# Mendelson Syndrome and Aspiration Prophylaxis

> Mendelson syndrome and aspiration prophylaxis for NEET-PG Obstetrics and Gynaecology, covering gastric aspiration risk and the standard prophylaxis regimen.

- Canonical URL: https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/mendelson-syndrome-and-aspiration-prophylaxis
- Exam / course: NEET-PG · Subject: Obstetrics and Gynaecology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: Chestnut's Obstetric Anesthesia; Morgan and Mikhail's Clinical Anesthesiology; Williams Obstetrics
- How to cite: "Mendelson Syndrome and Aspiration Prophylaxis", PrepElephant, https://prepelephant.com/topics/neet-pg/obstetrics-and-gynaecology/mendelson-syndrome-and-aspiration-prophylaxis

## Direct answer

Mendelson syndrome is chemical pneumonitis caused by aspiration of acidic gastric contents into the lungs, typically during induction of general anaesthesia. Pregnancy raises the risk because gastric emptying slows, lower oesophageal sphincter tone falls and intragastric pressure rises. Prophylaxis combines a non-particulate antacid, an H2 receptor antagonist and metoclopramide, given before induction.

## What you must remember

- **Mechanism:** Acidic gastric contents injure the alveolar lining, and severe injury is linked to a very low gastric pH.
- **Pregnancy:** Progesterone lowers lower oesophageal sphincter tone, while gastric emptying slows and intragastric pressure rises.
- **Labour:** Opioid analgesia slows gastric emptying further, so the risk rises during labour.
- **Sodium citrate:** A non-particulate antacid; 30 mL of 0.3 molar solution is given orally before induction.
- **Acid suppression:** An H2 receptor antagonist given before induction reduces gastric acid secretion.
- **Prokinetic:** Metoclopramide increases lower oesophageal sphincter tone and speeds gastric emptying when given before induction.

## Building the prophylaxis bundle before induction

Aspiration prophylaxis works by neutralising acid, reducing its production and speeding emptying, so each agent covers a different part of the risk. The sequence below is set before general anaesthesia for caesarean section.

1. **Identify the need:** Any caesarean under general anaesthesia, and any labouring woman at increased risk, needs prophylaxis.
2. **Give the antacid:** Oral sodium citrate 30 mL is given about 30 minutes before induction.
3. **Reduce acid secretion:** An H2 receptor antagonist is given before induction, by the route and timing set by local protocol.
4. **Speed emptying:** Metoclopramide is given before induction to increase sphincter tone and promote gastric emptying.
5. **Protect the airway:** Rapid sequence induction with a cuffed tube and suction ready shortens the period of aspiration risk.

Each agent addresses a different part of the mechanism, so exam answers should name all three drugs and the airway step. Omitting one component leaves a gap in protection.

## Where the exam tries to catch you

Aspiration questions usually test the risk factors in labour, the choice of antacid and the management of established aspiration.

- **Trap:** Assuming the stomach is empty in labour after a standard fasting period — **Correct:** Pregnancy, labour and opioids delay emptying, so the stomach may still contain food.
- **Trap:** Using a particulate antacid for prophylaxis — **Correct:** Only non-particulate antacids such as sodium citrate are used, because particles add to the lung injury.
- **Trap:** Giving prophylactic antibiotics as first-line treatment of established Mendelson syndrome — **Correct:** Treatment is supportive, with oxygen and ventilation, and antibiotics are added only if a secondary infection develops.

## Frequently asked questions

### What is the main cause of Mendelson syndrome?
Mendelson syndrome is caused by aspiration of acidic gastric contents into the lungs during anaesthesia or sedation. The acid causes chemical pneumonitis that can progress to hypoxia and respiratory failure. The risk is greatest during induction and recovery.

### Why is pregnancy a risk factor for aspiration?
In pregnancy, progesterone lowers lower oesophageal sphincter tone, gastric emptying slows and intragastric pressure rises. Opioids given in labour slow emptying further. These changes increase the chance of regurgitation at induction.

### What is the standard aspiration prophylaxis for caesarean under general anaesthesia?
The standard regimen uses a non-particulate antacid such as 30 mL of 0.3 molar sodium citrate, an H2 receptor antagonist and metoclopramide before induction. Rapid sequence induction is then performed. Each drug targets a different part of the mechanism.

### Is antibiotic therapy routine for Mendelson syndrome?
No, antibiotics are not routinely used for chemical pneumonitis from aspiration of acid. Treatment is supportive, with oxygen, ventilation when needed and close monitoring. Antibiotics are reserved for a suspected secondary bacterial infection.