Post-operative Nausea & Vomiting

Post-operative nausea and vomiting (PONV) is one of the most common and distressing complications of anaesthesia and surgery, occurring in 20–30% of all surgical patients and up to 70–80% of high-risk patients. Though rarely fatal, it is a major patient dissatisfier, a leading cause of unplanned admission after day-care surgery, and a recurrent favourite in NEET PG and INI-CET — especially the Apfel score and the drug of choice for prophylaxis.

Definition & scope

• PONV = nausea, retching or vomiting occurring within 24 hours of surgery.
• PDNV (post-discharge nausea and vomiting) = symptoms occurring after discharge from day-care/ambulatory surgery, often missed because it happens at home.
• PONV is distinct from delayed nausea (after 24 h, often opioid- or ileus-related).

The vomiting reflex is coordinated by the vomiting centre in the lateral reticular formation of the medulla. It receives afferents from five major sources, each with characteristic receptors that map directly onto antiemetic pharmacology.

Input pathway	Trigger	Key receptors	Targeting antiemetic
Chemoreceptor trigger zone (CTZ, area postrema)	Drugs, opioids, toxins	D2, 5-HT3, NK1	Ondansetron, droperidol, aprepitant
Vestibular apparatus	Motion, opioids	H1, M1 (muscarinic)	Cyclizine, hyoscine
Vagal/GI afferents (gut)	Gastric distension, irritation	5-HT3	Ondansetron
Higher cortical centres	Anxiety, pain, sights/smells	GABA, others	Benzodiazepines
Vomiting centre (final common path)	—	H1, M1, NK1, 5-HT	—

High-yield: The CTZ (area postrema) lies on the floor of the fourth ventricle, outside the blood–brain barrier — that is why circulating emetogenic drugs (opioids, chemotherapy) and toxins can directly trigger vomiting.

Risk factors & the Apfel score

Risk factors are best classified as patient, anaesthetic, and surgical.

Patient factors (the four Apfel predictors):

1. Female sex (strongest patient predictor)
2. Non-smoker
3. History of PONV or motion sickness
4. Younger age / postoperative opioid use (in the simplified Apfel score, the fourth item is postoperative opioids)

Anaesthetic factors:

• Volatile inhalational agents (dose-dependent — the single biggest anaesthetic trigger; the effect is most pronounced in the first 2–3 hours)
• Nitrous oxide (N₂O)
• High-dose intra- and post-operative opioids
• Neostigmine (high doses, by muscarinic/cholinergic effect)

Surgical factors:

• Type: laparoscopic, gynaecological, cholecystectomy, strabismus/ophthalmic, middle-ear/ENT, bariatric, breast.
• Duration: each additional 30 min increases risk by ~60% relatively.

The Apfel simplified risk score

The Apfel score (2004) is the single most examined item in this topic. It uses four independent predictors, each scoring 1 point:

Apfel predictor	Point
Female sex	1
Non-smoker	1
History of PONV / motion sickness	1
Postoperative opioid use	1

Predicted incidence of PONV by total score:

Apfel score	Approx. risk of PONV
0	~10%
1	~20%
2	~40%
3	~60%
4	~80%

High-yield: Apfel 0–1 = low, 2 = moderate, 3–4 = high risk. The number of prophylactic antiemetics given should match the score — low risk may need none, moderate needs 1–2 agents, high risk needs ≥2–3 agents plus a TIVA technique.

Mnemonic for Apfel — "FuN-HO": Female, Non-smoker, History of PONV/motion sickness, Opioids (post-op). (Some texts use the POVOC score for children.)

For children, the equivalent is the Eberhart / POVOC score (4 factors: surgery ≥30 min, age ≥3 years, strabismus surgery, history of POV in child or relatives).

Pathophysiology — why these triggers matter

• Volatile agents sensitise the CTZ and act on the vomiting centre directly; the relationship is dose- and duration-dependent, and they are the dominant cause of early (0–2 h) PONV.
• Opioids act on μ-receptors in the CTZ, delay gastric emptying, and sensitise the vestibular apparatus — explaining nausea worsened by movement.
• Nitrous oxide increases middle-ear and bowel-lumen pressure and stimulates the sympathetic system / opioid receptors centrally.
• Surgical stimulation of the gut releases serotonin (5-HT) from enterochromaffin cells, activating 5-HT3 receptors on vagal afferents — the rationale for ondansetron.

Pharmacological prophylaxis — drugs & doses

This is the highest-yield management table. Antiemetics work best when classes are combined (different receptors → additive/synergistic effect, not same-class duplication).

Drug	Class / receptor	Adult prophylactic dose	Timing	Key adverse effect
Ondansetron	5-HT3 antagonist	4 mg IV	End of surgery	Headache, constipation, QT prolongation
Dexamethasone	Corticosteroid	4–8 mg IV	At induction	Perineal itch on rapid push, transient hyperglycaemia
Droperidol	Butyrophenone, D2 antagonist	0.625–1.25 mg IV	End of surgery	QT prolongation (black-box), sedation, EPS
Aprepitant	NK1 antagonist	40 mg PO	Pre-induction	Most effective against vomiting; long acting
Metoclopramide	D2 antagonist + prokinetic	10 mg IV (weak)	End of surgery	EPS, dystonia
Cyclizine / promethazine	H1 antihistamine	25–50 mg	—	Sedation, anticholinergic
Hyoscine (scopolamine)	Anticholinergic (transdermal)	1.5 mg patch	Evening before/2 h pre-op	Dry mouth, visual blurring, confusion in elderly

High-yield: Ondansetron 4 mg IV at the end of surgery is the prototypical first-line antiemetic. Dexamethasone is given at induction (slow onset) while 5-HT3 antagonists are given at the end of surgery (work better closer to emergence). Mixing up these two timings is a classic MCQ trap.

High-yield: Both ondansetron and droperidol prolong the QT interval — avoid the combination, and avoid in congenital long-QT syndrome. The FDA black-box warning on droperidol is for torsades de pointes / QT prolongation.

Newer / supplementary agents

• Palonosetron — second-generation 5-HT3 antagonist with a long half-life (~40 h), useful against PDNV; negligible QT effect.
• Amisulpride (IV) — D2/D3 antagonist, newer rescue/prophylaxis agent.
• NK1 antagonists (aprepitant, fosaprepitant, rolapitant) — most potent at suppressing the vomiting component (vs nausea) and have the longest duration.

Stepwise approach to prevention

The modern strategy is multimodal and risk-stratified:

Step 1 → Assess risk (Apfel score) → Step 2 → Reduce baseline risk (avoid triggers) → Step 3 → Give prophylaxis matched to risk (number of agents = number of risk factors, roughly) → Step 4 → Rescue treatment if PONV occurs despite prophylaxis.

Step 2 — baseline risk reduction (most examinable list):

1. Use TIVA with propofol instead of volatile agents.
2. Avoid / minimise nitrous oxide.
3. Minimise volatile anaesthetics.
4. Minimise intra- and post-operative opioids — use multimodal/opioid-sparing analgesia (paracetamol, NSAIDs, regional blocks, dexmedetomidine).
5. Use regional anaesthesia in preference to general where feasible.
6. Adequate IV hydration (liberal crystalloids reduce PONV).
7. Avoid high-dose neostigmine (use sugammadex for reversal where available).

High-yield: Propofol-based TIVA is antiemetic — propofol has an intrinsic antiemetic action at subhypnotic doses and avoids the emetogenic volatile agents. TIVA is the preferred technique in high-risk patients and a guaranteed exam point ("which anaesthetic technique reduces PONV?" → propofol TIVA).

Step 3 — prophylaxis matched to risk:

• Low risk (Apfel 0–1): no routine prophylaxis, or a single agent.
• Moderate risk (Apfel 2): 1–2 agents (e.g., ondansetron + dexamethasone).
• High risk (Apfel 3–4): ≥2 agents from different classes + TIVA; consider adding NK1 antagonist and non-pharmacological methods.

Rescue (treatment of established PONV)

High-yield: For rescue, use a drug from a different pharmacological class than the one used for prophylaxis. Re-dosing the same drug within 6 hours is ineffective — its receptors are already occupied.

• If no prophylaxis was given → low-dose ondansetron 1 mg IV (rescue dose is lower than prophylactic 4 mg).
• If ondansetron was used for prophylaxis → switch to dexamethasone, droperidol, or an NK1 antagonist.
• Propofol 20 mg IV is an effective short-acting rescue in PACU.

Non-pharmacological methods

• P6 (Neiguan) acupoint stimulation at the wrist — proven to reduce PONV, equivalent to a single antiemetic. Classic eponymous fact.
• Adequate hydration, supplemental oxygen (variable evidence), aromatherapy (isopropyl alcohol sniff) for mild nausea.

Complications of PONV

• Aspiration pneumonitis (if airway not protected).
• Wound dehiscence, tension on suture lines, increased intra-abdominal/intra-ocular/intracranial pressure.
• Dehydration and electrolyte disturbance (hypokalaemia, metabolic alkalosis from vomiting).
• Bleeding (especially after thyroid, tonsillar, ophthalmic surgery).
• Delayed discharge / unplanned hospital admission after day-care surgery — a major healthcare cost and the commonest reason PONV is taken seriously.
• Oesophageal rupture (Boerhaave) — rare but catastrophic with violent retching.

Key differentials of post-operative nausea/vomiting

Not all post-op vomiting is "PONV" — exclude treatable causes before just giving antiemetics:

Cause	Clue	Action
Hypotension	Spinal/epidural sympathetic block, hypovolaemia	Fluids, vasopressor
Hypoxia / hypercarbia	Low SpO₂, raised EtCO₂	Oxygen, ventilation
Pain	Nausea correlates with severe pain	Treat pain (opioid-sparing)
Opioid excess	Sedation + pinpoint pupils	Reduce dose, multimodal analgesia
Raised ICP	Neurosurgery, headache, bradycardia	Imaging, neuro management
Ileus / bowel obstruction	Distension, absent bowel sounds, late onset	Decompress, surgical review
Hypoglycaemia / DKA	Diabetic, altered sensorium	Check glucose

High-yield: Always rule out hypotension and hypoxia first in a vomiting post-operative patient — reflexively giving an antiemetic without checking these is a classic clinical-vignette error.

Recently asked / exam angle

• Apfel score components and risk percentages are asked verbatim — memorise FuN-HO and the 10–20–40–60–80% ladder.
• Timing of antiemetics: dexamethasone at induction, ondansetron at the end of surgery — a frequent two-statement assertion-reason question.
• Which agents prolong QT? → ondansetron + droperidol (avoid combining).
• Most emetogenic anaesthetic factor → volatile agents (dose-dependent); technique that reduces PONV → propofol TIVA.
• Drug of choice for high-risk PONV → combination prophylaxis (ondansetron + dexamethasone ± droperidol/aprepitant) with TIVA.
• Site of CTZ → area postrema, floor of 4th ventricle, outside blood–brain barrier.
• P6 acupressure point location (wrist, Neiguan) and its proven efficacy.
• Children: strabismus surgery and adenotonsillectomy as highest-risk paediatric procedures; POVOC/Eberhart score.
• Most effective drug against the vomiting (not nausea) component → NK1 antagonists (aprepitant).
• Rescue principle: use a different class than prophylaxis; rescue ondansetron dose is 1 mg (lower than prophylaxis).

Rapid revision

1. PONV = nausea/retching/vomiting within 24 h of surgery; PDNV is the post-discharge form.
2. Apfel score (FuN-HO): Female, Non-smoker, History of PONV/motion sickness, Opioids post-op — 1 point each.
3. Risk ladder: score 0→10%, 1→20%, 2→40%, 3→60%, 4→80%.
4. CTZ = area postrema, on the floor of the 4th ventricle, outside the BBB.
5. Volatile agents are the strongest anaesthetic trigger (dose-dependent); female sex is the strongest patient predictor.
6. Ondansetron 4 mg IV at end of surgery; dexamethasone 4–8 mg IV at induction.
7. Ondansetron and droperidol both prolong the QT interval — droperidol carries an FDA black-box warning for torsades.
8. Propofol TIVA is antiemetic; minimise volatiles, N₂O, and opioids; reverse with sugammadex to avoid high-dose neostigmine.
9. NK1 antagonists (aprepitant) are most effective against the vomiting component and have the longest action.
10. Rescue with a different class; rescue ondansetron dose is 1 mg, and propofol 20 mg works in PACU.
11. P6 (Neiguan) acupoint stimulation reduces PONV — equivalent to one antiemetic.
12. Always exclude hypotension and hypoxia before treating "PONV" as simple nausea.