Acute Pain Management & Analgesic Ladder

Acute pain is a protective, self-limiting symptom of tissue injury that, if poorly controlled (especially postoperatively), drives morbidity — atelectasis, ileus, DVT, chronic pain and delayed recovery. The modern exam favours the multimodal analgesia concept and a "reverse" WHO ladder applied to surgical pain.

Definition & classification

Pain is defined by the IASP (revised 2020) as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage." The key NEET-PG-tested update is that pain is always subjective and a verbal report is not required to experience pain (relevant for neonates, demented, ventilated patients).

Acute pain is classified along several axes:

Axis	Categories	Example
Duration	Acute (<3 months) vs chronic (>3 months)	Post-op vs CRPS
Pathophysiology	Nociceptive (somatic / visceral) vs neuropathic	Incision pain vs phantom limb
Origin	Somatic (sharp, localised) vs visceral (dull, referred)	Skin cut vs colicky gut pain
Inflammatory	Nociceptive with sensitisation	Burns, pancreatitis

High-yield: Somatic pain is well-localised and sharp (A-delta fibres → first/fast pain). Visceral pain is poorly localised, dull, referred and carried by C fibres (second/slow pain, unmyelinated). A-delta are thinly myelinated; C fibres are unmyelinated and slowest.

Pain pathway (flow): Transduction (nociceptor) → Transmission (A-delta/C fibre → dorsal horn, substance P, glutamate at NMDA/AMPA) → Modulation (descending serotonergic/noradrenergic + gate control at substantia gelatinosa) → Perception (thalamus → somatosensory cortex). Each step is a drug target — this is the conceptual basis of multimodal analgesia.

Pain assessment scales

Assessment ("the 5th vital sign") must precede and follow every intervention.

Scale	Type	Best use
Visual Analogue Scale (VAS)	0–100 mm line	Adults, research; most sensitive
Numeric Rating Scale (NRS)	0–10 verbal	Quick bedside, most practical
Wong-Baker FACES	Faces 0–10	Children ≥3 yr, low literacy
FLACC	Behavioural (Face, Legs, Activity, Cry, Consolability)	Pre-verbal children 2 mo–7 yr
CRIES	Crying, Requires O₂, Increased vitals, Expression, Sleepless	Neonates
BPS / CPOT	Behavioural	Ventilated/ICU non-communicative

High-yield: NRS ≥4 (moderate) usually triggers intervention; the goal is a "functional" score (≤3) that allows coughing, mobilising and deep breathing — not necessarily zero pain.

The WHO analgesic ladder & its surgical adaptation

The original WHO 3-step ladder (1986) was designed for cancer pain and works bottom-up:

1. Step 1 — Mild pain: Non-opioid ± adjuvant. Paracetamol, NSAIDs.
2. Step 2 — Moderate pain: Weak opioid + non-opioid ± adjuvant. Codeine, tramadol.
3. Step 3 — Severe pain: Strong opioid + non-opioid ± adjuvant. Morphine, fentanyl, oxycodone.

Adjuvants (any step): antidepressants (amitriptyline, duloxetine), anticonvulsants (gabapentin, pregabalin), steroids, bisphosphonates.

High-yield: For acute/postoperative pain the ladder is used in REVERSE ("down the ladder") — start strong (Step 3) when pain is maximal immediately post-op and step down as pain subsides. The cancer ladder steps up.

A proposed 4th step adds interventional techniques: nerve blocks, epidural/intrathecal analgesia, neurolysis, PCA.

Mnemonic for ladder drugs — "Please Care, Make Pain Manageable": Paracetamol → Codeine → Morphine, with adjuvants throughout.

Multimodal (balanced) analgesia

The central modern concept: combine drugs acting at different points of the pain pathway to achieve additive/synergistic analgesia with lower doses of each → fewer side effects (especially opioid-sparing).

Typical postoperative multimodal regimen combines:

• Paracetamol (central COX, regular scheduled dosing) +
• NSAID / COX-2 inhibitor (peripheral + central anti-inflammatory) +
• Regional/local block (transduction & transmission block) +
• Opioid (rescue, PCA) +
• Adjuvants (gabapentinoids, ketamine, dexamethasone, alpha-2 agonists).

High-yield: The single most exam-relevant benefit of multimodal/regional analgesia is the opioid-sparing effect → reduces nausea/vomiting, sedation, respiratory depression, ileus and the risk of dependence.

This dovetails with ERAS (Enhanced Recovery After Surgery) protocols, which mandate opioid-sparing, multimodal, regional-based analgesia and early mobilisation.

Pre-emptive vs preventive analgesia

• Pre-emptive: analgesia given before the surgical stimulus to block central sensitisation/"wind-up."
• Preventive: broader — analgesia (before, during, after) that reduces post-op pain beyond its expected drug duration. Preventive analgesia has more robust evidence than strict pre-emptive dosing.

Key analgesic drug classes (drug-of-choice focus)

Drug	Mechanism	Notable point / max dose
Paracetamol	Central COX inhibition, ?cannabinoid/serotonergic	Max 4 g/day (3 g in liver disease/elderly); hepatotoxic in overdose (NAPQI)
NSAIDs (ibuprofen, diclofenac, ketorolac)	Peripheral & central COX-1/2 inhibition → ↓prostaglandins	Ketorolac potent parenteral NSAID; renal/GI/platelet risk
COX-2 (celecoxib, parecoxib)	Selective COX-2	Less GI bleed, no platelet effect; ↑CV/thrombotic risk
Tramadol	Weak μ-agonist + SNRI (serotonin/NA reuptake inhibition)	Seizures, serotonin syndrome; less respiratory depression
Morphine	μ-opioid agonist	Gold-standard strong opioid; histamine release, active metabolite M6G accumulates in renal failure
Fentanyl	μ-opioid, lipophilic	Short-acting, no active metabolite → opioid of choice in renal failure
Ketamine	NMDA antagonist	Sub-anaesthetic dose adjunct, anti-hyperalgesic, opioid-sparing
Gabapentin/pregabalin	α2δ Ca²⁺ channel	Neuropathic + reduces acute post-op opioid need
Dexmedetomidine/clonidine	α2-agonist	Sedation + analgesia, opioid-sparing

High-yield: Paracetamol acts mainly centrally and has negligible peripheral anti-inflammatory effect — that is why it is not a true anti-inflammatory like NSAIDs. Paracetamol overdose treated with N-acetylcysteine (replenishes glutathione); plot levels on the Rumack-Matthew nomogram.

Patient-controlled analgesia (PCA)

PCA is a microprocessor-controlled infusion pump that delivers a preset bolus of opioid when the patient presses a demand button, with safety lockout to prevent overdose.

Key programmable parameters:

1. Demand (bolus) dose — e.g. morphine 1 mg.
2. Lockout interval — minimum time between successful doses (5–10 min) — the central safety feature.
3. Background/basal infusion — usually OFF in opioid-naïve patients (continuous infusion ↑ respiratory depression risk).
4. 1-hour / 4-hour dose limit.

High-yield: PCA's intrinsic safety depends on the patient's own sedation — an over-sedated patient stops pressing the button, so no one else should press it for them ("PCA by proxy" is dangerous). The lockout interval is the key programmed safety parameter.

Most common PCA opioid is morphine; fentanyl/hydromorphone used in renal impairment. The commonest side effect is nausea/vomiting; the most feared is respiratory depression (treat with naloxone).

Regional & neuraxial techniques

• Epidural analgesia: local anaesthetic (bupivacaine/ropivacaine) ± opioid (fentanyl); gold standard for major thoraco-abdominal surgery; risks hypotension, urinary retention, epidural haematoma (esp. with anticoagulants).
• Peripheral nerve blocks / catheters: TAP block (abdominal), interscalene (shoulder), femoral/adductor canal (knee), erector spinae plane block (thoracic) — central to opioid-sparing ERAS.
• Local infiltration / wound catheters.

High-yield: Neuraxial opioids cause a biphasic respiratory depression — early (lipophilic, fentanyl) and delayed (hydrophilic, morphine, up to 6–24 h) due to rostral CSF spread. Watch morphine epidurals overnight.

Special situations — renal & hepatic impairment

Drug	Renal impairment	Hepatic impairment
Paracetamol	Generally safe (extend dosing interval if severe)	Reduce to ≤2–3 g/day; avoid in decompensated cirrhosis
NSAIDs	Avoid (worsen renal function, fluid retention, hyperkalaemia)	Avoid (GI bleed, hepatorenal)
Morphine	Avoid/reduce — M6G accumulates → prolonged sedation/respiratory depression	Reduce dose, ↑ interval (↓clearance)
Codeine/tramadol	Active metabolites accumulate — caution	Unpredictable activation (CYP2D6)
Fentanyl	Preferred — no active metabolites	Relatively safe (titrate)
Gabapentinoids	Renally cleared → reduce dose	Safe (not hepatically metabolised)

High-yield: In renal failure, the safest opioid is fentanyl (also alfentanil/remifentanil); avoid morphine, codeine, pethidine (pethidine → normeperidine, a renally-cleared seizure-causing metabolite). In hepatic failure, avoid NSAIDs and reduce all opioid doses.

Complications of inadequate vs excessive analgesia

Under-treated acute pain → tachycardia/hypertension, myocardial ischaemia, shallow breathing → atelectasis/pneumonia, immobility → DVT/PE, ileus, immunosuppression, sleep deprivation, and chronic post-surgical pain / central sensitisation.

Opioid excess → respiratory depression (↓RR is the danger sign), sedation, miosis, constipation, nausea, urinary retention, pruritus, tolerance and dependence.

High-yield: The earliest reliable clinical sign of opioid over-dose is increasing sedation, which precedes respiratory depression — hence sedation scoring (e.g. Pasero/Ramsay) is monitored alongside RR. Naloxone (0.04–0.4 mg titrated IV) reverses it but is short-acting → re-sedation possible; repeat/infuse.

Key differentials / distinguishing concepts

• Nociceptive vs neuropathic pain: neuropathic (burning, shooting, allodynia, hyperalgesia) responds poorly to opioids/NSAIDs and better to gabapentinoids/TCAs/SNRIs. Use a screening tool (DN4, LANSS).
• Pain vs anxiety/delirium in ICU — both cause agitation; assess with CPOT before escalating sedation.
• Acute pain crisis vs opioid-induced hyperalgesia — paradoxically more pain with escalating opioids; managed by reducing opioid and adding ketamine/adjuvants.
• Tolerance vs dependence vs addiction — pharmacological vs psychological; commonly confused in MCQs.

Recently asked / exam angle

• The WHO ladder is for cancer pain and goes upward; for acute post-op pain it is applied in reverse (downward) — repeatedly tested one-liner.
• Single best opioid in renal failure = fentanyl; avoid morphine and pethidine.
• Pethidine (meperidine) metabolite normeperidine causes seizures and accumulates in renal failure — classic negative-marking trap.
• Multimodal analgesia's chief advantage = opioid-sparing; ERAS protocols emphasise regional blocks + paracetamol + NSAID.
• PCA lockout interval is the principal safety mechanism; basal infusion increases respiratory depression risk in opioid-naïve patients.
• Paracetamol is centrally acting, max 4 g/day, antidote N-acetylcysteine, nomogram = Rumack-Matthew.
• A-delta = fast/sharp/first pain; C fibres = slow/dull/second pain (unmyelinated).
• Naloxone reverses opioid respiratory depression; shorter acting than morphine → monitor for re-sedation.
• FLACC for pre-verbal children, CRIES for neonates, Wong-Baker FACES ≥3 years.
• Neuraxial morphine → delayed respiratory depression (hydrophilic, rostral spread).

Rapid revision

1. IASP: pain is always subjective; verbal report not required.
2. Acute pain ladder = reverse WHO (start strong, step down); cancer ladder = step up.
3. WHO Step 1 non-opioid → Step 2 weak opioid → Step 3 strong opioid (± adjuvant at every step).
4. Multimodal analgesia = opioid-sparing, hits multiple pathway points; backbone of ERAS.
5. Paracetamol max 4 g/day (3 g in elderly/liver); central action; antidote NAC.
6. NSAIDs avoided in renal impairment, peptic ulcer, asthma (aspirin-sensitive), and 3rd-trimester pregnancy.
7. Fentanyl = safest opioid in renal failure; morphine's M6G and pethidine's normeperidine accumulate.
8. PCA: demand dose + lockout interval (key safety) + usually no basal infusion; never press by proxy.
9. Sedation precedes respiratory depression — earliest opioid-overdose sign; reverse with naloxone.
10. A-delta = sharp/fast/first pain (thin myelinated); C = dull/slow/second pain (unmyelinated).
11. Tramadol = μ-agonist + SNRI → risk of seizures and serotonin syndrome.
12. Neuraxial morphine → delayed (6–24 h) respiratory depression; FLACC (toddlers), CRIES (neonates), VAS most sensitive.