Postoperative Pain Management

Nimmaanrat S, MD, FRCAT, MMed (Pain Mgt)

Topics to be Covered

Definition

Neurobiology

Classification

Multimodal analgesia

Preventive analgesia

Step down approach

Measurement

Methods of treating postoperative pain (various routes)

Patient-controlled analgesia (PCA)

Non-opioids

Weak opioids

Strong opioids

Regional analgesia

Pain

“An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”

Neurobiology of Pain

Descartes (Cartesian)’s Model of Pain

Classification of Pain

Acute

Chronic

Duration

Nociceptive

Neuropathic

Pathophysiology

The Continuum of Pain

<1 month

Time to resolution

3-6 months

Acute Chronic

• Usually obvious tissue damage

• Increased nervous system activity

• Pain resolves upon healing

• Serves a protective function

• Pain for 3-6 months or more

• Pain beyond expected period of

healing

• Usually has no protective function

• Degrades health and function

Insult

Progression from Acute to Chronic Pain

Incidence of Chronic Pain after Surgery

Amputation

Thoracotomy

Mastectomy

Cholecystectomy

Inguinal hernia

Vasectomy

Dental surgery

30 - 85 %

5 - 67 %

11 - 57 %

3 - 56 %

0 - 63 %

0 - 37 %

5 - 13 %

Risk Factors for Chronic Postsurgical Pain

Preop factors Pain, moderate to

severe, > 1/12

Repeat surgery

Psychologic vulnerability

Workers’ compensation

Intraop factors Surgical approach with

risk of nerve damage

Postop factors Pain (acute,

moderate to severe)

Radiation to area

Neurotoxic chemotherapy

Depression

Psychologic vulnerability

Neuroticism

Anxiety

Adverse Consequences of Uncontrolled Postoperative Pain

Physiological effects

Psychological effects

Physiological Adverse Effects

Increase sympathetic activity

Increase risk of MI

Decrease GI motility (ileus)

Increase incidence of pulmonary complications (atelectasis, hypoxia)

Suppress immunity

Psychological Adverse Effects

Receive less attention than those asso. w chronic pain

But not less important

Failure to control postop. Pain

Anxiety

Insomnia

Inability to think and interact w others

etc.

Postoperative Pain Management

Multimodal analgesia

Preventive analgesia

Step down approach

Multimodal Analgesia

Clinically Meaningful Adverse Events – CMEs

The incidence of clinically meaningful adverse events is dose-related (level II)

Preventive Analgesia

Step Down Approach

Measurement of Pain

Pain Measurement

Pain is a subjective, personal experience

The logical and true assessment of pt’s pain must therefore be pt’s own report

Self report is gold standard

Unidimension

Multidimensions

Unidimensional Tools

Categorical scales

Numerical rating scales (NRS)

Visual analog scales (VAS)

Picture scales / pain drawings

Methods of Treating Postoperative Pain

Methods of Treating Postoperative Pain

Traditional administration of opioids

Parenteral administration of opioids

Non-parenteral administration of opioids

Local anesthetic techniques

Non-opioid analgesics

Non-pharmacological methods

Parenteral Administration of Opioids

Bolus IV administration

Continuous IV infusion

Patient-controlled analgesia (PCA)

Bolus IV

Bolus + infusion

Subcutaneous

Non-parenteral Administration of Opioids

Sublingual

Oral

Transmucosal

Rectal

Transdermal

Nasal

Inhalation

Intra-articular opioids

Non-opioid Analgesics

Non-steroidal anti-inflammatory drugs (NSAIDs)

Selective COX-2 inhibitors (COXIBs)

Paracetamol

NMDA antagonists

Central α2-adrenergic agonists

Patient-controlled Analgesia (PCA)

Patient-controlled Analgesia (PCA)

Patient-controlled Analgesia (PCA)

Advantages of PCA

Non-parenteral Opioid Administration

Non-parenteral Administration of Opioids

Sublingual

Oral

Transmucosal

Rectal

Transdermal

Nasal

Inhalation

Intra-articular opioids

Sublingual Opioids

Cooperation required

No need for painful injections

Popular for pts

Convenient for nurses

Buprenorphine

Partial agonist / ceiling effect

Oral Route

All opioids undergo extensive first-pass metabolism

Low oral bioavailability (20-30%)

Immediate postop. period : invariably reduction of gastric emptying

Oral Route

Absorption may be delayed, with poor analgesia

If given on regular basis : a danger of a large dose being propelled into upper GI tract when gastric motility returns to normal

Over dosage

Ventilatory depression

Transmucosal Route

Premedication in children

Onset of pain relief : 9/60

Rectal Route

Bioavailability varies according to site of suppository

Venous blood from lower part of rectum drains directly into systemic circulation

But upper part drains into portal circulation

Inhaled / Intranasal Route

Intranasal spray devices for fentanyl

Metered inhalers with improved pulmonary drug delivery systems and lockout times

Future : may allow noninvasive PCA administration

Pharmacological Approach

Classification of Pain Medications

Non-opioid analgesics

Opioid analgesics

Adjuvants

Paracetamol

Paracetamol (Acetaminophen)

Effective analgesic with antipyretic activity

Does not inhibit COX in peripheral tissues -> lack of anti-inflammatory activity

Mechanism of analgesic action remains unclear

Paracetamol

Generally well tolerated

Most serious adverse effect of acute overdosage is a dose-dependent

(150 mg/kg), potentially fetal, hepatic necrosis

Insufficient glutathione (liver disease, alcohol consumption> 3 units/day, malnutrition etc.)

NSAIDs

Mechanisms of Action

Selective COX-2 Inhibitors (COXIBs)

COXIBS

COX-2 is constitutively expressed in kidney

Maintenance of renal blood flow

Mediation of renin release

Regulation of Na+ excretion

COXIBs & NSAIDs have similar renal adverse effects

↑ed risk in pre-existing renal impairment,

hypovolumia, hypotension, use of

nephrotoxic agents & ACEIs

COXIBs

The pharmacological class of COXIBs appears to be associated with an increased risk of CV adverse events

The CV risks may increase with dose & duration of exposure

The shortest duration possible & the lowest effective daily dose should be used

COXIBs

Must not be used in pts with established

Ischemic heart disease

Cerebrovascular disease

Peripheral arterial disease

To exercise caution in pts with risk factors of heart disease

Hypertension

Hyperlipidemia

DM

Smoking

Opioids

Classification of Opioids

Weak opioids (mild - moderate pain)

Strong opioids (moderate - severe pain)

Weak Opioids

Codeine

Tramadol

Codeine

Classic weak opioid

Potency 1/10 of MO (CYP2D6, MO)

30-120 mg q 4/24

Dose limiting side effects (constipation, N/V,

confusion)

Fixed combination

Paracetamol vs Paracetamol plus Codeine

Tramadol

Dual-acting analgesic

Tramadol & M1 (CYP2D6) have affinity at

μ-opioid receptors

Also inhibits reuptake of serotonin &

noradrenaline

Potency 1/20 – 1/5 of MO

Tramadol

Max 400 mg/day

Max 200 mg/day in pts with hepatic /

renal impairment

Tramadol Retard (12/24)

Less sedation & constipation

Unfortunately, N/V frequently reported

Strong Opioids

Morphine

Pethidine

Fentanyl

Strong Opioids

Mainstay for the Rx of moderate to severe pain

Interpatient requirements vary greatly

Doses need to be titrated to suit each pt

In adults, age rather than weight is the predictor of requirement

Strong Opioids

All full agonists given in equianalgesic doses produce the same analgesic effects & side effects

One opioid is not superior over others but some are better in some pts (level II)

The incidence of clinically meaningful adverse effects is dose – related (level II)

Pethidine should be discouraged

Opioids

Assessment of sedation level is a more reliable way of detecting early opioid-induced respiratory depression

Morphine

M3G & M6G are main metabolites, excreted via kidney

M6G

Opioid agonist

May potent than morphine

M3G

No analgesic activity

May antagonise analgesic effect

May cause hyperalgesia, allodynia & muoclonus

Pethidine

Widely used even though it has multiple disadvantages

Despite common belief that it is the most effective opioid in treatment of renal colic, it is no better than morphine

Pethidine & morphine have similar effects on sphincter of Oddi & biliary tract

No evidence that pethidine is better

in the treatment of biliary colic

Norpethidine

Metabolized in liver to several inactive compounds and norpethidine

Accumulation leads to neuroexcitatory

Nervousness

Tremors

Twitches

Multifocal myoclonus

Seizures

Pethidine / Norpethidine

Impaired renal function → ↑ half-life of

norpethidine

Patients in renal failure are at ↑ed risk of

norpethidine toxicity

Naloxone not reverses & may ↑ norpethidine

toxicity

Overall, the use of pethidine should be discouraged in favor of other opioids

Fentanyl

High lipid solubility

Potency 100X of morphine

Lack of active metabolite

Fast onset

Short duration of action

Guideline for Postoperative Pain Management

(Example)

Regional Analgesia

Epidural Analgesia

Peripheral Nerve Block

Ultrasound-guided Peripheral Nerve Block

supraclavicular brachial plexus block

Peripheral Nerve Block

Single shot

Continuous infusion