# A nurse prepares to administer ondansetron 8 mg IV to a client receiving chemotherapy. Which baseline finding most warrants holding the dose?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=391807&lang=en  
> language: en  
> subject: Medical Emergencies  
> category: PA

## Question

A nurse prepares to administer ondansetron 8 mg IV to a client receiving chemotherapy. Which baseline finding most warrants holding the dose?

## Option

1. QTc 510 ms with serum potassium 3.3 and concurrent methadone — significant QT prolongation risk **✔ Correct answer**
2. Temperature 37.0°C
3. Hemoglobin 11.5 g/dL
4. Blood pressure 124/78

**Correct answer: 1**

## Explanation

Correct (2): Ondansetron prolongs QT. With baseline QTc ≥500 ms, electrolyte abnormalities (low K/Mg), and other QT-prolonging drugs (methadone), risk of torsades de pointes is high. Hold ondansetron, correct K/Mg, notify provider; alternative antiemetic (e.g., scopolamine, granisetron with lower QT effect, or aprepitant) may be considered. (1,3,4) Normal findings.

## In-depth explanation

Memory Tip 5-HT3 antagonist = "OND-ansetron" QT longer; granisetron less. Correct K/Mg before serotonin antagonists; constipation also common.

## Clinical scenario

Scenario 55-year-old female receiving chemotherapy; ondansetron 8 mg IV ordered for nausea. ECG: QTc 510 ms, K+ 3.3, also on methadone.

## Key concepts

- **5-HT3 antagonist** — Ondansetron, granisetron, palonosetron, dolasetron; serotonin receptor antiemetics.
- **QT prolongation** — Risk of torsades de pointes; thresholds QTc >500 ms or change >60 ms warrant action.
- **Drug-induced torsades** — Polymorphic VT from QT-prolonging drugs; treated with IV magnesium.

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