Normal QTc Interval Values
Normal QTc values are ≤450 ms for men and ≤460 ms for women, with values above these thresholds considered borderline prolonged (451-480 ms for men, 461-480 ms for women) and values >480 ms considered prolonged for both sexes. 1
QTc Normal Ranges by Gender
According to the American College of Cardiology, American Heart Association, and Heart Rhythm Society guidelines, QTc intervals are categorized as follows:
| Category | Men | Women |
|---|---|---|
| Normal | ≤450 ms | ≤460 ms |
| Borderline prolonged | 451-480 ms | 461-480 ms |
| Prolonged | >480 ms | >480 ms |
| High-risk prolongation | >500 ms | >500 ms |
| Abnormally short | ≤390 ms | ≤390 ms |
QT Interval Correction Formulas
The choice of correction formula is critical for accurate QTc measurement:
- Bazett's formula (QTcB = QT/√(RR)) is recommended only for heart rates between 60-80 bpm
- Fridericia's formula (QTcF = QT/∛(RR)) is preferred for heart rates <60 or >80 bpm
Using the wrong formula can lead to significant errors in QTc assessment. Bazett's formula, despite being the most commonly used, tends to overcorrect at high heart rates and undercorrect at low heart rates 1, 2. This can result in falsely prolonged QTc values, particularly in tachycardic patients 3.
Clinical Implications and Pitfalls
- QTc prolongation risk: Values >480 ms are associated with increased risk of torsades de pointes and sudden cardiac death
- High-risk prolongation: QTc >500 ms requires immediate attention regardless of symptoms or family history 1
- Context matters: First-time ECGs obtained after syncopal episodes should be interpreted cautiously, particularly with borderline QT intervals 4
Common Pitfalls to Avoid:
- Single measurement reliance: Approximately one-third of pediatric patients in emergency settings may exhibit QTc values ≥440 ms that normalize on follow-up 4
- Formula selection errors: Using Bazett's formula outside the 60-80 bpm range can lead to incorrect QTc assessment 1, 2
- Measurement technique inconsistency: The American Heart Association recommends:
- Measuring from the beginning of QRS to the end of T-wave
- Using leads with well-defined T-wave ends (typically II, V3, or V5)
- Using the same lead for serial measurements
- Using the same recording system and reader when possible 1
Special Populations
- Women: Typically have longer baseline QTc intervals than men and may experience greater drug-induced QT prolongation 1
- Athletes: Often have longer uncorrected QT intervals due to lower resting heart rates, but QTc is usually within normal limits 1
- Elderly patients: Require more cautious interpretation of QTc values 1
- High altitude exposure: Can lengthen a healthy adult's QTc interval by more than 20 ms 5
Practical QTc Estimation
For quick clinical estimation when Fridericia's formula is preferred but not readily available, QTc-Fridericia can be approximated by:
- Adding 5% to the uncorrected QT for each 10 beats/min increase in heart rate above 60 bpm
- Subtracting 5% from the uncorrected QT for each 10 beats/min decrease in heart rate below 60 bpm 3
This provides a practical method for rapid QTc assessment when precise calculation tools are unavailable.