From the Guidelines
The normal QT interval is typically considered to be less than 450 ms in men and less than 460 ms in women, as suggested by the European Society of Cardiology 1.
Key Points to Consider
- The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization, measuring the total time of electrical activation and recovery of the ventricles.
- The QT interval varies with heart rate, becoming shorter at faster heart rates and longer at slower heart rates, which is why it's often corrected for heart rate (QTc) using formulas like Bazett's or Fridericia's formula 1.
- A prolonged QT interval may indicate an increased risk of dangerous arrhythmias, particularly when the QTc is greater than 500 ms or when there is a significant increase from baseline 1.
- Conditions that can provoke torsades de pointes, such as hypokalaemia and extreme bradycardia, should be avoided in patients with drug-induced QT prolongation 1.
Clinical Implications
- A 12-lead ECG should be recorded at baseline, and the QT interval should be obtained in all patients, with consideration for repeated ECGs in those with a history of QT prolongation or relevant cardiac disease 1.
- Treatment discontinuation or alternative regimens should be considered if the QTc is greater than 500 ms, if there is significant QTc prolongation, or if dysrhythmias are encountered 1.
- The QT interval should be evaluated before initiation of treatment and during titration of dose, especially in elderly patients or those with cardiac risk factors 1.
From the Research
Normal QT Interval
The normal QT interval is not explicitly stated in the provided studies. However, the studies discuss the corrected QT interval (QTc) and its relation to the risk of torsades de pointes.
Corrected QT Interval (QTc)
- The QTc interval is a measure of the efficiency of repolarization of the left ventricle, and excessive prolongation can result in a potentially fatal ventricular arrhythmia known as torsades de pointes 2.
- The normal QTc interval is typically considered to be less than 440 ms for males and less than 460 ms for females, although the exact values are not provided in the studies.
- A QTc interval of 470-500 ms for males or 480-500 ms for females is considered borderline, and dose reduction or discontinuation of the offending drug should be considered if the QTc interval increases by 60 ms or more from pretreatment values 3.
- A QTc interval of 500 ms or more is considered prolonged and requires careful monitoring and potential discontinuation of the offending drug 3, 2.
Factors Influencing QT Interval
- The QT interval is influenced by heart rate, autonomic factors, electrolyte levels, gender, and age 4.
- Certain drugs, such as antiarrhythmic, antibacterial, and psychotropic agents, can prolong the QT interval and increase the risk of torsades de pointes 4, 5.
- Hypokalemia, hypomagnesemia, and bradycardia can also contribute to QT interval prolongation 3.