QTc Calculator (Corrected QT Interval)

Calculate corrected QT interval to assess cardiac arrhythmia risk using multiple validated formulas.

Calculate QTc

Or use heart rate below

What is QTc?

QTc (corrected QT) is the QT interval from the electrocardiogram adjusted for heart rate. The QT interval represents the time of ventricular depolarization and repolarization. Since QT varies with heart rate, the correction (QTc) allows standardized comparison across different heart rates. QTc prolongation is associated with increased risk of severe ventricular arrhythmias, especially Torsades de Pointes, and sudden cardiac death.

Correction Formulas

There are several formulas to calculate QTc, each with its advantages:

Bazett (1920)

QTc = QT / √RR

Most widely used formula, but overestimates QTc in tachycardia and underestimates in bradycardia

Fridericia (1920)

QTc = QT / ∛RR

Better for extreme heart rates (very high or low)

Framingham (1992)

QTc = QT + 0.154 × (1000 - RR)

Developed in general population, good for normal HR

Hodges (1983)

QTc = QT + 1.75 × (HR - 60)

Uses heart rate directly instead of RR interval

Result Interpretation

Normal QTc values differ between men and women:

Men

  • < 430 ms: Normal
  • 430-450 ms: Normal/Borderline
  • 450-470 ms: Borderline - Monitor
  • > 470 ms: Prolonged - Increased risk
  • > 500 ms: Very prolonged - High risk of Torsades

Women

  • < 450 ms: Normal
  • 450-460 ms: Normal/Borderline
  • 460-480 ms: Borderline - Monitor
  • > 480 ms: Prolonged - Increased risk
  • > 500 ms: Very prolonged - High risk of Torsades

Clinical Significance

Prolonged QTc can be:

  • Congenital: Congenital long QT syndrome (LQTS)
  • Acquired: Medications (antiarrhythmics, antibiotics, antipsychotics), electrolyte disturbances (hypokalemia, hypomagnesemia), myocardial ischemia
  • Risk: For every 10 ms increase above 500 ms, Torsades risk increases 5-7%

How to Measure QT and RR on ECG?

For accurate electrocardiogram measurements:

  • 1QT Interval: From the start of Q wave to the end of T wave (return to baseline)
  • 2RR Interval: From one R peak to the next R peak
  • 3Leads: Measure preferably in lead II or V5 (where T is most evident)
  • 4Average: Use the average of 3-5 consecutive beats
  • 5Rhythm: Ideally in regular sinus rhythm
  • 6T wave end: Intersection of descending tangent with baseline

Causes of Prolonged QTc

Congenital

  • •Long QT syndrome type 1 (LQTS1 - potassium channel)
  • •Long QT syndrome type 2 (LQTS2 - potassium channel)
  • •Long QT syndrome type 3 (LQTS3 - sodium channel)
  • •Romano-Ward syndrome
  • •Jervell and Lange-Nielsen syndrome

Acquired

  • •Medications: Antiarrhythmics (class IA and III), antibiotics (macrolides, fluoroquinolones), antipsychotics, antidepressants
  • •Electrolyte disturbances: Hypokalemia, hypomagnesemia, hypocalcemia
  • •Cardiac: Myocardial ischemia, cardiomyopathies, severe bradycardia
  • •Others: Hypothyroidism, stroke, liquid diets, anorexia nervosa

Related Topics

  • Long QT Syndrome
  • Torsades de Pointes
  • Ventricular Arrhythmias
  • Clinical Electrocardiography
  • Sudden Cardiac Death
  • Cardiotoxic Medications
  • Electrolyte Disturbances
  • Cardiovascular Risk

Limitations

This calculator provides an estimate of QTc based on validated mathematical formulas. It does not replace specialized medical evaluation or electrocardiogram interpretation by a cardiologist. QT measurement can be difficult in some conditions (atrial fibrillation, prominent U waves, bundle branch block). Different formulas may give slightly different results. In case of prolonged QTc, consult a cardiologist for complete evaluation and appropriate management.

Frequently Asked Questions

Which QTc formula should I use?

Bazett's formula is most widely used and recommended by guidelines, but overestimates in tachycardia and underestimates in bradycardia. For extreme heart rates (< 60 or > 100 bpm), consider using Fridericia or Framingham. In clinical practice, use the formula adopted by your service to maintain consistency in interpretation.

When is QTc considered dangerous?

QTc > 500 ms is considered high risk for Torsades de Pointes, regardless of gender. Between 470-500 ms in men or 480-500 ms in women indicates increased risk and requires evaluation. Risk increases progressively with higher values, being critical above 550 ms. Any increase > 60 ms above baseline is also concerning.

Which medications prolong QTc?

Main classes: antiarrhythmics (sotalol, amiodarone, quinidine), antibiotics (azithromycin, clarithromycin, fluoroquinolones), antipsychotics (haloperidol, ziprasidone, quetiapine), antidepressants (citalopram, escitalopram), antifungals (fluconazole), antihistamines (terfenadine), and methadone. Always check drug interactions in patients with borderline QTc.

What is Torsades de Pointes?

It is a characteristic polymorphic ventricular tachycardia, with QRS complexes that appear to 'twist' around the baseline. It is strongly associated with prolonged QTc. It can be self-limited (causing syncope) or degenerate into ventricular fibrillation and sudden death. Acute treatment includes IV magnesium, electrolyte correction, and heart rate increase (isoproterenol or pacemaker).

Do women have longer QTc than men?

Yes, women naturally have QTc about 10-20 ms longer than men due to hormonal differences (testosterone shortens QT, estrogen prolongs it). Therefore, reference values are different. Women also have higher risk of drug-induced QTc and higher incidence of drug-related Torsades de Pointes.

Scientific References

  1. 1. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370.
  2. 2. Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009;119(10):e241-e250.
  3. 3. Goldenberg I, Moss AJ, Zareba W. QT Interval: How to Measure It and What Is Normal. J Cardiovasc Electrophysiol. 2006;17(3):333-336.
  4. 4. Drew BJ, Ackerman MJ, Funk M, et al. Prevention of Torsade de Pointes in Hospital Settings. Circulation. 2010;121(8):1047-1060.
  5. 5. Priori SG, Blomström-Lundqvist C, Mazzanti A, et al. 2015 ESC Guidelines for the management of patients with ventricular arrhythmias. Eur Heart J. 2015;36(41):2793-2867.