QT Interval Measurement and Interpretation
ECG Calibration and Measurement
- Standard ECG paper runs at 25 mm/second, where each small square (1 mm) represents 0.04 seconds (40 milliseconds) 3
- Each large square (5 mm) represents 0.20 seconds (200 milliseconds) 3
- At a heart rate of 60 beats per minute with an RR interval of 1.0 second, a normal uncorrected QT interval of 400 ms would span exactly 10 small squares 3
QT Interval Correction and Normal Values
- The American College of Cardiology recommends using Bazett's formula (QTc = QT/√RR) or other correction formulas for clinical interpretation 4, 1
- Normal QTc values are: Men: ≤450 ms, Women: ≤460 ms 4, 1, 2
- Children <12 years: approximately 450 ms regardless of gender 3, 2
Measurement Approach and Pitfalls
- The American Heart Association and American College of Cardiology recommend measuring QT interval from the earliest onset of the QRS complex to the latest offset of the T wave, ideally using global measurements across multiple leads 5, 6
- Measure in leads II, V5, or V6, using the longest value obtained 3, 7
- Avoid measuring in leads with prominent U waves; if unavoidable, use leads aVR or aVL where U waves are typically absent 7
- Failing to account for QRS prolongation in bundle branch blocks, which artificially lengthens the QT interval 4, 7
Clinical Interpretation
- Clinical interpretation requires rate correction using appropriate formulas (Fridericia is more accurate than Bazett at non-normal heart rates) 2
- Gender-specific thresholds after puberty (8-10 ms difference emerges at ages 12-16) 10, 2
- Recognition that athletes may have longer uncorrected QT intervals due to bradycardia, but normal QTc values 10, 2