Kate Lawler, BSc (Hons) Nursing, RN, DipN.
Clinical Nurse Specialist in Pain Management, James Cook University Hospital, Middlesbrough
Freedom from pain should be a basic human right, yet lack of knowledge can limit our ability to achieve this for patients (Liebeskind and Melzack, 1987). A knowledge of the basic principles of analgesia, appropriate drugs and routes of administration enable ward teams to practise effectively in this respect.
The term analgesia - derived from the Greek word meaning painlessness - can be achieved in several ways. Drugs from each of the major analgesic groups can be used in combination, together with techniques such as transcutaneous nerve stimulation and acupuncture. This Factfile gives an overview of these groups of analgesics.
Balanced analgesia, combining one of each of the major groups of analgesics, is a key principle of effective analgesia, especially for acute or postoperative pain. The use of non-opioid drugs (paracetamol, non-steroidal anti-inflammatory drugs) alongside opioids affords improved analgesia while minimising opioid side-effects (Wall and Melzack, 1999; Sofaer, 1998). At James Cook University Hospital in Middlesbrough, analgesic guidelines are used that are based on an expanded analgesic ladder, complete with simple drug formulary and incorporating a pain score (Figure 1).
Major drug groups
These are active at receptor sites, inhibiting or modulating the transmission of pain signals. Some have a weak affinity for receptors, such as codeine, or a strong affinity, such as morphine. Receptor sites are widely distributed in the brain and spinal cord, and at the periphery. Morphine, as a pure opiate agonist, binds readily to all ‘-receptor sites. The antagonist naloxone, which ’unhooks’ morphine from receptor sites and renders it inactive, easily reverses its action. Untoward side-effects such as respiratory depression are resolved in this way. Naloxone is also useful in the management of opiate-induced urinary retention.
Morphine has long been the ‘gold-standard’ analgesic, having a straightforward mode of action that is well-recognised and understood. Morphine has potentially distressing side-effects such as nausea, constipation and pruritus - early recognition and prompt action is important.
Fentanyl has a short half-life, and does not have toxic metabolites, so it is useful for patients with renal impairment. As well as intravenous preparations, it is available for transdermal use as a patch. Release of the drug takes place over 72 hours so acute use is not recommended. The product is licensed for cancer pain only at present.
Pethidine is seen by some as not being particularly useful, having a short-lived effect, being less potent, and having a greater tendency to cause side-effects commonly associated with opioids, such as nausea, sedation, dysphoria and hallucinations.
Tramadol is a relatively new opioid with a dual action, having a ‘-receptor agonist action and an inhibiting effect on noradrenaline re-uptake at the synapse. It is not a controlled drug. Tramadol is perhaps less constipating than other opioids (Charlton, 1999) and might be regarded as ’double codeine’, having about one-fifth the potency of morphine when given orally, due to its greater bioavailability (Wall and Melzack, 1999).
Codeine is an alkaloid of morphine, having about one-tenth its potency (Wall and Melzack, 1999). Dihydrocodeine is a little different, having a slightly greater potency, and a higher incidence of adverse effects at high doses (Hawthorn and Redmond, 1999). A variety of doses can be used as compounds with paracetamol, and both are constipating.
Non-steroidal anti-inflammatory drugs
NSAIDs inhibit the production of cyclo-oxygenase, used in prostaglandin synthesis. Clinical decision-making related to the use of NSAIDs is therefore more complex than that of opioids, and the Royal College of Anaesthetists has produced guidance (1999).
As prostaglandin is a key sensitising agent in the inflammatory process, excellent pain relief is afforded by its reduction. However, prostaglandin has several other important functions. Prostaglandins are necessary for the inhibition of gastric acid secretion and vasodilator activity, controlling blood flow through the renal medulla. NSAID use is therefore contraindicated in patients with gastric irritation or renal impairment.
Herxheimer (1998) reports increased incidence of gastric bleeding attributable to NSAID use and suggests promoting their use for symptomatic relief only.
The new generation of COX II selective anti-inflammatory drugs (or coxibs) inhibit only one of the cyclo-oxygenase enzymes. This drug group does not interrupt the inhibition of gastric acid secretion, and is recommended where NSAID use is clinically indicated in patients who are at risk of gastric irritation (National Institute for Clinical Excellence, 2001).
Platelet aggregation is inhibited, and caution is advised where there is any tendency to bleed. Some NSAIDs slow the metabolism of warfarin, so monitoring of clotting time is necessary, or use may be precluded. NSAIDs interact adversely with some other drugs, such as antihypertensives, potassium-sparing diuretics, and ACE inhibitors.
Confusion over the use of NSAIDs for patients with asthma arises from the fact that, for some, aspirin sensitivity exacerbates the condition. This applies to only 5-10% of middle-aged patients; the majority of people with asthma can take NSAIDs with no adverse effects, but caution is advised (RCA, 1998).
OTHER MEANS OF ACHIEVING PAIN RELIEF
- Local anaesthetics alter the action potential of nerves, thus blocking transmission. Their use in regional blocks in A&E, and epidural infusions postoperatively, are a mainstay of acute pain management (McLure and Wildsmith, 1991)
- Entonox for ‘incident’ pain is cheap, safe, and virtually free from side-effects. It should not be used for patients who may have any sealed, air-filled space (such as pneumothorax, colonic obstruction) as the nitrous oxide will permeate the space and expand within it (BOC Gases (Medical), 2001)
- Transcutaneous nerve stimulation is effective for somatic pain, and works by ‘pain-gating’. Stimulation of A beta fibres that are touch sensitive overrides some of the painful stimuli from A delta fibres. The principle is the same as rubbing a painful area (Bowsher, 1993)
PRINCIPLES OF ANALGESIA AND NURSING RESPONSIBILITIES
- Pain assessment is fundamental to planning and evaluating analgesia
- Informing patients of the importance of regular analgesia, and adherence with this advice, allows attainment and maintenance of the therapeutic range of drugs used
- Patient education on the importance of pain management to recovery encourages adherence
- Maintaining a working knowledge of commonly used analgesics contributes to effective practice
FACTORS TO CONSIDER
- Age of the patient: ‘start low, go slow’ is a useful maxim for elderly people, and for paediatric patients doses are calculated according to weight (Clayton and Stock, 2001; Downie, 1999)
- Other health problems can affect drug absorption or metabolism (such as liver or renal problems) (Clayton and Stock, 2001; Downie, 1999)
- Different routes of administration affect the bioavailability of drugs (Clayton and Stock, 2001; Downie, 1999)
- Patient preference should be taken into account when considering suitable routes of administration
- Price, availability, and product licence.
Research into the use of nabilone for chronic pain is growing. Cannabis has been recognised for its analgesic properties since the mid-1800s, and Queen Victoria is said to have used it for dysmenorrhoea (Haigh, 2001). Interestingly, cannabinoids are travelling a similar historical path to morphine. Both were originally a herbal remedy, have naturally occurring receptor sites in the body, and both have a reputation for abuse (Wall, 2000). Whether or not cannabinoids will follow morphine in becoming a ‘gold standard’ for the future remains to be seen.
- Medicines Control Agency website www.mca.gov.uk/index.htm
- Oxford Pain Internet site www.jr2.ox.ac.uk: 80/Bandolier
- McQuay, H., Moore, A. (1998) An Evidence-based Resource for Pain Relief. Oxford: Oxford University Press.
Paracetamol, when given regularly, has an efficacy that is underestimated, but it is useful to emphasise to patients that it takes about four doses to reach a therapeutic range. The mode of action is uncertain, but some inhibitory effect on prostaglandin synthesis is reported (Hawthorn and Redmond, 1999). Irreversible liver damage occurs at high doses (10-15g) and the risk of exceeding the safe dose of up to 4g a day must be emphasised.
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