Regional anaesthetic techniques used for surgery may have positive respiratory and cardiovascular effects associated with reduced blood loss and excellent pain relief which can improve convalescence. Clearly, any technique that can be used for the surgical procedure will provide near perfect postoperative pain relief if it can be prolonged beyond the time of the surgery. There are many straightforward local anaesthetic techniques which can be continued into the postoperative period to provide effective pain relief. Most of these can be carried out with minimal risk to the patient and include local infiltration of incisions with long-acting local anaesthetics, blockade of peripheral nerves or plexuses and continuous block techniques peripherally or centrally. It is a mistake to expect 100% analgesia in every patient using a local anaesthetic technique alone as postoperative pain has many sources. The true place of local anaesthetic techniques is as part of a prepared plan for overall management that employs these techniques in conjunction with appropriate analgesic drugs. As pain is multifactorial in origin it is logical that management should consist of a combination of approaches in order to achieve the best results.

Spinal anaesthesia provides excellent analgesia for surgery in the lower half of the body and pain relief can last many hours after completion of the operation if long-acting drugs containing vasoconstrictors are used. Continuous analgesia using the spinal route has been tried but epidural analgesia is used more widely. The use of the epidural technique requires experienced practitioners and specific training for nursing staff in the postoperative management of patients. In addition, great care must be taken to maintain sterility if a continuous technique is to be used. Epidural catheters can be placed in either the cervical, thoracic or lumbar regions but lumbar epidural blockade is the most commonly used. Although continuous infusions of local anaesthetic may produce very effective analgesia, they may also produce undesirable side effects such as hypotension, sensory and motor block, nausea and urinary retention. Combination of local anaesthetic drugs with opioids given centrally may reduce some of these problems (see intrathecal and epidural opioids)

The following table lists suggested concentrations, "safe" maximum doses and block characteristics of the most widely available local anaesthetics. All doses assume healthy adult patients and maximum permissible dosage should be calculated on the basis of body weight, particularly in the case of children (see section on paediatric pain).

Intravascular injection of local anaesthetic drugs can produce serious or life-threatening effects at much smaller doses than the maxima quoted.

Local anaesthetic injections at any site can form part of balanced analgesia where a mixture of techniques provides pain relief. This has the advantage of decreasing the dosage of each drug needed and diminishing the likelihood of side effects. The small delay that results from performance of the blocks is outweighed by the benefit to the patient.

Toxicity

The most important factor in the prevention of local anaesthetic toxicity is the avoidance of intravascular injection. Careful aspiration is vital especially if the needle is moved. However, a negative aspiration test is not an absolute guarantee of correct needle placement. Inject slowly and watch carefully for signs of toxicity such as buzzing in the ears, a feeling of numbness in the face and lips and a feeling of muscle twitching. If toxicity is suspected the injection should be stopped and the patient's respiration and circulation assessed. Provided hypoxia is avoided little other treatment is needed. Cardiovascular depression should be treated by raising the legs, giving intravenous fluids and administering a vasopressor such as ephedrine. Major collapse requires full resuscitation. Convulsions may occur and need management of airway, breathing and circulation as well as control of the fitting with diazepam or thiopentone.

 
Non-opioid analgesics

The most commonly used analgesic agents throughout the world are drugs in this group such as aspirin, paracetamol and the non-steroidal anti-inflammatory drugs (NSAIDs). These are the main analgesic treatment for mild to moderate pain.

Aspirin is an effective analgesic and is widely available throughout the world. It is active orally within a short period as it is rapidly metabolised into salicylic acid which has analgesic and, probably, anti-inflammatory activity. Salicylic acid has a half life of about four hours at therapeutic doses. Excretion is dose dependent and high doses will be excreted more slowly. The length of action may be reduced if aspirin is given with antacids.

Aspirin has major gastrointestinal side effects and may cause nausea, sickness or gastrointestinal bleeding because of antiplatelet effects which are irreversible. For this latter reason the use of aspirin after surgery should be withheld if alternatives are available. Diflunisal and Choline salicylate are related compounds without this latter problem.

Aspirin has an epidemiological association with Reye's syndrome and should not normally be used to provide analgesia in children under the age of 12 years.

Doses range from a minimum of 300mg orally, 4 hourly, to a maximum of 8g, orally daily.

Paracetamol has analgesic and antipyretic properties but little anti-inflammatory effect. It is well absorbed orally and is metabolised almost entirely in the liver. It has few side effects in normal dosage and is widely used for the treatment of minor pain. It causes hepatotoxicity in overdosage by overloading the normal metabolic pathways with the formation of a toxic metabolite.

Doses range from a minimum of 500mg, orally, 4 hourly to a maximum of 4g, orally daily.

NSAIDs have both analgesic and anti-inflammatory actions. Their mechanism of action is predominantly by inhibition of prostaglandin synthesis by the enzyme cyclo-oxygenase which catalyses the conversion of arachidonic acid to the various prostaglandins that are the chief mediators of inflammation. All NSAIDs work in the same way and thus there is no point in giving more than one at a time. In addition, there is a widespread individual variation in response to these agents and thus there is no drug of choice. NSAIDs are, in general, more useful for superficial pain arising from the skin, buccal mucosa, joint surfaces and bone. They may be usefully combined with opioids due to their different modes of action.

The choice of a NSAID should be made on the basis of availability, cost and length of action (see Table 2). If pain is likely to be persistent over a long period of time it may be logical to choose an agent with a long half life and prolonged clinical effect. However, this group of drugs has a high incidence of side effects with prolonged use and caution should be exercised. All NSAIDs have antiplatelet activity leading to increased bleeding time. These drugs also inhibit prostaglandin synthesis in the gastric mucosa and may thus produce gastric bleeding as a side effect. Care should be exercised when using these drugs in patients with asthma or impaired renal function.

The following should be regarded as relative contraindications to the use of NSAIDs: Any history of peptic ulceration, gastrointestinal bleeding or bleeding diathesis; operations associated with high blood loss, asthma, moderate to severe renal impairment, dehydration and any history of hypersensitivity to NSAIDs or aspirin.

NSAIDs are available in a variety of formulations: tablet, injection, topical cream and suppository. The incidence of side effects and adverse reactions with an individual drug is similar regardless of the route of delivery.

Ibuprofen is the drug of choice if the oral route is available. It is clinically effective, cheap and has a lower side effects profile than other NSAIDs. Alternatives are diclofenac, naproxen, piroxicam, ketorolac, indomethacin and mefenamic acid. Where the oral route is not available the drug may be given by another route such as suppository, injection or topically. Aspirin and most of the NSAIDs are available as suppositories and are well absorbed.

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