From the Guidelines
A normal QTc interval is typically considered to be less than 450 ms in men and less than 460 ms in women. The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization in the cardiac cycle, and it must be corrected for heart rate (hence "QTc") because the QT interval naturally shortens at faster heart rates 1. Several formulas exist for this correction, with Bazett's formula being the most commonly used in clinical practice, though it may overcorrect at high heart rates and undercorrect at low heart rates.
Key Points to Consider
- QTc values above 450 ms in men and 460 ms in women are generally considered prolonged, with values exceeding 500 ms raising significant concern for risk of dangerous arrhythmias like Torsades de Pointes 1.
- The QTc is important to monitor when starting medications known to prolong the QT interval, in patients with congenital long QT syndromes, and in cases of electrolyte abnormalities, particularly hypokalemia, hypomagnesemia, and hypocalcemia, which can all prolong the QT interval and increase arrhythmia risk 1.
- It is essential to note that the risk of Torsades de Pointes increases exponentially with the length of the QTc interval, and a QTc >500 ms is associated with a 2- to 3-fold higher risk for TdP 1.
Clinical Implications
- Monitoring the QT/QTc intervals alone may be inadequate to accurately predict TdP, and other premonitory ECG signs such as T-wave alternans and T-U-wave distortion should also be considered 1.
- In patients with drug-induced LQTS, the QT interval may be prolonged during normal sinus rhythm without adverse effect, but after a pause, QT-interval prolongation and T-U deformity become markedly exaggerated, and TdP is triggered 1.
From the Research
Normal QTc Interval
The normal QTc interval is a topic of interest in various medical studies.
- A study published in the Journal of Electrocardiology in 2004 2 compared different QT correction formulae and found that the upper normal limits of corrected QTc were:
- Bazett: 483 ms
- Fridericia: 460 ms
- Framingham: 457 ms
- Hodges: 457 ms
- Another study published in the Clinical Journal of Sport Medicine in 2019 3 proposed new QT interval thresholds for detecting those at risk of sudden cardiac death, recommending the use of Fridericia correction with thresholds of:
- 460 ms for men
- 470 ms for women
- 485 ms for marked prolongation
- It's worth noting that the definition of a normal QTc interval can vary depending on the population being studied and the correction formula used.
Factors Affecting QTc Interval
Several factors can affect the QTc interval, including:
- Heart rate: A study published in the Journal of Electrocardiology in 2004 2 found that the QTc interval has a linear trend with heart rate.
- Gender: The same study found that the upper normal limits of QTc interval vary between males and females.
- Age: A study published in Pediatrics in 2011 4 found that the average QTc interval in children and adolescents was 428 ± 28 milliseconds.
- Medications: A study published in the Psychiatric Quarterly in 2003 5 found that antipsychotic drugs can prolong the QT interval, with some drugs causing greater prolongation than others.
Clinical Significance
The QTc interval is an important measurement in clinical practice, particularly in the diagnosis and management of long QT syndrome.
- A study published in the World Journal of Emergency Medicine in 2023 6 found that acquired long QT syndrome is a prevalent condition in the emergency department, with a prevalence of 7.82% in patients admitted to the ED.
- The same study found that patients with QT intervals greater than 500 ms were more frequently female and were more frequently on QT-prolonging drugs.