in the presence of a continuous, low frequency, C fibre (i.e. painful) input. In this case, the C-fibre input has ‘sensitized’ the dorsal horn cell, and animal models have demonstrated that NMDA (N-methyl-D-aspartate) receptor activation is a key feature. This process is termed ‘central sensitization’.
Allodynia is the term for pain induced by a previously non-painful stimulus. Hyperalgesia is increased pain from a previous painful stimulus. Both occur following peripheral and central sensitization, and they should be regarded as normal physiological processes following tissue injury, designed to encourage the organism to protect the injury.
The receptive field of a dorsal horn neurone refers to the area in the periphery where stimulation triggers action potentials in that dorsal horn neurone. Following tissue injury, the receptive field expands, and minor changes in excitability cause major changes in the size of the receptive field.
Spinothalamic tract: probably the most important tract for pain transmission, this tract projects to several nuclei in the thalamus. It is the target for treating intractable cancer pain with cordotomy.
Descending inhibitory pathways, originating in particular from the periaqueductal grey matter in the midbrain and the rostral ventromedial medulla, may modulate transmission across spinal cord synapses. Both these areas contain high concentrations of endogenous opioids and opioid receptors. Activation of these receptors increases activity in descending monoamine (serotonin and noradrenaline) pathways (indirectly, by reducing stimulation of inhibitory interneurones), and reduces transmission across dorsal horn synapses.
Afferent C fibres from abdominal viscera travel with autonomic nerves, particularly sympathetic, and synapse with dorsal horn cells. Thus, sympathetic denervation may be useful for intractable visceral pain (e.g. coeliac plexus block for pancreatic cancer). There is no distinct spinothalamic tract for visceral afferents.
This is pain arising from an abnormality or change in either the peripheral or central nervous systems. It is usually accompanied by other sensory (numbness, paraesthesiae), motor (weakness) or autonomic dysfunction, and so may be distinguished from referred pain. Radicular pain is pain arising in the distribution of a spinal nerve, and may be caused by compression of the spinal cord, nerve root, or plexus.
Referred pain is experienced at a site distant from the pain source, and occurs because of convergence of different pain afferents on to common dorsal horn neurones. Segmental embryonic innervation remains throughout growth and accounts for the distance between the source of pain and referred site. Referred pain is often described as an ache, and is not accompanied by any other sensory abnormality.
Changes occur within the nervous system following any prolonged, noxious stimulus, as in the postoperative period where the surgical wound sends afferent neuronal information to the central nervous system for some time. Both ‘peripheral’ and ‘central’ sensitization occurs and alters the body’s response to further peripheral sensory input.
Moreover, surgery generates a catabolic state by changes in endocrine hormonal control, with increased secretion of catabolic hormones and decreased secretion of anabolic hormones. The results include: pain; nausea, vomiting and intestinal stasis; alterations in blood flow, coagulation and fibrinolysis; alterations in substrate metabolism; alterations in water and electrolyte handling by the body; and increased demands on the cardiovascular and respiratory systems.
New principles of pain management have been developed to improve analgesia after surgery in the light of our improved understanding of the processes involved. These include the recognition of the adverse effects of unrelieved pain, the need for an experienced and flexible approach to the problem by medical and nursing staff, and the necessity of informing the patient about the pain relief process. It is best to make plans for analgesia before surgery takes place, and this is especially important in short-stay surgery, when the patient is discharged home soon after the operation. The safety and efficacy of postoperative pain management may be improved by frequent assessment of the patient, good education of the staff and patients about the techniques and drugs used, preparation of protocols and guidelines for staff to follow, and regular evaluation by quality assurance programmes. These principles have been developed and incorporated into Acute or more recently Inpatient Pain Teams. Whilst acute postoperative pain normally settles over a relatively short time period, it is recognized that there is a significant incidence of chronic, severe pain after surgery including thoracotomy, mastectomy, limb amputation, and the less invasive operation of vasectomy. The aetiology of ongoing severe pain after surgery must lie in the pathophysiological changes, described above, which occur after tissue damage.
Since the publication of the Royal College of Surgeons’ and College of Anaesthetists’ report on postoperative pain in 1990, there has been a gradual and effective formation of Acute Pain Teams; more recently, these have become Inpatient Pain Teams. Their role is multidimensional, multidisciplinary and aimed at improving analgesia, maintaining safety, education and, in the long term, reducing morbidity, mortality and potentially bed days.
The teams are usually multidisciplinary but managed by senior nursing staff trained in Pain Management, who often have postgraduate qualifications in the management of pain. Guidance and assistance, which is both practical and educational, is provided by consultant anaesthetists trained in Pain Management, and usually anaesthetists in training. Further assistance at ward level is provided by a system of link nurses who have a special interest in Pain Management on their ward.
A patient’s experience of pain as illustrated in the previous Venn diagram (Fig. 41.1) is multidimensional, requiring input from a wide variety of professionals experienced in pain management.
Using patient-controlled analgesic apparatus (see below), it has been shown that there is marked interindividual variation in analgesic requirements. Thus, after open cholecystectomy, some patients may require no morphine within the first 24 h, whereas others may require as much as 120 mg. Unfortunately, there is no way of predicting in advance the extent of opioid requirements of an individual patient. In clinical practice, requirements are assessed on a trial-and-error basis; anaesthetists are therefore in an ideal position to be involved in prescribing postoperative analgesia, as they obtain a ‘feel’ for dose requirements during the management of anaesthesia.
In general, upper abdominal surgery produces greater pain than lower abdominal surgery, which in turn is associated with greater pain than peripheral surgery. This generalization is not entirely accurate; operations on the richly innervated digits may be associated with quite severe pain.
The type of pain may differ with different types of surgery. Operations on joints are associated with sharp pain. In contrast, abdominal surgery is associated with three types of pain: a continuous dull nauseating ache (which responds well to morphine); a spasmodic, intermittent cramping pain originating from motile viscera, which may respond to hyoscine (Buscopan); and a sharper pain induced by coughing and movement (which responds poorly to morphine). Pain associated with surgery on the digits may respond relatively poorly to opioids but well to non-steroidal anti-inflammatory drugs. There is increasing evidence that minimally invasive, laparoscopic surgery produces less-prolonged postoperative pain than do traditional techniques, but the pain may still be severe, especially in the immediate postoperative period.
Table 41.2 provides an approximate guide to the duration and severity of postoperative pain.
|Site of Operation
|Duration of Opioid Use (h)
|Severity of Pain (0–4)
|Up to 48
|Up to 36
|Up to 48
|Up to 24
|Up to 48
The analgesic requirements of males and females are identical for similar types of surgery, but variations in opioid receptor type (more OPRK1 [κ] receptors in females) may influence gender response to different opiates. However, there is a reduction in analgesic requirements with advancing age. Consequently, it is essential that the anaesthetist reduces the dosage of opioid drugs in elderly patients. For example, reasonable starting doses for intramuscular (or subcutaneous) postoperative morphine administration would be 7.5–12.5 mg for patients aged 20–39 years, but the dose should be reduced to only 2.5–5.0 mg for patients of 70–85 years of age.
The established anaesthetic practice of prescribing the potent opioid drugs on a milligram or microgram per body weight basis lacks scientific validity. There is no evidence to suggest that variations in body weight in the adult population affect opioid requirements significantly.
The patient’s personality affects pain perception and response to analgesic drugs. Thus, patients with low anxiety and low neuroticism scores on a personality scale exhibit less postoperative pain and require smaller doses of opioid than patients who rate highly on these scales. Patients with high scores may exhibit a higher incidence of postoperative chest complications (Table 41.3).
Personality – more pain if high neuroticism/extroversion
These psychological factors help to explain the efficacy of preoperative psychotherapy. Anxiety and postoperative analgesic requirements are reduced if the preoperative assessment by the anaesthetist includes an explanation of forthcoming perioperative events and details regarding the provision of pain relief.
After the intramuscular injection of an opioid, there is a three- to sevenfold difference among patients in the rate at which peak plasma concentration of the drug occurs and a two- to fivefold difference in the peak plasma concentration achieved. This is illustrated in Figure 41.4, which shows the mean change in plasma concentration after the first and second, and seventh and eighth injections. The variability in the plasma concentration is reflected by the large standard deviation of the mean. In addition, average concentrations increase after each of the first few injections; oscillation around a steady mean concentration does not occur until after approximately the fourth injection.
FIGURE 41.4 Blood concentrations of pethidine and pain score after surgery; a pain score of 0 indicates no pain. Doses of pethidine 100 mg have been given 4-hourly. Mean blood concentration of pethidine continues to rise for 24 h before a plateau is reached. Little pain relief is provided by the first dose. Even after 24 h, significant pain is present 3 and 4 h after each injection, as blood concentrations decline.
Although there are widespread pharmacokinetic variations among patients in response to administration of opioids, the major reason for variation in opioid sensitivity is pharmacodynamic, i.e. a difference in the inherent sensitivity of opioid receptors.
Using continuous infusions of opioids to achieve equilibrium between receptor drug concentration and plasma concentration, it is possible to define a steady-state plasma concentration of opioid at which analgesia is produced. This is termed the minimum effective analgesic concentration (MEAC); values of MEAC for the commonly available opioids are shown in Table 41.4. MEAC levels vary four- to fivefold among individual patients and are affected by age and differences in psychological profile.
|MEAC (ng mL–1)
Although pain measurement is subjective, it is a useful tool to aid assessment of the patient and the effect of treatment. Patient and doctor recall of the effectiveness of previous treatment is an unreliable way of monitoring response to therapy, and it is much better to record some type of repeatable measure. Recently, pain it has been named as the fifth vital sign and many hospitals encourage/require nursing and medical staff to record a ‘Pain Score’ along with temperature, respiratory rate, pulse rate and blood pressure. Some simple ways of measuring pain are listed below. It is important to involve the patient in the assessment process because individual responses to treatment are so variable. Pain should be assessed on movement, not simply when the patient is lying perfectly still in an attempt to minimize discomfort. A typical standard movement is to ask the patient to reach over to the other side of the bed while in a lying or semirecumbent position.
The Visual Analogue Scale (VAS) and the numeric rating scale (NRS) are validated in clinical use, but some patients find them more difficult to understand than scales using word descriptors. These scales are often understood better as a vertical score with increasing pain at the top and less pain at the bottom. The face pain scoring system used for children can be usefully applied to adults, particularly if their first language is not English.
A simple numerical scale is often used. For example 0–3, 0 representing no pain and 3 the worst pain ever. The advantage of this scale is that it is easy for the patient to remember the options and easy for nursing staff to record.
All of these scoring systems function best for the patient’s subjective feeling of pain intensity at a specific time i.e. present pain intensity. They can be used for pain in the last 24 h or over the last week, but are limited by the patient’s memory of pain and altered by other events in the intervening time period.