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QTc normalises the QT interval for heart rate. Prolonged QTc (>450ms men, >470ms women) signals risk of torsades de pointes, a potentially fatal arrhythmia.
Formel
QTc (Bazett) = QT (ms) / √RR (seconds); QTc (Fridericia) = QT / ∛RR; Normal < 450 ms (men), < 460 ms (women)
- QT
- QT interval duration (Milliseconds)
- RR
- RR interval (heart rate) (Seconds)
- QTc
- Corrected QT interval (Milliseconds)
Trinn-for-trinn guide
- 1Bazett: QTc = QT / √RR (RR in seconds)
- 2Fridericia: QTc = QT / RR^(1/3) — more accurate
- 3RR (sec) = 60 / HR (bpm)
- 4Critical: QTc > 500ms requires urgent review
Løste eksempler
Inndata
QT = 380ms, HR = 60 bpm (RR = 1.0s)
Resultat
QTc (Bazett) = 380ms — Normal
Ofte stilte spørsmål
Why correct QT for heart rate?
QT interval shortens with faster heart rate. Raw QT depends on HR. Correction isolates intrinsic QT duration. Allows comparison across patients/time.
What does prolonged QTc mean?
Risk factor for torsades de pointes (dangerous arrhythmia). Causes: medications (antipsychotics, antibiotics, antiemetics), electrolyte abnormalities (K, Ca, Mg), congenital (Long QT syndrome).
When should I check QTc?
Baseline ECG before starting QT-prolonging medications. Repeat if adding second drug, or symptoms (syncope, palpitations). Monitor electrolytes (K, Mg critical).
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