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Introduction Ondansetron is a highly selective serotonin 5-HT3 receptor antagonist. In emergency medicine it represents a modern, safe alternative to older antiemetics (metoclopramide, thiethylperazine), particularly due to the absence of extrapyramidal side effects and high efficacy against severe vomiting.

1. Drug Identification

Active substance: Ondansetronum (as ondansetron hydrochloride dihydrate).
Trade names in Slovakia: Ondansetron Accord, Zofran, Emeset.
ATC classification: A04AA01 (Serotonin 5-HT3 antagonists).
Dosage form in EMS: Injection solution 4 mg/2 ml or 8 mg/4 ml (ampule).
Pharmacological group: Antiemetic.

2. Mechanism of Action (Pharmacodynamics) Ondansetron blocks type 3 serotonin receptors at two key levels:

Peripheral effect: Blockade of 5-HT3 receptors on vagal afferent endings in the GI tract. Serotonin released from the small-intestine mucosa (e.g., due to irritation or ischemia) can no longer stimulate the vagal vomiting reflex.
Central effect: Blockade of 5-HT3 receptors in the area postrema (chemoreceptor trigger zone) in the medulla.
Result: Strong suppression of nausea and vomiting without affecting dopamine receptors, eliminating the risk of facial muscle spasms.

3. Hemodynamic Effects – IN DETAIL

BP and HR: The drug has no direct effect on blood pressure or heart rate at standard doses.
Cardiac interval: A significant risk is QT prolongation. Ondansetron can affect myocardial repolarization, increasing the risk of the malignant arrhythmia Torsades de Pointes (TdP).
Safety: Extra caution is needed in patients with electrolyte imbalances (hypokalemia, hypomagnesemia), which is common in vomiting patients.

4. Pharmacokinetics (from a Paramedic’s Perspective)

Routes of administration: IV (slow injection), IM, and in hospital also PO (dispersible tablets).
Onset of action: Very fast with IV administration (within 5–10 minutes).
Duration of effect: 4–8 hours.
Metabolism: Extensive hepatic metabolism (CYP450 enzymes).

5. Indications in Prehospital Practice

Symptomatic treatment of persistent vomiting: Particularly in acute gastroenteritis, where rapid dehydration is a risk (excellent profile in children).
Oncology conditions: Nausea from cytotoxic chemotherapy or radiotherapy during transport.
First-line treatment failure: If metoclopramide or thiethylperazine fails to suppress symptoms.

6. Dosing in EMS

Adult: 4 mg to 8 mg IV (slow administration over 2–5 minutes).
Pediatrics: 0.15 mg/kg body weight IV (usual dose up to a maximum of 4 mg).
Scope of practice: Physician (RLP). A basic-crew paramedic (RZP) — in Slovakia, ondansetron is increasingly included in standard treatment protocols for crews without a physician, due to its safety.

7. Contraindications

Absolute: Hypersensitivity to ondansetron or other 5-HT3 antagonists, concurrent administration with apomorphine (cases of severe hypotension and loss of consciousness have been reported).
Relative: Congenital long QT syndrome, congestive heart failure, bradyarrhythmias.

8. Adverse Effects

Common: Headache (frequently reported by patients after administration), a sensation of warmth or flushing, constipation.
Serious: QT prolongation, transient visual disturbances (with rapid IV administration).

9. Drug Interactions in EMS

Apomorphine: Strictly contraindicated combination.
QT-prolonging agents: (e.g., amiodarone, sotalol, certain neuroleptics) — ECG monitoring is required.
Serotonergic agents (SSRIs, SNRIs): Risk of serotonin syndrome (confusion, tachycardia, hyperreflexia).