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Physiology of Pain and Principles of Pain Relief
Dr Dushyanthi Perera MD FRCASenior Lecturer in AnaesthesiologyFaculty of MedicineUniversity of Kelaniya
Pain is one of the commonest reason for a patient visiting a doctor.
Definition of painPain pathwayPost op painAnalgesia for post op painPain in labour Chronic Pain
Cancer pain Non malignant
Ethical issues
DEFINITION
“An unpleasant sensory and emotional experience associated with potential or actual tissue damage.”
EMOTIONAL RATIONAL
PHYSICAL
Psychological
Anxiety
Prev. experiences
Insight
Motivation
Factors influencing the pain experienceAgeGenderPersonalityCultureLearned behaviour from past
experiencesBeliefs / AttitudesReligiousAnxiety and fears
Pain is whatever the patient says it is, existing whenever the patient says it does.
The patient’s self reporting of pain is the single most reliable indicator of pain.
Assessment of PainA detailed historyA comprehensive physical examinationAn understanding of the pathophysiology
of painMethodical documentationMeasurement of Pain
Pain HistorySiteIntensityDurationElsewhere – radiation
Associated factorsRelieving factorsManagement to date
SIDE ARM
Pathophysiology of Pain
Types of painAcute ( brief pain that subsides with
healing) Labour Post op
Chronic Pain Cancer Non malignant pain
Types of painNociceptive pain ( stimuli from somatic
and visceral structures)
Neuropathic (abnormal processing of the nervous system)
NociceptionTerm used to describe how pain becomes
conscious
TransductionTransmissionPerceptionModulation
Transduction – changing of the noxious stimuli in sensory nerve endings to impulses
Transmission – movement of these impulses to the brain
Perception – recognizing, defining and responding to the pain
Modulation – activation of descending pathways that exert inhibitory effects on pain transmission
Peripheral Receptors (somatic or visceral)High threshold cutaneous receptors
‘Silent’ nociceptors
Peripheral opioid receptors ( anti nociceptors)
Inflammatory Mediator soupNeurogenic eg Substance P
Tissue mediated eg Bradykinin prostoglandins, 5HT, histamine, K and H ions
Substance P and Prostoglandins further sensitize the nociceptors including activating the ‘silent’ type.
PHYSIOLOGY OF PAINTRAUMA
Nociceptors
Skin Deep tissues Viscera
MechanoreceptorsPolymodel Symp and Parasymp
Ad (sharp,localized) C (dull,poorly localized, aching)
Inflammatory mediators- SP, Hist, Bradykinin
++ +
PHYSIOLOGY OF PAIN
DORSAL HORN substance P,glutamate,ATP
SPINOTHALAMIC TRACTS
THALAMUS
CORTEX Descending tracts
Exact location in the brain where pain is perceived is unclear
Reticular activating system- symp response
Somatosensory cortex – localizes and characterizes
Limbic system – emotional and behavioural response
Central Spinal Processing
Gate Control Theory
‘Wind Up’ phenomenon (AMPA and NMDA receptors)
Descending inhibitory control ( endogenous opioids, 5HT, Noradrenaline, GABA)
Higher Centres
T cell
SG
Large fibresAb C fibres
+ -
+ +
--
Gate Control Theory of Melzack and Wall
Harmful Effects of PainCVS – Increased heart rate and BP.
HypercoagulabilityRS – inadequate ventilationStress response – release of multitude
of hormones and hyperglycaemiaGIT – nausea and vomitingGUT – fluid and urinary retention Immune – depressed immunityMuscle spasm
REQUIREMENTS
CONTINUOUS ANALGESIA
ANALGESIA TITRATABLE TO PAIN
AVAILABLE ANALGESIC TECHNIQUES OPIODS
Morphine, Pethidine,Codeine NSAIDS
Paracetamol, Diclofenac sodium LOCAL ANAESTHETICS
Local infiltration, Epidural, EMLA OTHERS
Acupuncture, Entonox, TRAMADOL
OPIOIDS
ROUTES- im, iv, sc,continuous infusion, PCA SIDE EFFECTS
VENTILATORY DEPRESSION SEDATION VASODILATATION COUGH SUPPRESSION NAUSEA AND VOMITING TOLERENCE AND DEPENDENCE CONSTIPATION
NSAID
OPIOID SPARINGCONTRA INDICATIONS
GASTRIC ULCERS BLEEDING DIATHESIS RENAL DYSFUNCTION
HYPOVOLAEMIC, ELDERLY, INADEQ. RESUSC POST OP
HYPERSENSITIVITY
LOCAL ANAESTHETIC
EXPERTISE MONITORING Low Tech – Topical
Wound infiltration Peripheral nerve blocks Plexus blocks
High Tech – Spinal , Epidural
Advantages of epidural
Prolong the duration of analgesia by use of the catheter
Decreases the incidence of DVTLess sedation therefore early
mobilization and feedingEarly return of bowel function
Disadvantages
Expensive Expertise Monitoring Hypotension, urinary retention Resp depression with added opioids
If opioids are added they must not be given by any other route.
Ideally keep on O2.
Balanced analgesia
Combination of appropriate analgesics.Act on different sites in the pain
pathwayDecrease individual doses and thereby
decrease the incidence of side effects
PRE EMPTIVE ANALGESIA
Factors to consider when choosing analgesicsAppropriateness of the intervention for
the painCoexisting illnessAvailable staffAvailable equipmentRisks and side effectsCost / Benefit ratio
MONITORINGPAIN SCORE
1 NO PAIN AT REST OR MOVEMENT2 NO PAIN AT REST SLIGHT PAIN ON
MOVEMENT3 PAIN AT REST. MOD. PAIN ON
MOVEMENT4 CONTINUOUS PAIN AT REST AND
SEVERE PAIN ON MOVEMENTVisual Analogue Score
MONITORING
SEDATION0 NONE1 DROWSY. EASYY TO ROUSE2 ASLEEP .EASY TO ROUSE3 SOMNOLENT AND DIFFCULT TO
ROUSE
MONITORING
RESPIRATORY RATE
NAUSEA AND VOMITING
CVS - HEART RATE AND BP
Steps to Successful Management of Pain
Regular assessment and recording of pain and side effects
Protocols for monitoring and treating pain Protocols for monitoring and treating side
effects Use a safe and simple balanced analgesic
regime Appropriate backup by identified personnel Continuing in service training and education.