Question 1

What's the difference between Bazett and Fridericia QTc formulas?

Accepted Answer

**Bazett** (`QT / √RR`) is the most commonly used and reported on most ECGs, but it overestimates QTc at high heart rates and underestimates at low heart rates. **Fridericia** (`QT / ∛RR`) is more accurate when HR is outside 60–100 bpm and is preferred for serial QTc tracking, especially in tachycardic patients.

Question 2

At what QTc is torsades de pointes a concern?

Accepted Answer

- **<440 ms**: normal
- **440–460**: borderline
- **460–500**: prolonged
- **>500**: high risk for torsades de pointes

A QTc >500 ms (or an increase of >60 ms from baseline) generally warrants holding QT-prolonging medications, replacing electrolytes (K, Mg), and considering telemetry.

Question 3

Which medications are most likely to prolong the QT interval?

Accepted Answer

Common inpatient QT-prolongers include certain antiarrhythmics (amiodarone, sotalol, dofetilide), antibiotics (azithromycin, fluoroquinolones, fluconazole), antipsychotics (haloperidol, quetiapine, ziprasidone), antiemetics (ondansetron, droperidol), and methadone. CredibleMeds maintains the authoritative classification (known / possible / conditional risk).

Question 4

Does a normal QTc rule out drug-induced torsades risk?

Accepted Answer

No. Risk also depends on electrolyte derangements (hypokalemia, hypomagnesemia, hypocalcemia), bradycardia, female sex, structural heart disease, and concurrent use of multiple QT-prolonging drugs. A patient with a normal baseline QTc on a single agent can still develop torsades when conditions stack.

QTc + Drug Check

References

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