Introduction to pain
Definition of pain |
An unpleasant sensory & emotional experience associated with actual/potential tissue damage |
Purpose of pain |
1. As a protective mechanism |
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2. Cause individual to react to remove pain stimulus |
Nociceptive pain mechanism
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Wide spread in superficial layers of skin & certain internal tissues |
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Excited by 3 different stimuli : mechanial, thermal, chemical |
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Pain is related to degree of receptor stimulation by processes causing tissue injury (more receptor stimulated=more pain) |
2 nociceptor systems |
1. A delta fibres (faster) |
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2. C fibres (slower) |
Chemicals that stimulate nociceptors |
Histamine |
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Bradykinins |
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5-HT (serotonin) |
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some metabolic substances released from damaged cells (lactic acid,ATP) |
Sources of nociceptive pain |
Somatic pain |
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Visceral pain |
Pain from internal structures, poorly localised, often radiates or referred to other areas |
Neuropathic pain
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Pain resulting from pathophysiologic changes in peripheral or CNS |
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A state of chronic pain is sustained |
Idiopathic pain
Patient's psychological state contribute to pain |
May be due to anzxiety,depression, other psychological disorders |
Pharmacotherapy in management of pain
1. Opioid analegesics |
2. NSAIDs |
3. Local anaesthetics |
4. Alpha 2 agonists |
Opioids
MOA |
Binds to opioid receptors and inhibit action on neurons |
Type of opioid receptors |
1. Mu receptors |
effects: analgesia,respiratory&physical depression,miosis,reduced GI motility |
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2. Kappa receptors |
effects: sedation,miosis |
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3. Delta receptors |
effects: dysphoria,hallucinations |
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Classification of opioids
Strong agonists |
Morphine |
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Pethidine |
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Methadone |
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Fentanyl |
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Sufentanil/Alfentanil |
Mild to moderate agonists |
Codeine |
Mixed agonist-anatagonists |
Pentazocine |
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Buprenorphine |
Strong agonist opioid's desirable effects
Analgesia |
Centrally mediated |
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Alters emotional perception of pain |
Sedation |
Sense of well being |
Cough supression |
Reduce GI motility |
Can help with diarrhoea |
Strong agonist opioid's adverse effects
Respiratory depression |
Dose related |
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Most important side effect which limits clinical use |
Miosis |
Constriction of pupil |
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Decreases ability to see in dim light |
Orthostatic hypotension |
Nausea & vomiting |
Constipation |
Anorexia |
Sedation |
Development of dependence |
Cause addiction |
Caution:
*Eldery are more prone to adverse effects of narcotic analgescis, thus lower dose is required
Tolerance of strong agonist opioids
Due to regular/intermittent use |
Regular administration of fixed dose of drug give rise to progressively decreasing effect |
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Progressively higher dose has to be administered to achieve the same effect |
Develops gradually |
Cross tolerance between opioids |
Will develop tolerance to drugs of similar pharmacological action |
Clinical uses of strong agonists opioids
Severe pain |
Pre-medication for anaesthesia |
Methadone |
Substitution therapy in drug dependence clinics |
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Chronic use: long term treatment in terminal cancer patients |
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Mild-moderate agonists (CODEINE)
Indications |
Mild-moderate pain |
Usually in combination with non-opioid analgesics |
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Cough supression |
At lower dose than that for analgesia |
Mixed agonist-antagonist opioids
Opioids with full agonist activity at one receptor subtype but behaves like an antagonist or partial agonist at another receptor subtype |
Examples: |
Pentazocine |
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Buprenorphine |
Clinical uses |
Chronic severe pain |
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Drug abusers |
Advantages of mixed agonist-antagonist
Less adverse effects mediated by specific receptors |
Less prone to cause dependence and abuse |
Caution:
*Should not be given to patients that are already on treatment with pure strong agonist as it may precipate severe withdrawal syndrome
Tramadol
Chemically unrelated to other opioid drugs |
MOA |
Partial mu agonist |
Less affinity than morphine |
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Inhibition of serotonin and noradrenaline reuptake |
Levels of serotonin&noradrenaline increase |
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Block nociceptor impulse at spinal level |
Clinical use |
Mild to moderate pain |
Adverse effects |
Less constipation,less respiratory depression,less dependence than opioids |
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Dizziness,sedation,nausea,vomiting |
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Constipation,headache |
Counselling points for opioids
Drug may cause drowsiness,dizziness,blurring of vision |
Do not drive or operate heavy machinery |
Avoid alcohol |
If patient experience GI effects |
Drug can be taken with food |
Seek medical attention if |
Experience severe nausea,vomiting,constipation |
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