How to Measure QT Interval on ECG
Complete step-by-step guide - from theory to clinical practice
The QT Interval in ECG Interpretation
For those learning to read ECGs, the QT interval is one of the most important — and often overlooked — parameters. Systematic ECG interpretation involves analyzing rhythm, rate, axis, P waves, PR intervals, QRS complexes, ST segments and T waves. The QT interval integrates T wave analysis and is fundamental because it represents the heart's "electrical vulnerability."
Even if you can already identify a myocardial infarction, atrial fibrillation, or bundle branch block, failing to measure the QT could mean missing a potentially fatal condition — especially in patients taking common medications like antibiotics, antiarrhythmics, or antipsychotics.
This guide was created for physicians, nurses, residents and students who want to master QT measurement and interpretation as an essential part of complete ECG reading.
Why include QT in your ECG analysis routine?
- It's the only parameter that alerts you to fatal arrhythmia risk from medications
- Complements ST segment and T wave analysis
- Differential diagnosis in unexplained syncope
- Essential in complete 12-lead ECG assessment
- Increasingly tested in residency and board exams
What is the QT Interval
The QT interval represents the total time of ventricular depolarization and repolarization on ECG. It starts at the beginning of QRS and ends at the end of T wave. Correct measurement is essential to identify risk of arrhythmias like Torsades de Pointes.
Step by Step: How to Measure QT
Step 1: Obtain quality ECG
Ensure the tracing is clear, with stable baseline and well-defined T waves. Standard speed should be 25 mm/s.
Step 2: Choose appropriate lead
Preferably use DII or V5-V6, where T waves are best visualized. Avoid leads with flat or biphasic T waves.
Step 3: Identify QRS complex onset
Mark the exact point where QRS complex deviates from baseline. This is the starting point of QT measurement.
Step 4: Identify T wave end (tangent method)
Draw a tangent line to the descending part of T wave. The point where this tangent crosses the baseline is the QT end. Important: do not include U wave in measurement.
Step 5: Measure QT interval in milliseconds
Measure distance from QRS onset to T wave end. At 25mm/s speed: 1mm = 40ms (1 small square). Measure at least 3 consecutive complexes and average.
Step 6: Measure RR interval
Measure distance between two consecutive R peaks. RR interval in seconds is used in correction formulas. At 25mm/s: RR(s) = RR(mm) × 0.04
Step 7: Apply correction formula
Calculate QTc using appropriate formula. QTc Calculator automates this calculation with 5 validated formulas.
QT Correction Formulas
Bazett: QTc = QT / √RR (ideal for HR 60-100 bpm)
Fridericia: QTc = QT / ∛RR (better for tachycardia)
Framingham: QTc = QT + 0.154 × (1 - RR) × 1000 (linear, versatile)
Hodges: QTc = QT + 1.75 × (FC - 60) (uses HR directly)
Dmitrienko: Advanced logarithmic formula
QTc Reference Values
Men
Normal: <440ms
Borderline: 440-460ms
Prolonged: >460ms
Women
Normal: <460ms
Borderline: 460-480ms
Prolonged: >480ms
Common Pitfalls in QT Measurement
- U Wave: Never include in QT measurement - confuses and overestimates value
- Wide QRS (LBBB/RBBB): Use Bogossian correction (subtracts 48.5% of excess QRS)
- Atrial fibrillation: Irregular RR - measure multiple QT and average at least 5 cycles
- Non-standard speed: Calibrate correctly if ECG is not at 25mm/s
QT-Prolonging Medications
CredibleMeds is the world reference for consulting medications that prolong the QT interval. Maintained by AZCERT (Arizona Center for Education and Research on Therapeutics), it categorizes drugs by risk level:
- Known Risk of Torsades de Pointes
- Possible Risk of Torsades de Pointes
- Conditional Risk of Torsades de Pointes
- Drugs to Avoid in Long QT Syndrome