Normal QTc Interval for Women
The normal QTc interval for women is less than 460 ms, with values ≥460 ms considered prolonged. 1, 2
Gender-Specific Normal Values
Women have a longer normal QTc threshold than men (women <460 ms vs. men <450 ms), reflecting a physiological gender difference that emerges during adolescence 1
The gender difference ranges from 6-10 ms in older adults and 12-15 ms in younger adults, appearing when testosterone shortens the rate-adjusted QT in adolescent boys while girls experience minimal change 1
After age 40, the gender difference becomes minimal and practically disappears in elderly populations, with women ≥70 years having the same upper limit of 460 ms 3
Abnormal QTc Thresholds in Women
Prolonged QTc
QTc ≥460 ms is considered prolonged in women and warrants clinical attention 1, 2
QTc >480 ms represents the 99th percentile and should be considered abnormally prolonged in postpubertal women 3
QTc >500 ms is highly abnormal regardless of gender and requires immediate action due to significant arrhythmia risk 2, 3
Short QTc
Clinical Risk Stratification
The American College of Cardiology provides severity grading for QTc prolongation 2:
- Grade 1: 450-480 ms
- Grade 2: 481-500 ms
- Grade 3: >500 ms
A QTc >500 ms or an increase >60 ms from baseline carries increased risk for Torsades de Pointes, a potentially fatal ventricular arrhythmia 2, 3
Important Measurement Considerations
QT Correction Formulas
Bazett's formula (QTc = QT/√RR) overcorrects at high heart rates (>80 bpm) and undercorrects at low heart rates (<60 bpm), potentially leading to false prolongations 1, 4, 5
Fridericia's formula (QTc = QT/∛RR) is more accurate, especially at higher heart rates, and is recommended by the FDA 2, 5
For optimal accuracy, measure QT in leads II, V3, or V5, using the longest value obtained 2
Special Populations
In pregnant women, QTc remains in the normal range but may be slightly longer than non-pregnant controls, though still <460 ms 6
In children <12 years, the 98th percentile is approximately 450 ms without gender differentiation, with the 8 ms gender difference appearing in the 12-16 year age group 1, 2
Common Pitfalls to Avoid
Do not use Bazett's formula at heart rates >80 bpm or <60 bpm as it produces unreliable corrections that may falsely suggest prolongation 4, 5
Account for QRS duration in bundle branch blocks or conduction delays by using the JT interval (QT minus QRS duration) with JT-specific normal values 1, 2
Recognize that time of day influences QT intervals when comparing sequential tracings 1, 3
Women are at higher risk for malignant arrhythmias in long QT syndrome compared to men, making accurate QTc assessment particularly important 1
Electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) can prolong QTc and should be corrected before attributing prolongation to other causes 2