The Corrected QT Interval (QTc) is a clinical measurement used in cardiology to account for the influence of heart rate on the QT interval observed on an electrocardiogram (ECG). The QT interval represents the time between the onset of ventricular depolarization (Q wave) and the end of ventricular repolarization (T wave). Because the QT interval varies with heart rate, the corrected QT (QTc) provides a standardized value that allows meaningful comparison across patients and conditions.
QTc is crucial in evaluating the risk of life-threatening arrhythmias, especially Torsades de Pointes, ventricular tachycardia, and sudden cardiac death. It is commonly used in clinical practice to monitor patients on medications that can prolong repolarization, as well as in inherited cardiac channelopathies.
Normal Ranges / Interpretation
Normal QTc values differ slightly between men and women, but in general:
| Population | Normal QTc (ms) | Borderline | Prolonged |
|---|---|---|---|
| Men | < 440 | 440–460 | > 460 |
| Women | < 460 | 460–480 | > 480 |
| High risk (both sexes) | > 500 ms is strongly associated with increased risk of Torsades de Pointes | ||
A shortened QTc (< 350 ms) is also clinically significant, associated with short QT syndrome and increased risk of atrial and ventricular arrhythmias.
Calculation Methods
Several formulas have been developed to correct the QT interval for heart rate:
- Bazett’s Formula: QTc = QT / √RR
Most commonly used, but overestimates QTc at high heart rates and underestimates at low heart rates. - Fridericia’s Formula: QTc = QT / (RR)1/3
Improved accuracy at extremes of heart rate compared to Bazett’s. - Framingham Formula: QTc = QT + 0.154 × (1 − RR)
Derived from population data, less widely used but more precise at moderate rates. - Hodges Formula: QTc = QT + 1.75 × (HR − 60)
Simpler calculation, sometimes applied in research or automated systems.
Where QT is measured in milliseconds and RR is the interval between consecutive R waves in seconds.
Clinical Significance
QTc measurement is central to cardiovascular risk assessment. Its clinical significance includes:
- Drug safety monitoring: Many antiarrhythmics, antibiotics (e.g., macrolides, fluoroquinolones), antipsychotics, and antidepressants prolong QTc. Regular monitoring helps prevent arrhythmic complications.
- Inherited syndromes: Long QT syndrome (LQTS) and short QT syndrome are genetic channelopathies associated with sudden cardiac death, especially in young individuals.
- Risk stratification: QTc prolongation is a marker of increased risk for ventricular arrhythmias and is used in making decisions about implantable cardioverter-defibrillators (ICDs).
- Critical care: Electrolyte imbalances (hypokalemia, hypomagnesemia, hypocalcemia) that prolong QTc require rapid identification and treatment.
- Regulatory relevance: QTc monitoring is mandated in drug development trials to ensure cardiac safety of new medications.
Indications for Use
QTc should be measured and interpreted in the following situations:
- Patients presenting with syncope, palpitations, or seizures of unknown origin.
- Routine ECG evaluation in patients on QT-prolonging medications.
- Screening for congenital long QT syndrome, especially with family history of sudden cardiac death.
- Monitoring during electrolyte disturbances or metabolic crises.
- Hospitalized or critically ill patients receiving multiple medications with arrhythmogenic potential.
Limitations
While valuable, QTc has limitations:
- Formula variability: Different correction methods yield different results, and none is perfect across all heart rates.
- Measurement error: Manual measurement of the QT interval is prone to interobserver variability. Automated ECG systems may miscalculate QT in abnormal waveforms.
- Heart rate dependence: Despite correction, QTc remains less reliable in extreme tachycardia or bradycardia.
- Clinical context required: A prolonged QTc does not always indicate pathology, and a normal QTc does not rule out arrhythmic risk.