41
Postoperative Pain
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.
FIGURE 41.1 The interrelationship between emotional, rational and physical components of pain. Perceived pain is represented by the area of intersection of all three components.
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.
Because the measurement of pain is difficult, it is seldom possible to adjust the dose of drug to match the extent of pain.
There are enormous variations in the extent of analgesic requirements depending upon the type of surgery, pharmacokinetic variability, pharmacodynamic variability, etc.
PHYSIOLOGY
Nociceptors
Primary Afferent Fibres
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).
Dorsal Horn Neurones
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).
Ascending Tracts and Supraspinal Systems
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.
Spinoreticular tract: terminates in the reticular nuclei in the brainstem.
Spinomesencephalic tract: terminates in the mesencephalic reticular formation and periaqueductal grey.
INPATIENT PAIN TEAMS
In broad terms, these teams have the following functions:
Standardization of orders of analgesic prescription and monitoring of patients
Education of nurses, doctors and staff allied to medicine who deal with patients in acute pain
Provision and monitoring of new or specialist analgesic methods
Advice to staff on managing acute pain
Constant evaluation of analgesic regimens e.g. efficacy, side-effects and safety
Secondary to a disease process
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.
CAUSES OF VARIATION IN ANALGESIC REQUIREMENTS
Site and Type of Surgery
Table 41.2 provides an approximate guide to the duration and severity of postoperative pain.
TABLE 41.2
Duration and Severity of Postoperative Pain
Site of Operation | Duration of Opioid Use (h) | Severity of Pain (0–4) |
Abdominal | ||
Upper | 48–72 | 3 |
Lower | Up to 48 | 2 |
Inguinal | Up to 36 | 1 |
Thoracotomy | 72–96 | 4 |
Limbs | 24–36 | 2 |
Faciomaxillary | Up to 48 | 2 |
Body wall | Up to 24 | 1 |
Perineal | 24–48 | 2 |
Hip surgery | Up to 48 | 2 |
Psychological Factors
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).
TABLE 41.3
Psychological Factors Which Influence Postoperative Analgesic Requirements
Personality – more pain if high neuroticism/extroversion
Social background
Culture
Motivation
Preoperative psychotherapy
Pharmacokinetic Variability
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.
Pharmacodynamic Variability
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.
TABLE 41.4
Minimum Effective Analgesic Concentration (MEAC) in Blood for a Number of Analgesic Drugs. (Note the Wide Range of Values for Each Agent)
Drug | MEAC (ng mL–1) |
Fentanyl | 1–3 |
Alfentanil | 100–300 |
Pethidine | 300–650 |
Morphine | 12–24 |
Methadone | 30–70 |
MEASUREMENT OF PAIN
Verbal rating scales tend to be short, easy to administer and easy to understand (e.g. ‘none, mild, moderate or severe’), but not as accurate as the methods below.
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 binary scale is one of the simplest to use. For example, ‘Is your pain at least 50% relieved?’, to which the patient answers yes or no.
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.
TREATMENT OF POSTOPERATIVE PAIN
Treatment should aim to fulfil the following criteria:
1. Easy to administer and understand
2. Easy to assess effectiveness and side-effects
3. Be administered using the gastrointestinal tract if possible
4. Easy to train nursing and medical staff to use, monitor, and know of side-effects