Q: Can hypokalaemia alone cause TdP without QT-prolonging drugs?

Hypokalaemia (K+ <3.5 mmol/L) prolongs repolarisation and can trigger TdP even in the absence of QT-prolonging drugs, especially in patients with underlying heart disease or female sex. Maintaining serum K+ ≥4.5 mmol/L is the single most important corrective intervention when using QT-prolonging therapies.

Question 1

Which QTc correction formula does the tool use?

Accepted Answer

The tool primarily uses Bazett (QTcB = QT / √RR), the most widely used formula in both clinical practice and CredibleMeds. However, Bazett overcorrects at high heart rates. For patients with tachycardia, consider using Fridericia (QTcF = QT / ∛RR) or Framingham, which the tool also displays.

Question 2

Is there a safe QTc threshold for prescribing QT-prolonging drugs?

Accepted Answer

A QTc <450 ms (men) or <460 ms (women) is generally safe. QTc 460–500 ms: use QT-prolonging drugs with caution, correct electrolytes, monitor ECG. QTc >500 ms: avoid all additional QT-prolonging drugs, correct reversible causes, and consider cardiology consultation.

Question 3

Can hypokalaemia alone cause TdP without QT-prolonging drugs?

Accepted Answer

Hypokalaemia (K+ <3.5 mmol/L) prolongs repolarisation and can trigger TdP even in the absence of QT-prolonging drugs, especially in patients with underlying heart disease or female sex. Maintaining serum K+ ≥4.5 mmol/L is the single most important corrective intervention when using QT-prolonging therapies.