What is a Prolonged QTc Value?
A prolonged QTc interval is defined as ≥470 ms in men and ≥480 ms in women (99th percentile), though borderline prolongation begins at ≥450 ms in men and ≥460 ms in women after puberty. 1
Sex- and Age-Specific Thresholds
The definition of QTc prolongation varies by sex and age:
Adults (Post-Puberty)
- Borderline prolonged: ≥450 ms (men), ≥460 ms (women)
- Definitely prolonged: ≥470 ms (men), ≥480 ms (women)
- High-risk threshold: ≥500 ms (both sexes) - associated with increased risk of torsades de pointes 1
Children (Pre-Puberty)
- Normal range: Similar for boys and girls until adolescence
- Prolonged: Approximately ≥450 ms (98th percentile) in children <12 years 2
- Post-puberty: Gender differences emerge at 12-16 years, with females having 8-12 ms longer QTc values 1
Clinical Significance by QTc Duration
The risk stratification follows a clear pattern:
- QTc ≥480 ms: Generally considered clearly abnormal and warrants clinical attention 1
- QTc ≥500 ms: Associated with significantly increased risk of ventricular arrhythmias, specifically torsades de pointes 1
Critical Context for Interpretation
QTc prolongation alone is insufficient for diagnosis of Long QT Syndrome (LQTS). 1 You must evaluate:
- Clinical symptoms: Unexplained syncope (especially with exercise, swimming, loud noise), seizures, resuscitated cardiac arrest
- Family history: Unexplained sudden death in relatives
- T-wave morphology: Abnormal patterns suggest congenital LQTS (broad-based in LQT1, low-voltage/biphasic/notched in LQT2, late-onset in LQT3) 1
- Reversible causes: QT-prolonging medications, electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), hypothermia 1
Important Measurement Caveats
Automated QTc measurements are frequently inaccurate and require manual verification, particularly with: 1
- Irregular heart rhythms
- Widened QRS complexes
- Abnormal ST-T wave configurations
- Post-cardiac arrest (QT can remain prolonged for days) 1
Manual measurement technique: Use the tangent method in leads II or V5, measuring from QRS onset to where the tangent of the T-wave downslope intersects the baseline, then correct for heart rate using Bazett's or other formulas 1
Overlap Between Normal and Pathologic
There is significant overlap between the upper limit of normal QTc distribution and the lower limit in LQTS patients. 1 This means borderline values (450-480 ms range) require careful clinical correlation and may warrant exercise stress testing to unmask "concealed" LQTS, where resting QTc appears normal but becomes abnormally prolonged post-exercise (typically measured at 4 minutes recovery). 1