From the Guidelines
The normal QTc interval is generally considered to be less than 430 ms in males and less than 450 ms in females. The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization in the heart's electrical cycle, and its measurement is crucial because prolonged QT intervals can predispose individuals to dangerous arrhythmias like Torsades de Pointes 1. This measurement is affected by heart rate, so clinicians often use a corrected QT interval (QTc) which adjusts the measurement based on heart rate using formulas like Fridericia's, recommended by the FDA 1.
Key Considerations
- The QT interval varies with heart rate and has to be adjusted by the RR interval to calculate QTc 1.
- A normal QTc interval is essential to prevent potentially lethal ventricular arrhythmias, and its prolongation can be induced by various conditions, including the administration of QT-prolonging cardioactive drugs 1.
- Monitoring the QT interval is essential when prescribing medications that can prolong the QT interval, especially in patients with other risk factors for QT prolongation 1.
Clinical Implications
- A 12-lead ECG should be recorded, and the QT interval, corrected for heart rate, should be obtained in all patients at baseline 1.
- Patients with a history of QT prolongation or relevant cardiac disease should be monitored by repeated 12-lead ECG 1.
- Conditions known to provoke torsade de pointes, especially hypokalaemia and extreme bradycardia, should be avoided in patients with drug-induced QT prolongation 1.
From the Research
Normal Range of QT Interval
The normal range of the QT interval is a topic of interest in cardiology, and several studies have investigated this issue.
- The QT interval extends from the beginning of the QRS complex to the end of the T wave, and its measurement is typically performed in lead II of an electrocardiogram (ECG) 2.
- The corrected QT interval (QTc) is used to account for variations in heart rate, and several formulas have been developed to calculate QTc, including Bazett's formula, Fridericia's formula, and others 3, 4.
QT Interval Correction Formulas
Different correction formulas can yield varying results for the upper normal limits of QTc.
- A study comparing four QT correction formulas (Bazett, Fridericia, Framingham, and Hodges) found that the upper normal limits of QTc were: Bazett 483 ms, Fridericia 460 ms, Framingham 457 ms, and Hodges 457 ms 3.
- Another study established age- and sex-specific criteria for prolonged QTc using formulas that minimize QTc bias at the upper normal limits, with the following criteria: Age < 40 years, men 430 ms, women 440 ms; Age 40 to 69, men 440 ms, women 450 ms; Age ≥ 70 years, men 455 ms, and women 460 ms 4.
Clinical Implications
The normal range of the QT interval has important clinical implications, as prolongation of the QT interval can increase the risk of ventricular arrhythmias.
- A study found that hypokalemia was associated with lengthening of the QT interval in psychiatric patients, highlighting the importance of screening for hypokalemia in this population 5.
- Another study found that the QT interval was often prolonged in patients with primary renal hypokalaemia-hypomagnesaemia, confirming that potassium and magnesium depletion can prolong the duration of the action potential of the cardiomyocyte 6.