approaches to management of acute pain 1

7/29/2019 Approaches to Management of Acute Pain 1

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APPROACHES TO MANAGEMENT OF ACUTE PAINIntroductionPain has been officially described by the International Association for the Study of Pain (IASP) as "an

unpleasant sensory and emotional experience associated with actual or potential tissue damage, or

described in terms of such damage". It is important to distinguish betweenpain perception

thresholdwhich is relatively constant (approximately 44-45 deg. C applied to the skin is recognised as

painful by most subjects) and thepain tolerance thresholdwhich is the maximum amount of pain that

the subject is prepared to tolerate and is very variable. It is the latter that is most important in the

patient's perception and attitude to pain, and thus whether he or she will seek treatment.

Pain pathwaysThe experience of pain requires four distinct stages: transduction, transmission, modulation and

perception.

Transduction : the sensation of pain is transduced at the body surface by three types of nerve endings

capable of nociception; mechanoreceptors, thermoreceptors and polymodal nociceptors. The latter are

the most important, and as their name suggests they may respond to any stimulus provided it is strong

enough to produce the sensation of pain, including traumatic and surgically induced injury. Following the

'acute pain' of the skin incision ('fast pain'), inflammation occurs in the injured tissue, a process which is

induced by substance P (SP) and autacoids (e.g. histamine, bradykinin and 5-hydroxytryptamine)

release from damaged cells. The nociceptive nerve endings continue to be stimulated by these

autacoids ('slow pain') a process which is sensitised by prostaglandins. Non-steroidal anti-inflammatory

drugs (NSAIDS) work by inhibition of prostaglandin production.

Transmission : impulses from the nociceptors travel in two distinct fibres in sensory nerves to thedorsal horn of the spinal cord. The acute, sharp pain is transmitted in the small, myelinated A delta ('fastpain') fibres, whilst the prolonged pain due to inflammation and tissue damage is transmitted byunmyelinated C fibres ('slow pain'). A delta fibres exist on the surface of the body (including mouth andanus) whilst C fibres can originate from all parts of the body outside the central nervous system (CNS)itself. The latter also transmit visceral pain sensation (e.g. from nociceptors in the mesentery and heart)in fibres which run with other C fibres of postganglionic sympathetic nerves.The biological value of the localised 'fast pain' is to induce a localised reflex flexion withdrawal of the

affected part away from the injury causing stimulus, thus limiting tissue damage. Anaesthetists spend

most of their time ablating this response to surgery with general anaesthetics and muscle relaxants or


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regional blockade with local anaesthetics! C fibre pain is not well localised and is often 'referred' to

somatic afferents on the body surface (see later). Being associated with pain and swelling, immobility is

induced which forces rest and promotes healing of the affected part.

In the spinal cord A delta and C fibres synapse in dorsal horn laminae such as lamina II, the substantia

gelatinosa (SG), decussate and then pass up the contra lateral spinothalamic tract to the thalamus. The

synaptic transmitter involved in many C fibres (not A delta) is SP whose release can be presynaptically

inhibited by enkephalins (see later). Thus, opioids have relatively little effect on A delta pain (except in

overdose where central effects predominate) but act very specifically to ease the pain associated with C

fibre activity.

Modulation : opioids interact with the two important pain modulating pathways present in the CNS:

A centrally activated, descending pathway, the descending anti-nociceptive tract (DANT) originates inthe periventricular (PVG) and periaqueductal grey areas (PAG) of the hypothalamus and midbrain andterminates in laminae I, II (SG) and V where pre-synaptic inhibition of SP production is mediated by therelease of enkephalin, an endogenous opioid peptide. Opioid receptors are concentrated in both thePVG, PAG and the spinal laminae. Electrical stimulation or opioids applied in the location of the PAGresults in increased activity in the DANT and prolonged analgesia in rats and humans. This central paininhibitory pathway also mediates the influence that changes in mentation, mood, anxiety state and so oncan have on the perception of pain. The presence of naturally occurring morphine-like neurotransmitters(enkephalins, endorphins and dynorphins) and receptors in the central nervous system underlines the

central actions of opioid analgesics.

A spinal mechanism whereby opioids, especially when applied directly to the spinal laminae by

intrathecal or epidural injection, result in inhibition of SP release directly.

In addition, mechanoreceptor input from large A beta fibres can also inhibit pain transmission and thus

provide an explanation for analgesia induced by 'rubbing' the affected part and for techniques such as

transcutaneous electrical nerve stimulation (TENS). The latter involves placing pads over the nerve

distribution of the pain and applying a small electric current through the pads. This sets up electrical

activity in the large fibres which cause inhibition of pain transmission through the posterior horn.

Subsidiary spinal inhibitory pathways also involve catecholamines and this may explain the

augmentation of analgesia achieved by concomitant use of the tricyclic antidepressants in both acute

and chronic pain.

The 'gate theory' of pain originally proposed by Melzack and Wall in 1965 ties all these facts together by

proposing that pain transmission through the spinal laminae is not an 'all or none' phenomenon but can


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be modulated centrally and locally. Both A delta and C fibre pathways can be blocked by local

anaesthetics (see later).

Perception : of pain occurs in the thalamus and sensory cortex. The spinothalamic (STT) tracts

segregate into two pathways as they approach the thalamus. The medial STT projects to the medial

thalamus and reticular formation and is concerned with the autonomic and emotional response to pain.

The axons of the lateral STT arise from cells in laminae I to V which have small receptive fields. Having

synapsed in the lateral thalamus they project to the sensory cortex in a highly organised manner which

corresponds to distinct topographical parts of the body. This allows sensory discrimination and

localisation of pain, especially from A delta fibres.

Pathophysiology of acute clinical (surgical) pain

The main distinction between acute clinical pain and experimental pain is that the former leads to:

Tissue damage leading to inflammatory changes (sensitising soup)Nerve damage leading to neuropathic pain

This can leads to peripheral AND central sensitisation of pain

Mechanisms of peripheral and central sensitisation

Peripheral: a sensitising soup is released by tissue damage leading to nociceptor sensitisation byinvolvement of inflammatory mediators and other factors such as nerve growth factor and neuropeptidesand increased sympathetic nervous activity in the affected area

Central: an increased excitability of neurones in the spinal cord leading to increased duration and area

of response due to abnormal connections between A-beta and A delta and C fibres due to neuro-

anatomical re-organisation following nerve injury. 'Wind-up' is a phenomenon occuring in this situation

where repetitive stimuli from normal or hyperalgesic cutaneous areas give rise to pain.

Clinical implications of peripheral and central sensitisation

The result is:


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Primary hyperalgesia: an exaggerated responsiveness to noxious stimuli in the injured area Secondary hyperalgesia: a spread of hyper-responsiveness to surrounding non-injured tissue

Allodynia: a reduction in the intensity of stimuli necessary to induce pain so that stimuli not normally

producing pain (e.g. touch or pressure) now do so

Prolonged neuropathic pain unresponsive to opioids: although neuropathic pain is often associated

with pain occuring weeks or even months after surgery (e.g. post thoracotomy pain due to intercostal

nerve damage and neuroma formation) it may occur within hours or days and is particularly difficult to

treat. Anti-depressants, local anaesthetics and NSAIDs may be more effective than opioids

Pre-emptive analgesia: is often employed in an attempt to reduce these effects by local anaestheticinfiltration before skin incision, long acting opioids prior to surgery and NSAIDs. There is as yet very littlefirm evidence of a definite benficial effect in humans.Update on opioid receptors

Opioid analgesic receptor subtypes are divided into mu, delta and kappa

Endogenous ligands: for mu, delta and kappa receptors are beta endorphin, enkephalins and

dynorphin respectively.

Exogenous ligands: are morphine, DPDPE and enadoline (the latter two are not used clinically

Exogenous antagonist: is naloxone

Opioid receptor mechanisms: these are G protein (Gi) coupled. Activation by opioids reduces

neurotransmission by presynaptic inhibition of transmitter (e.g. substance P) release. The receptor

effects result in increased K+ conductance (hyperpolarised), inactivation of calcium channels (N type)

and inhibition of adenylyl cyclase

Analgesics

These are substances which raise the pain threshold, but unlike local anaesthetics they do not

specifically block sensory nerves . Analgesics may be divided into 4 main groups, the non-steroidal anti-

inflammatory drugs (NSAIDs), the opioid agonists and high efficacy partial agonists, the opioid agonist-


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antagonists and miscellaneous drugs whose site of action is unknown.

It should be remembered that these drugs, as a rule, do not attempt to cure the condition causing the

pain, but are purely symptomatic, e.g. following surgery, where natural healing processes eventually

remove the cause of pain. Other drugs may produce analgesia by actually removing the pain causing

condition, e.g. steroids in rheumatoid arthritis, nitrates in angina, carbamazepine in trigeminal neuralgia

and ergot in migraine. In addition, general anaesthetics like nitrous oxide and methoxyflurane can

achieve analgesia without loss of consciousness

Non-steroidal anti-inflammatory drugs (NSAIDs)

These include aspirin, paracetamol (although not a peripherally acting anti-inflammatory drug), and the

newer drugs such as diclofenac, ketorolac and piroxicam. Their analgesic action is complimentary to

opioids, being particularly effective in pain produced from bony injury, inflammation or movement, an

area where opioids are relatively ineffective. However, unlike the opioids, C fibre pain arising from

hollow viscera is usually not relieved. Given the fact that perioperative pain is often a combination of

these two broad categories it is not surprising that NSAIDs are now combined with opioids (and local

anaesthetics) in perioperative pain management.

Mechanism of action

Tissue injury results in activation of lipooxygenase and breakdown of cell wall phospholipid to

arachadonic acid. Release of autacoids such as histamine and bradykinin initiate inflammation and

stimulate receptors (such as bradykinin receptors) on nociceptive nerve endings causing pain, a

process which is sensitised by prostaglandins (see earlier). NSAIDs limit the conversion of arachidonic

acid to the unstable cyclic endoperoxide PGG2, which is an intermediate in prostaglandin production, by

inhibition of the enzyme cyclo-oxygenase (COX). COX exists in two forms, the constitutive form (COX1)

and the inducibe form, COX2 which is increased as a result of injury (see later).

Thus, inhibition of prostaglandin production by NSAIDs results in their analgesic action (see above). The

reaction with cyclo-oxygenase is dependent on the NSAID involved, aspirin having a particularly

profound effect as it irreversibly acetylates the enzyme. Other drugs such as diclofenac act as

competitive blockers. Formation of nerve growth factor (NGF) as a result of tissue injury also causes


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sensitisation of nociceptors which can be inhibited by NGF inhibitors. A promising area of research!

Side effects and contraindications of NSAIDs

These side effects do not apply to paracetamol.

Bleeding tendency: platelets have a high concentration of cyclo-oxygenase, necessary for thegeneration of the potent aggregating and constricting agent, thromboxane. Once released from thebone marrow they are unable to re-generate new enzyme, so aspirin (but not other NSAIDs), even inthe small doses used in myocardial infarction prophylaxis, inhibits the enzyme for the life of the platelet(7-11 days). As this can result in a pronounced bleeding tendency following major surgery, aspirinshould be stopped for 2 to 3 weeks prior to major surgery to allow for the production of adequatenumbers of unaffected platelets. A mild increase in bleeding time has been noted with diclofenac andketorolac when used intraoperatively but an overt bleeding tendency is unusual.

Gastric and intestinal ulceration and bleeding: inhibition of the cytoprotective gastric prostaglandins

PGI2 and PGE2 results in decreased gastric secretion of protective, acid inhibiting mucous which

renders the stomach more susceptible to injury. This can lead to occult blood loss and anaemia.

Although this is more of a problem in chronic usage, they are generally contraindicated in patients with

known gastric or duodenal ulcers. In selected patients with ulcers, these drugs may be combined with a

PGE1 analogue, misoprostol, which markedly reduces the incidence of ulcer formation and allows

continuation of NSAID use in those patients in whom it is essential (eg. severe rheumatoid arthritis).

Renal impairment and failure: vasodilator prostaglandins are responsible for optimising the amount

and distribution of renal blood flow in hypovolaemia, chronic renal and heart failure and ascites by

opposing the effects of vasoconstrictors such as angiotensin and noradrenaline. Inhibition by both

COX1 and 2 and specific COX 2 inhibitor NSAIDs can result in a severe decrease in renal function and

may even precipitate renal failure. All NSAIDs should be avoided in these patients and in those in whom

major bleeding cannot always be avoided, e.g. major aortic surgery. Ability to excrete a water and

sodium load may also be affected resulting in a decreased urine output in the perioperative period.

'Allergic' reactions: ranging from rhinorrhea to full blown asthma or anaphylaxis occur in a small

percentage of patients. Although bronchospasm may be due to an imbalance in production of dilator

versus constrictor prostaglandins, the actual mechanism is unknown. Affected individuals may react to

any NSAID. Up to 25% of atopic or asthmatic individuals are susceptible, so these drugs should

probably be avoided in this group of patients unless they have known untoward exposure to the drug


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being used in the doses needed for postoperative analgesia.

Use and doses used in perioperative pain

Apart from the contraindications above, NSAIDs can be employed throughout the perioperative period

alongside opioids and local anaesthetics. They are particularly useful as adjuncts in day surgery where

long acting opioids should be avoided due to side effects such as nausea and vomiting which may delay

discharge for the day surgery unit. In the immediate postoperative period, gastric emptying is delayed

so either a large loading dose is given pre- or intra-operatively to cover this period (e.g. diclofenac 100

mg. by suppository in the adult (12.5 - 25 mg. in the child, an unlicensed use) or parenterally, e.g. 1 mg.

kg-1 diclofenac i.v. diluted in 100 mls of normal saline or 0.15 mg. kg-1 ketorolac i.m.) Subsequently the

oral preparation can be given e.g. (in the 60 kg. adult) 50 mg. diclofenac b.d. or ketorolac 30 mg. qds.

In less severe pain, paracetamol is the drug of choice as it does not have the side effects of other drugs

in this group, e.g. gastric irritation and bleeding or platelet dysfunction (but see Chapter 21). It is given in

a dose of two tablets (1g) 6 hourly. In children it may be given as an elixir of 120 mg. in 5 mls. in a dose

of 15 mg.kg-1 b.d. The availability of paracetamol in an i.v. preparation has considerably increased the

use of this drug as a perioperative analgesic. 1 g i.v. (in the adult) is usually administered about 20

minutes before the end of surgery.

COX 1 and COX 2

There are two distinct types of cyclo-oxygenase, types 1 and 2 (COX 1 and COX 2). COX 1 is the

constitutive enzyme which is normally present in tissues and results in the formation of prostaglandins

which are protective (as seen above). COX 2 is the inducible form which is produced following injury

and is responsible for the formation of abnormal amounts of prostaglandins which sensitise nociceptors.

Specific COX 2 inhibitors have been developed which it was hoped would relieve pain without the side

effects of COX 1 inhibition as described above. The evidence available so far suggests that the promise

of substantially approved safety has yet to be convincingly demonstrated in practice (DTB November

2000 Vol 38 no 11 p. 84). They are also much more expensive than the non-selective NSAIDs such as

ibuprofen. It should also be remembered that since these selective COX 2 inhibitors drugs lack anti-

platelet activity they do not provide protection against ischaemic cardiovascular events (Current

Problems in pharmacovigilance Volume 27 February 2001 p.5). In addition, COX 2 is an important


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enzyme in maintaining optimum renal blood flow during hypovolaemia. Clearly a specific COX 2 inhibitor

would be no better than a non-specifice COX and 2 inhibitor in this situation. However, they do have a

lesser effect on bleeding tendency and gastro-intestinal side effects (see below).

Opioid agonists and high efficacy partial agonists

History of Pain: The use of opium

The use of opium dates from time immemorial and is well described in ancient civilisations. The first

reference to its use was found on a Sumerian tablet (4000 BC). In Egypt, the juice ofPapaver

somniferum (the white poppy) was recognised as an efficacious analgesic and indeed poppy fields

existed around Thebes (from which the name thebaine is derived). In Ancient Greece it is interesting to

note that Morpheus was Ovids God of Dreams which is where the name morphine is derived. Juice of

the white poppy was called Opion. The first written reference to Opium was by Theophrastus in the 3rd

century BC. In Ancient Rome, the specific use of opium as a pain killer and sedative was known and

described by Galen and Dioscorides. Much of this knowledge was lost during the dark ages and opium

was repopularised in Europe by Paracelsus (1493-51). Thomas Sydenham introduced 'laudanum' or

'tincture of opium' in the 17th century and he accurately described laudanum as 'a highly efficacious

(and toxic) mixture of opiate alkaloids'. It's widespread use in the American Civil War led to a major

spread of addiction. The development of the syringe by Pravaz in 1853 and the hollow needle by Wood

in 1855 allowed it to be employed parenterally. The increased efficacy of giving these compounds in this

way (bypassing the problems with absorption, decreased gut motility, high first pass metabolism and

nausea and vomiting) lead to an increased incidence of side effects.

The alkaloid content of opium

Naturally occurring opium consists of the following:

Morphine 9 - 17%Noscapine (Narcotine) 2 - 9%

Codeine 0.3 - 4%


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Thebaine 0.1 - 0.8%

Papaverine 0.5 - 1%

N.B. Only morphine and codeine should strictly speaking be referred to as opiates (naturally occurring in

opium). Other analgesics in the same class such as diamorphine and pethidine should be referred to as

opioids. Although morphine can be synthesised it is still obtained from opium commercially.

Present day opioids

These include morphine, buprenorphine, diamorphine, oxycodone and pethidine and the intra-operative

analgesics fentanyl, alfentanil and remifentanil.

Actions

They work both spinally and in areas of the brain stem rich in naturally occurring opioid peptides (see

above). Beneficial actions, apart from analgesia, include euphoria and anxiolysis.

Problems with conventional opioids

They have many side effects which limit their effectiveness for the treatment of severe pain, e.g.

Respiratory depression which is usually dose related and means that the drug cannot be prescribed

regularly (except in an HDU or ICU). In addition, although respiratory depression is usually maximal

within 10 - 15 minutes of administration of an opioid by the i.v. route, it may take up to 24 hours when

the same drug (e.g. morphine) is given spinally.

Short duration of action which means that these drugs are relatively ineffective when given by

intermittent i.m. injection (see text for exceptions)

Nausea and vomiting, especially in the ambulant patient and this limits their use for day surgery

Tolerance and addiction potential leading to proscribed use

Sphincter of Oddi spasm, especially with morphine and its congeners makes them unsuitable for


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alleviating the pain of biliary colic (see text for exceptions)

Constipation and adynamic ileus is a serious problem in the chronic pain patient and the ICU patient

on i.v. opioids for sedation

Side effects not directly associated with opioid activity such as histamine release (morphine and

pethidine) and chest wall stiffness (e.g. fentanyl)

All these result in limitations being imposed on the amount of analgesics that patients are prescribed

and actually receive. This is justifiable when related to their tendency to cause respiratory depression

but not as regards their addictive potential in acute usage. However, the latter does impose logistic

restrictions on drug administration (vide infra). In particular, the great variability in pain perceived and

effectiveness of opioids between patients requires that they are conventionally prescribed 'on demand'

(see later). Patients have to be told that they must ask for the drug when they are in pain. Most other

drugs are prescribed regularly. Nausea and vomiting are very common side effects which means that

they are usually prescribed with antiemetics. There is little evidence to suggest that one opioid is

superior to another in this respect.

N.B. as a partial agonist with a long duration of action, buprenorphine has a different profile to other

opioids . Although respiratory depression is not so severe and there is a lessened tendency to cause

apnoea, a bolus dose of buprenorphine still potentiates both the respiratory

depression andanalgesia produced by other opioids (except when these have been given in overdose

where most of the effects are reversed) and respiratory depression of other sedative drugs such as

benzodiazepines. It has a long duration of action due to enhanced receptor avidity which means that

analgesia is much less dependent on maintenance of a steady concentration of buprenorphine in the

biophase. As a result of enhanced receptor avidity it's effects are not consistently reversed by

naloxone which may be required in high dosage. However, doxapram is usually effective although very

infrequently required.Histamine release is only moderate and not clinically significant. Spasm of the

sphincter of Oddi does not occurso it can be used in biliary colic. Constipation is not a feature and

this is a significant advantage when used for chronic pain. It is much less likely to cause

addiction but can substitute for other opioids such as diamorphine in mild to moderate drug abusers.

For this reason it's use is subject to schedule 3 control under the 'Misuse of Drugs Act' (CD). Although it

is usually supplied with other more addictive (schedule 2) opioids it is not legally subject to the same


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regulations, e.g. requiring documentation of it's use. As with other opioids, buprenorphine

causes nausea and vomiting although there is no evidence in the literature to suggest that it is any

worse than other long acting opioids in this respect. It is slowly but completely absorbed by

the sublingual (s.l.) route throughout the perioperative period. This removes the necessity for repeated

(painful) i.m. injections. Buprenorphine can be used alongside the other agonists such as papaveretum .

Administration of Opioids for postoperative pain

There is evidence that prescribing habits result in patients receiving less analgesia than they need. But,

patients are usually uncomplaining, so just because they don't make a fuss does not mean that they

aren't in pain! Various methods of drug delivery are available in the postoperative period. It is important

to note that opioids are mainly effective against steady, dull pain and are less consistently effective

against the pain on movement and coughing.

Nurse administered

P.r.n. (pro re nata, as needed) prescription of an i.m. opioid. This means that the drug will be

administered according to patient demands but within the limits of the prescription.

This may seem simple to the prescriber but in practice it may be fraught with difficulties. Unfortunately,

to give a controlled drug on a general ward may take up to 30 minutes involving:

complicated protocols and extra staff due to controlled drug regulations

the use of syringes and needles (adding to expense, time and patient discomfort)

if the drug is short acting (e.g. 2 hours for pethidine) then up to 25% of the nurses time may be

engaged in administering postoperative pain relieving drugs.

the onset time of an i.m. drug (30 - 60 minutes) may be too slow to catch up with the rapidly

declining analgesia of a short acting intraoperative opioid (vide infra)

The result is that the patient ends up in pain! In addition, doctors often under prescribe analgesics and

nurses reduce these doses still further.


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Pharmacokinetic and pharmacodynamic principles dictate that it is impossible to expect 3 to 4 hrly.

p.r.n. i.m. regimes of parenteral opioids (except buprenorphine and methadone) to produce consistently

good postoperative analgesia. A better solution is to use either a long acting (due to

pharmacodynamics) analgesic such as buprenorphine which consistently gives equivalent (or better)

analgesia to controlled drugs with less administrative effort or to titrate i.v. a long-acting (due to

pharmacokinetics) analgesic such as methadone.

Although oral absorption of opioids on demand is feasible following surgery, delay in gastric emptying

and ileus may result in loss of effectiveness initially, followed by overdosage when motility returns. S.l.

administration of buprenorphine avoids these difficulties but is unlikely to compensate for the rapid

decline in analgesia in the immediate perioperative period when fentanyl has been used as the primary

intraoperative analgesic. Thus, slow absorption from the s.l. route may fail to produce adequate

analgesia unless it has been preceded by a loading dose of a long-acting opioid. Doses of opioid

agonists used (70 kg patient) are shown in table 13.1

Regular pre-emptive opioids

Excellent analgesia is obtainable by regular(pre-emptive) use of opioids in the postoperative period.

This has included a variety of techniques and agents, such as by i.m. injection, s.l. or by nurse

controlled i.v. infusion. The results are good, with both s.l. buprenorphine and i.m. morphine proving

equally satisfactory to patient controlled analgesic (PCA, see next section). However, these techniques,

particularly i.v. infusion, have the inherent danger of respiratory depression and must only be carried

out in a high-dependency environment with nurse/patient ratios much higher than is to be found on a

normal ward.

Patient controlled analgesia (PCA)

This was first introduced in 1970 into obstetric practice where the patient simply controlled the

administration of i.v. pethidine by operation of a spring loaded clamp on an infusion set! In a way, it

mimics the use of the PCA 50:50 nitrous oxide/oxygen gas mixture (Entonox) (see later). The rationale

of PCA is that an improvement in analgesic efficacy is obtained by allowing the patient (rather than the

nurse) to manage analgesic requirements. Although the availability of custom made apparatus, with in-

built safety features, have increased the popularity and applicability of the technique, the cost of the


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apparatus remains prohibitive. The normal route of administration is i.v., but the i.m., s.c. and epidural

routes are also used. The patient must be given clear instructions on how to use the apparatus for

maximal benefit. To avoid inadvertent toxicity, the following parameters must be correctly pre-set by the

operator:

Dose and concentration of opioid used is set-up in the machine, this allows calculation and storage of

doses administered to the patient.

Demand dose administered (e.g. 15 mg. for i.v. pethidine)

Lockout interval is the interval from an administered dose until the next dose is given by the machine,

despite patient requests. It is designed to allow enough time for the maximal effect of the drug to be

reached, usually about 10 minutes if a drug such as diamorphine or oxycodone is given i.v. It also

prevents repeated doses at short intervals and thus limits toxicity but also the amount of drug the patient

can receive.

Maximum hourly or 4 hourly dose (e.g. 50 mg. hr-1 for pethidine)Background infusion of opioid, if required. This may be fixed or interactively adjusted according to the

frequency of patient demands

Much effort has been expended on coming up with suitable doses of analgesics and the time intervals

of administration. For maximum efficacy a loading dose of the appropriate analgesic must be

administered before allowing the machine to take over as these regimes only cater for maintenance

requirements. Once the apparatus is satisfactorily set-up, the great advantage claimed for PCA over

techniques of postoperative analgesia, such as opioids by infusion, regular administration and by spinal

techniques, is absence of need to monitor the patient in a high dependency environment, always

providing that a background infusion is not used, .

Although the technique has proved acceptable, poor results for PCA can occur and may be explained

by:

Failure to adequately explain the apparatus to the patient


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Failure to give a loading dose

The patient chooses to have pain rather than suffer the adverse effects of opioids, e.g. nausea

and excessive sedation

Excessive pain following a period of sleep during which there has been no administration of

analgesic. During the 'catching up' period, the patient will be in pain.

The latter problem is partially addressed by utilising apparatus which has the ability to provide a

background infusion of opioid, but this is not entirely effective and leads to increased opioid

consumption and need for high dependency care. Thus, although finding favour with some, PCA

techniques have not come into widespread use for two reasons:

The expense and complexity of the apparatus

The introduction, at about the same time, of spinal opioid techniques which were 'automatically'

assumed to be more efficacious.

It is only in the last few years that direct comparison has been made between PCA and epidural opioids,

the former being slightly less efficacious but equally, if not more, acceptable to the patient (see later). Is

PCA the ideal way to administer opioids for postoperative pain relief? The very narrow dividing line

between serious respiratory depression, good analgesia and no pain relief at all, suggests that it might

be. However, it must be said that regular sublingual buprenorphine or i.m. morphine give equivalent

analgesia to i.v. pethidine or fentanyl via PCA.

Spinal opioids

The discovery of endogenous opioid receptors and ligands in the brain and spinal cord (see above) led

to the feasibility of their use spinally. Intrathecally applied narcotics produced a long lasting elevation in

the nociceptive threshold in animals. Use of this technique in man followed in 1979. At the outset, it is

most important to establish the rationale for the spinal administration of opioids. There must be firm

evidence that this route is preferable, in terms of analgesic effectiveness, without an increase in side

effects such as respiratory depression, when compared with the simpler and less invasive enteral or

parenteral administration. If questioned, patients may not wish to accept increased risks for the possible
http://www.dr-green.co.uk/teaching%20topics/Chapter%2014%20Regional%20analgesia.htm#Epiduralhttp://www.dr-green.co.uk/teaching%20topics/Chapter%2014%20Regional%20analgesia.htm#Epiduralhttp://www.dr-green.co.uk/teaching%20topics/Chapter%2014%20Regional%20analgesia.htm#Epidural


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benefit of improved analgesia . It should also be remembered that none of the opioids are specifically

licensed for use by the epidural or intrathecal route.

Crucial to the subsequent popularity of spinal opioids was the observation that the effects were primarily

limited to pain perception, selective spinal analgesia, with little effect on motor, sensory (e.g.

touch and pin prick) and autonomic function as occur with LA. Adverse effects of spinal and parenteral

opioids are similar and hopes that respiratory depression and nausea and vomiting would be less have

not been fulfilled. This is due to the fact that, by the epidural route, systemic uptake of both morphine

and more lipophilic agents such as fentanyl, is as great as if given i.m. or i.v., so blood levels are similar.

In fact, depending on the opioid used, side effects may be more severe, especially when using

morphine intrathecally. In addition, pruritus and urinary retention are more common.

Intrathecal opioids

Although large individual series of intrathecal opioids have been reported, there has been little work

comparing their use with more conventional techniques in randomised, double-blind, prospective

studies. Diamorphine and low dose intrathecal morphine [0.1 to 0.3 mg.] are effective and may be less

prone to respiratory depression than larger dose techniques. This route is much less popular than the

epidural route.

Epidural opioids

Morphine has a slow onset of action and a long duration of action. The contribution from systemic

uptake is modest. Diamorphine is more lipophilic than morphine with a faster onset of action and a

large [early] contribution from systemic uptake. Duration of action is less than with morphine [although

its conversion to morphine makes it longer than would be expected from the physical

properties].Fentanyl has a very rapid onset of action, mainly due to systemic absorption. Recent

evidence suggests that it is as effective by the i.v. route so the use of the epidural route has been

questioned although it is still commonly used by this route.

Monitoring

Whenever spinal administration of opioids are used, the patient is at increased risk from opioid induced

respiratory depression, thus diligent monitoring must be employed, e.g.

Frequent measurement of blood pressure, pulse rate, respiration and pulse oximetry (SpO2). Clear


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instructions must be available indicating who to call if defined limits are exceeded (e.g. systolic BP < 80,respiratory frequency < 8 breaths per minute, SpO2 < 90%).What to do in an emergency, e.g. use of naloxone and oxygen if respiratory embarrassment occurs;

use of fluids, ephedrine and tipping the bed if hypotension occurs.

Who to call if there is inadequate pain control. It is most important that the nurses have a

straightforward and reliable line of communication, particularly to the anaesthetic resident on duty.

Bleep numbers (and alternates) should be left with the ward staff. Many patients endure hours of

needless suffering due to inadequate communication between nurses and medical staff. This is

particularly important as, for safety reasons, no alternative i.m. opioid regime is usually prescribed

Close monitoring of these patients is essential and is often carried out in a high dependency unit (HDU)

or ICU, although some units have dispensed with this requirement. This is best achieved by production

of a special chart with all the details alluded to above, thus facilitating patient management and

acceptance of the technique if it is used on general wards. Nurses and resident surgical staff must be

fully aware of the complications and be able to initiate treatment if they should occur.

Conclusion

The effectiveness of opioids on their own by the epidural route (versus parenteral) has increasingly

been questioned. They are now administered more frequently by infusion in combination with local

anaesthetics with which the effects appear synergistic. A suitable regime is to mix 5 or 10 mg. of

diamorphine with 50 mls. of 0.25% bupivacaine in a 50 ml. syringe driver. A rate of 1 to 6 ml. hr-1 is

usually sufficient (following a loading dose to obtain satisfactory analgesia).

Other Uses

As they are addictive (less so for buprenorphine), their use is restricted to severe pain, such as with

myocardial infarction, renal colic and cancer. Their euphoric side effects make them useful

premedicants, and, together with the tendency to depress respiration, have led to their use in ICU for

sedating patients undergoing mechanical ventilation (see Chapter 18).

Opioid agonist-antagonists

Actions


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The side effects of conventional opioid agonists led to the development of a new class of compound, the

antagonist analgesics. They were so called because they originated from the morphine antagonist

nalorphine, which was only later discovered to have analgesic properties of its own. Because of its

unfortunate propensity to cause dysphoria it was not clinically useful as an analgesic, but it did not seem

to produce as serious respiratory depression or addiction as the conventional opioids such as morphine.

It led to the development of more clinically useful compounds such as pentazocine and nalbuphine.

Their ability to antagonise opioid agonists, and yet have analgesic properties of their own led to the

proposition, initially on clinical grounds, of three different types of opioid receptors, named mu, kappa

and sigma. (Subsequent pharmacological localisation in the central nervous system of these three

receptors subtypes, as well as delta and epsilon receptors, has confirmed the original clinical

proposition, see above). Conventional opioids are agonists and produce analgesia at the mu (named

after morphine) receptor at which pentazocine and nalbuphine are mu antagonists. The latter are

thought to produce analgesia at the kappa (keto-cyclazocine) receptor and dysphoria at the sigma (SFK

drug, normetazocine) receptor. This would explain why nalorphine is unable to antagonise the

respiratory depression of pentazocine, but naloxone (a pure mu and kappa antagonist) is. Side effects

include dysphoria, nausea and vomiting (pentazocine more than nalbuphine) and respiratory

depression. They are not as efficacious as the opioid agonists.

Uses

These are similar to the opioid agonists. However, pentazocine is not recommended for myocardial

infarction as it tends to increase right heart work. The alternating use of the opioid agonists and the

agonist-antagonists is to be deprecated. Nalbuphine is not a CD and is classified as a prescription only

medicine (POM).

Miscellaneous drugs

Nefopam and meptazinol are unique strong analgesics whose site of action is less clear than with other

opioids, but there appears to be a reduced tendency to produce respiratory depression. Other side

effects are unpredictable.

Ketamine is an NMDA antagonist (like nitrous oxide) and a powerful analgesic in sub anaesthetic doses


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of 20-40 mg; in anaesthetic doses )e.g. 120 mg for a 60 kg patient) it can cause sympathetic stimulation

and marked hallucinations (probably via the sigma receptor) which can be attenuated with diazepam.

Inhalation of a 50:50 mixture of nitrous oxide and oxygen (Entonox) is a very useful analgesic which is

given 'on demand' by the patient placing the mask on the face and actuating a demand valve by taking

a breath in. If the patient becomes drowsy, the mask falls away and gas delivery stops. It is particularly

useful for use in ambulances and for the intermittent pains of labor but can still be utilised

postoperatively for manoeuvres associated with acute pain such as physiotherapy and dressing change.

Tramadol has a potency and efficacy very similar to pethidine. However, it has a lessened tendency to

cause respiratory depression, it is much less addictive (not a CD, available as a POM - prescription only

medicine) and is available orally. Its main effects are via mu agonism but it also has spinal effect on

5HT receptors. The significance of the latter is not entirely clear. As a POM it is obviously easier to

prescribe for day surgery patients than pethidine or morphine although like other opioids, nausea and

vomiting may be troublesome.

Miscellaneous techniques

TENS, hypnosis and acupuncture have been used with success in some patients but have never been

widely employed for acute pain relief.

Use of opioid antagonists

Severe respiratory depression is an ever present problem when opioids are used for postoperative pain

relief. In most cases this can be avoided by careful patient assessment prior to administration of opioid

by whatever route. A respiratory rate below 6 -8 in the adult or 10 - 15 in the child necessitates

withholding further drug until the rate increases. Supportive measures such as additional oxygen,

together with assessment of blood pressure, pulse and oxygen saturation are all that are usually

necessary. If the respiratory rate falls to below 4, the specific antagonist naloxone should be

administered in a dose of 2 - 6 ug. kg-1 i.v. bolus. The effects are immediate but short lived so the dose

may need to be repeated. A dose related reversal of analgesia occurs so the smallest effective dose

should always be administered. Non-specific respiratory stimulants such as doxapram (1 - 2 mg.kg-1 i.v.

bolus) cause arousal and improve ventilation without reversing analgesia.


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Strategies for effective postoperative analgesia

Introduction

Many recent reports have emphasised the inadequacies of postoperative analgesic regimes. Up to 70%

of patients complain of moderate to severe pain in the postoperative period following major surgery.

What has gone wrong? The reasons for inadequate pain relief are multifactorial and include:

Patient expectations are often conflicting as to the amount of pain that they should expect to have inthe postoperative period. This ranges from the patient who expects to be totally pain free to the patientwho expects to have moderate to severe pain and is thus 'prepared to put up with it'. They are usuallynot informed about how them might obtain postoperative analgesics and that they are only given 'ondemand'. Patients also vary in the analgesic response to a given level of opioid in the plasma, somestudies showing a ten fold variation in drug level to produce the desired analgesic effect.Nurse expectations are also conflicting as to the amount of pain a patient should be expected to suffer.

Some feel that any pain is bad whilst others are more cautious and try to help the patient cope with the

'inevitable' pain of the postoperative period.

Operating surgeons do not always concern themselves with the issue!

Anaesthetists are often only concerned with a smooth intraoperative course and a rapid awakening of

the patient so that he or she can be rapidly returned to the ward. This often involves the use of ultra

short acting potent opioids, ideal for obtaining smooth operative conditions, but guaranteeing that the

patient will be in severe pain very early on in the postoperative period. Add to this the problem of limited

efficacy of i.m. opioid administration 'as required' on the ward and one has the perfect scenario for

inadequate postoperative pain control!

Resident surgical staffseem not to fully understand the pharmacology of opioids, expect them to be a

panacea, and generally just 'leave it all up to the nurse'!

This represents the 'worst possible' case but it is not unusual to find these attitudes engrained in clinical

practise. Putting matters right does not require vast expenditure on time and equipment. Acute pain

teams, use of epidural catheters and administering opioids and LA or expensive PCA computers,

although effective, do not address the problems of the majority of patients. They have tended to divert

our attention away from a more rational approach which must include all patients.


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Finding a solution

This approach includes the following and emphasises that close co-operation must be maintained

between all the parties involved:

Patients must be educated to appreciate the effects of the operation in producing pain (especially onmovement and coughing) and the effect this will have on their mobility and recovery. The types of painrelief available and how it can be obtained must be explained (usually by the anaesthetist in thepreoperative visit) and a strategy outlined.Nurses must be aware of the methods chosen and must be fully supported by the medical staff

particularly if methods such as epidural infusions of opioids are to be used.

Operating surgeons can contribute by utilising nerve blocks and LA infiltrations which significantly

reduce the requirement for opioid analgesics in the postoperative period.

Anaesthetists should employ techniques which allow a smooth transition from good operative to good

postoperative analgesia. This can involve eithercareful titration of longer acting opioids so that there is

not a sudden offset of analgesia just as the patient is leaving the operating room orinsertion of epidural

catheters which can be used in the postoperative period.

Resident surgical staffmust form the link between all these parties and should be fully informed of the

techniques used and the problems that may be encountered.

Recording postoperative pain

It is perhaps surprising that so little attention is given to recording postoperative pain. Whilst blood

pressure and pulse rate are meticulously charted the patient is writhing in agony without a quantitative

record being made! The two commonest pain assessments used are:

Visual linear analogue scale (VLAS). This is a 10 cm. horizontal line drawn on a piece of paper with oneend indicating 'no pain' and the other 'the worst possible pain that the patient could imagine'. From thisscale, a score of 0 indicates 'no pain' and 10 'worst possible pain'. This is presented to the patient whois asked to make a mark on the line corresponding to the pain being experienced. Although the line hasno gradations, a rough score can be obtained from the patient's mark by use of a ruler with 1 cm.gradations. A score of 2 to 3 is acceptable whilst a score of 7 to 8 is obviously not.Nominal, five point scale where 0 = no pain, 1 = slight pain, 2 = moderate pain, 3 = severe pain and 4


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= very severe pain. A score of 0 or 1 is ideal whereas 3, 4 or 5 are not. In practise, the commonest

score is 2!

In addition, the analgesic effectiveness of the prescribed analgesic should be recorded on a nominal

scale as: No relief = 0, Slight relief = 1, Moderate relief = 2, Good relief = 3 and Complete relief = 4. This

allows the doctors and nurses to assess the effect of the prescribed analgesic and will indicate whether

a change in dose, dosage interval or type of analgesic is required.

A postoperative pain record should always be kept and reviewed at frequent intervals so that action

can be taken if pain control is inadequate.

Analgesic techniques for a few common procedures

The following is a simple guide to achieving good postoperative analgesia. It is assumed that the patient

is a 60 kg. fit adult and that there are no contraindications to the drugs suggested. All these regeimes

can be supplemented with i.v. paracetamol (see above).

Major abdominal surgery:A NSAID, e.g. diclofenac 100 mg. p.r. or 100 mg. p.o. is given with thepremedication or 75 mg. i.v. during the procedure. The anaesthetic technique includes either a shortacting analgesic such as fentanyl or the ultra short acting remifentanil which is given by infusion and can

be adjusted to the analgesia required suring the procedure. Some anaesthetists us a long acting i.v.opioid such as morphine (10 to 20 mg.). LA infiltration of the wound with up to 40 mls. of 0.25%bupivacaine is given by the surgeon at the end of the procedure prior to skin closure. The anaesthetistensures that sufficient analgesia is present at the end of the procedure and, on awakening, the patientis comfortable and able to take deep breaths to command. This often involves trained recovery staffadministering small i.v. increments of the opioid, e.g. morphine 2.5 to 5 mg. at 5 to 10 minute intervalsup to a maximum dose of (say) 20mg. Once sufficient analgesia has been obtained, this is followed byprn. morphine 10 - 15 mg. i.m. 4 hourly or PCA. The dose of diclofenac is repeated the following day.This regime provides good to excellent analgesia in the majority of patients. Alternatively, an epiduralinfusion of opioids and LA can be considered for more difficult patients.Inguinal herniorrhaphy and varicose veins: analgesia is best achieved by NSAIDs such as

diclofenac 100 mg. by mouth or suppository (p.r.) 1 hr. preoperatively or 75 mg. iv intraoperatively,

together with LA infiltration and block with bupivacaine by the surgeon at the operative site. Short acting

opioids can be given intraoperatively as long acting opioids have little place in postoperative analgesia

as the majority of patients will be day cases. If necessary, a small dose of opioid (e.g. 50 - 75 mg. i.m.

pethidine with an anti emetic, or PCA fentanyl) may be given in the early postoperative period.

Diclofenac 50 mg. p.o. b.d. is continued from the following day for 3 more days.

Cystoscopy, dilatation and curettage: postoperative pain is best achieved by perioperative use of


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NSAIDs. A short acting opioid is used for the operative procedure but plays no part in the provision of

postoperative pain control.

approaches to management of acute pain 1

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