Ondansetron in Prehospital Care
Ondansetron (brand name Zofran) is a serotonin 5-HT3 receptor antagonist widely used in EMS for the management of nausea and vomiting. It is one of the most frequently administered medications in prehospital care because nausea accompanies a wide range of conditions, from motion sickness and gastroenteritis to myocardial infarction and head injury.
Mechanism of Action
Ondansetron blocks serotonin (5-HT3) receptors in the chemoreceptor trigger zone (CTZ) in the brain and in the vagal nerve terminals in the GI tract. By blocking these receptors, it prevents the signals that trigger the vomiting reflex. Unlike older antiemetics such as promethazine (Phenergan), ondansetron does not cause significant sedation, dystonic reactions, or respiratory depression, making it a safer choice in the prehospital setting.
Dosing and Routes
- IV: 4 mg administered slowly over 2 to 5 minutes. May repeat once after 15 minutes if needed.
- IM: 4 mg in the deltoid or lateral thigh.
- ODT (orally disintegrating tablet): 4 mg placed on the tongue, where it dissolves without water. This is the preferred route for patients with mild nausea who have functioning gag reflexes, as it avoids the need for IV access.
- Pediatric: 0.1 mg/kg IV (maximum 4 mg) for children over 6 months of age. Weight-based dosing is essential.
Clinical Considerations
While ondansetron is generally safe, providers should be aware of several clinical considerations:
- QT prolongation: Ondansetron can prolong the QT interval. Use caution in patients with known QT prolongation, those taking other QT-prolonging medications, or those with electrolyte abnormalities.
- Headache: The most common side effect, occurring in approximately 10 to 15% of patients.
- Constipation: More relevant in hospital settings with repeated dosing.
- Masking symptoms: Treating nausea without addressing the underlying cause can mask important clinical findings. Always investigate the cause of nausea and vomiting rather than simply treating the symptom.
When to Use Ondansetron in EMS
Ondansetron is appropriate for nausea and vomiting associated with virtually any condition encountered in EMS. However, consider whether treating the underlying cause would resolve the nausea: a hypoglycemic patient may stop vomiting after dextrose, and a patient in pain may stop vomiting after analgesic administration. Use ondansetron as an adjunct to treating the underlying condition, not as a substitute for a thorough assessment.