QTc Calculator

Understanding QT Interval

The QT interval spans from the onset of the QRS complex (ventricular depolarization) to the end of the T wave (ventricular repolarization) on an electrocardiogram. Measuring this interval matters because its lengthening correlates with increased risk of polymorphic ventricular tachycardia and sudden cardiac death.

A key challenge is that QT duration shortens as heart rate increases and lengthens as heart rate decreases. An interval acceptable at 100 bpm might indicate danger at 60 bpm. This rate dependency necessitates mathematical correction to produce a standardized value—the QTc—suitable for clinical comparison across patients with different rhythms.

QTc Correction Formulas

Four main equations exist for QT correction. Bazett's method, published in 1920, remains most widely used but tends to overcorrect fast rates and undercorrect slow rates. Fridericia, Framingham, and Hodges formulas were developed to reduce these systematic errors.

• QT — Duration of QT interval in milliseconds (from start of Q wave to end of T wave)
• RR — Duration of RR interval in seconds (between successive R wave peaks)
• Heart rate — Beats per minute; used to calculate RR interval if not directly measured
• QTc — Corrected QT interval in milliseconds, adjusted for heart rate

ECG Measurement Technique

Accurate QT measurement requires careful electrode selection. Use the lead showing the clearest, most prolonged QT interval—typically leads II, V2, or V3. Identify the Q wave's first deflection downward (or the R wave's onset if no Q is present) and follow the baseline until the T wave returns to it.

Modern ECG machines print at 25 mm/s paper speed: each small square (1 mm) equals 40 milliseconds, and each large square (5 mm) equals 200 milliseconds. Calipers are preferable to rulers for precision. When the T wave merges with U waves, measure to the nadir before the U wave rise. For irregular rhythms, average multiple complexes.

QT Prolongation and Arrhythmia Risk

A prolonged QTc—exceeding 450 ms in males or 470 ms in females—signals prolonged ventricular repolarization and increased susceptibility to early afterdepolarizations. These abnormal depolarizations can trigger torsades de pointes, a twisting polymorphic ventricular tachycardia that may degenerate into ventricular fibrillation.

Risk multiplies when QT prolongation coexists with hypokalaemia, hypomagnesaemia, female sex, bradycardia, or medications that block cardiac potassium channels. Common culprits include macrolide antibiotics, fluoroquinolones, antipsychotics, antiemetics, and Class IA and III antiarrhythmics. Always review concomitant medications and electrolyte status before prescribing additional QT-prolonging agents.

Clinical Considerations for QTc Assessment

Reliable QTc interpretation requires attention to measurement technique, formula choice, and individual risk factors.

1. Choose the appropriate correction formula — Bazett's formula dominates clinical practice but systematically overcorrects at heart rates above 100 bpm and undercorrects below 60 bpm. Fridericia performs better at extremes of rate. For borderline results, comparing multiple formulas may clarify whether prolongation is genuine or artefactual.
2. Verify electrolyte and medication status — Hypokalaemia and hypomagnesaemia independently prolong QT and potentiate arrhythmia risk. Similarly, drug interactions—especially combining QT-prolonging agents—elevate risk nonlinearly. Always obtain serum potassium and magnesium before attributing QT prolongation to cardiac pathology alone.
3. Account for sex and heart rate variation — Women have longer baseline QT intervals; QTc thresholds of 450 ms (men) versus 470 ms (women) reflect this. Bradycardic rhythms and pauses require individual assessment. A single QTc measurement at a specific heart rate may not capture risk during sleep or exercise, so consider trends across serial ECGs.
4. Recognize congenital versus acquired prolongation — Congenital long QT syndrome (LQTS) presents with marked prolongation, often >500 ms, and family history of sudden death. Acquired prolongation typically develops after medication initiation or electrolyte derangement. Genetic testing is warranted when LQTS is suspected and family screening may prevent sudden death in relatives.