From the Guidelines
A normal QTc interval is generally considered to be less than 450 milliseconds for men and less than 460 milliseconds for women, as suggested by the 2017 European Society of Cardiology (ESC) position paper 1. The QT interval on an electrocardiogram (ECG) typically ranges from 350 to 440 milliseconds in adults, though this can vary slightly between men and women. Women generally have slightly longer QT intervals than men. Since the QT interval varies with heart rate, clinicians often use the corrected QT interval (QTc), which adjusts the measurement based on heart rate. Some key points to consider when evaluating the QT interval include:
- The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization, essentially measuring the electrical activity involved in contraction and relaxation of the ventricles.
- Various factors can affect the QT interval, including certain medications (such as some antiarrhythmics, antipsychotics, and antibiotics), electrolyte abnormalities (particularly low potassium, magnesium, or calcium), and congenital conditions like Long QT Syndrome.
- QT intervals longer than the normal values are considered prolonged and may indicate an increased risk of dangerous arrhythmias, particularly torsades de pointes.
- The most recent and highest quality study, the 2017 ESC position paper 1, provides guidance on the assessment of the QT interval and associated risk factors for QT prolongation, and suggests that QTc intervals ≥450 ms in men and ≥460 ms in women are considered to be of particular concern. It is essential to note that the measurement of the QT interval should be made from the beginning of the QRS complex to the end of the T wave, and that the end of the T wave can be difficult to determine, especially if the T wave is notched or if there are prominent U waves that are fused with the T wave, as discussed in the 2009 American Heart Association (AHA) scientific statement 1 and the 2014 European Heart Journal article 1. In clinical practice, it is crucial to visually validate QT-interval prolongation reported by a computer algorithm, as emphasized in the 2009 AHA scientific statement 1, and to consider the clinical context and potential causes of QT prolongation, such as electrolyte abnormalities or medication use, as highlighted in the 2010 Circulation article 1.
From the Research
Normal QT Interval
The normal QT interval is a topic of interest in the medical field, particularly in relation to the use of antipsychotic medications and other drugs that can affect the heart's electrical activity.
- A normal QT interval is typically considered to be between 300-440 milliseconds for men and 300-460 milliseconds for women 2.
- However, the definition of a normal QT interval can vary depending on the source and the specific population being studied.
Factors Affecting QT Interval
Several factors can affect the QT interval, including:
- Genetics: Individual genetic factors can play a significant role in determining the QT interval 3.
- Age: Age is also a significant factor, with the QT interval tend to increase with age 3.
- Sex: Sex can also affect the QT interval, with women tend to have longer QT intervals than men 3.
- Electrolyte imbalances: Electrolyte imbalances, such as hypokalemia and hypomagnesemia, can also affect the QT interval 2.
- Medications: Certain medications, including antipsychotics and anti-arrhythmic drugs, can also affect the QT interval 4, 5, 6, 2.
QT Interval Prolongation
QT interval prolongation is a condition where the QT interval is longer than normal, which can increase the risk of torsades de pointes and other arrhythmias.
- Antipsychotic medications, such as clozapine and olanzapine, can cause QT interval prolongation 6.
- Anti-arrhythmic drugs, such as amiodarone and sotalol, can also cause QT interval prolongation 5.
- The risk of QT interval prolongation can be increased by combining multiple medications that affect the QT interval 2.