From the Guidelines
The normal QTc interval is generally considered to be less than 450 milliseconds (ms) for men and less than 460 ms for women, as suggested by the 2017 European Society of Cardiology (ESC) position paper 1. This interval represents the time from the beginning of the QRS complex to the end of the T wave, corrected for heart rate. The QTc is an important measurement because prolongation can indicate an increased risk of potentially dangerous ventricular arrhythmias, particularly torsades de pointes. Several formulas exist for calculating the QTc, with Bazett's formula being the most commonly used in clinical practice, though it tends to overcorrect at high heart rates and undercorrect at low heart rates. The QTc interval can be affected by various factors including electrolyte abnormalities (particularly hypokalemia, hypomagnesemia, and hypocalcemia), certain medications, congenital long QT syndromes, and structural heart disease. When interpreting QTc values, it's essential to consider the clinical context and any factors that might influence the measurement, as emphasized in the 2016 ESC position paper 1 and the 2017 Journal of the American College of Cardiology article 1. Key points to consider include monitoring patients with a history of QT prolongation, relevant cardiac disease, or those treated with QT-prolonging drugs, and minimizing exposure to other QT-prolonging drugs in patients treated with potentially QT-prolonging chemotherapy, as recommended by the 2016 ESC position paper 1. Additionally, conditions known to provoke torsade de pointes, especially hypokalaemia and extreme bradycardia, should be avoided in patients with drug-induced QT prolongation, as highlighted in the 2017 European Heart Journal article 1. It is also crucial to note that a QTc ≥ 500 ms or > 60 ms change from baseline has been associated with an increased risk for torsades de pointes, as defined by the CTCAE.4 and reported in the 2017 Journal of the American College of Cardiology article 1. Therefore, it is essential to carefully evaluate and monitor QTc intervals in clinical practice, taking into account the various factors that can influence this measurement, to minimize the risk of potentially life-threatening arrhythmias.
From the Research
Normal QTc Intervals
- Normal QTc intervals are typically <450 ms for men and <460 ms for women 2
- The QTc interval is used to adjust for the variation in QT interval with heart rate
Factors Affecting QTc Intervals
- Patient-related risk factors, such as female sex, age >65 years, and uncorrected electrolyte disturbances, can affect QTc intervals 2
- Certain medications, including anti-arrhythmic drugs, can prolong the QT interval and increase the risk of arrhythmic events 3, 4
Management of QT Prolongation
- Management of QT prolongation typically involves reducing the medication dosage or discontinuing it altogether, and in some cases, employing selective beta blockers 4
- Close monitoring of the QT interval is recommended for patients receiving anti-arrhythmic therapy, and consideration should be given to patient-specific risk factors for LQTS, including age, sex, and electrolyte imbalances 4, 5
Beta-Blockers and QT Dynamics
- Beta-blockers have heart-rate-dependent effects on the QT and QTc intervals in LQTS, increasing them at slower heart rates and shortening them at faster heart rates during exercise 6