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.