Normal QT Interval on ECG
The normal corrected QT interval (QTc) is <450 ms in men and <460 ms in women after puberty, with lead V5 or lead II being the preferred leads for measurement. 1
Normal QTc Values by Sex
The QT interval shows important sex-dependent differences that emerge after puberty:
- Men: Upper normal limit is <450 ms 1
- Women: Upper normal limit is <460 ms 1
- Both sexes: QTc ≥500 ms is considered highly abnormal and associated with increased risk of torsades de pointes 1
The 99th percentile values (definitively abnormal) are 470 ms for males and 480 ms for females. 2, 1
Before puberty, QTc values are similar in boys and girls, but after adolescence women develop longer QTc values due to hormonal changes. 1 This gender difference diminishes after age 40 and practically disappears in older adults. 3
Best Lead for QT Measurement
Lead V5 or lead II should be used for manual QT measurement, as these leads typically provide:
- The longest absolute QT interval 1
- Good reflection of left ventricular activation 1
- Larger T waves that are more readily measured 1
- Better separation of T waves from U waves (especially lead II) 4
Lead V3 is also highly accurate, with research showing 94% overall accuracy for predicting QT prolongation compared to 89% for lead II. 5 Anteroseptal leads (V2 or V3) provide the closest approximation to the maximum QT interval across all leads. 6
Critical caveat: Always use the same lead for serial measurements in a given patient over time to ensure accurate comparison. 2, 7
Measurement Technique
Manual Measurement Using the Tangent Method
- Identify the baseline (TP line) 1
- Measure from the onset of the QRS complex to the end of the T wave 1, 2
- Define the end of the T wave by drawing a tangent at the maximum slope of the T wave's downslope where it intersects the baseline 1, 2
- Average 3 consecutive QT intervals and their preceding R-R intervals 1
- Apply Bazett's formula: QTc = QT/√RR (both in seconds) 1, 7
Important Measurement Pitfalls
- Do not include U waves in the QT measurement, as this falsely prolongs the interval 1, 2
- Automated measurements can be inaccurate with irregular rhythms, widened QRS, or abnormal ST-T wave configurations 1
- Bazett's formula has limitations: overcorrects at heart rates <60 bpm and undercorrects at rates >100 bpm 1, 2
- Serial measurements should use the same ECG machine to avoid variability from different algorithms 1
Clinical Context
The QT interval represents ventricular depolarization and repolarization, corresponding to the myocardial action potential. 1 While these normal values provide guidance, clinical interpretation requires consideration of:
- Heart rate: Bazett's formula is most accurate between 60-100 bpm 1
- QRS duration: Bundle branch blocks artificially prolong QT; consider measuring JT interval instead 2
- T wave morphology: Abnormal T wave patterns may suggest congenital long QT syndrome even with borderline QTc values 1
- Clinical symptoms: Unexplained syncope, seizures, or family history of sudden death warrant investigation even with normal QTc 1