From the Guidelines
The normal QT interval on an electrocardiogram (ECG) is typically considered to be less than 450 milliseconds in men and less than 460 milliseconds in women when corrected for heart rate (QTc) 1. This interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization, essentially measuring the duration of electrical activation and recovery of the ventricles. The QT interval is important to monitor because prolongation can predispose patients to dangerous arrhythmias like torsades de pointes. Several factors can affect the QT interval, including:
- Electrolyte abnormalities (particularly low potassium, magnesium, or calcium)
- Certain medications
- Congenital long QT syndromes
- Structural heart disease. When interpreting QT intervals, it's essential to consider the patient's specific context, including their baseline ECG, current medications, and clinical condition 1. The corrected QT interval (QTc) is more useful for clinical purposes as it adjusts for heart rate variations, with normal values being less than 450 milliseconds in men and less than 460 milliseconds in women 1. It is also important to note that the QT interval should be measured from the beginning of the QRS complex to the end of the T wave, and discrete U waves should not be included in the QT interval 1. In clinical practice, the most commonly used QT correction formula is the one introduced by Bazett, QTC = QT interval divided by the square root of the R-R interval measured in seconds 1. However, it is crucial to recognize that normal limits established using the upper and lower limits of actual percentile distributions of the rate-adjusted QT are preferable to those as mean values ±2SD because these distributions are strongly skewed 1. Therefore, a normal QTc interval is less than 450 milliseconds in men and less than 460 milliseconds in women, and any prolongation beyond these values should raise suspicion for potential arrhythmias or other underlying conditions 1.
From the Research
Normal QT Interval
The normal QT interval is typically measured relative to the heart rate, and it often requires "correction" for the current heart rate to correctly stratify the risk of torsades de pointes 2.
QT Interval Measurement
- The QT interval depends on the heart rate, and a corrected QT (QTc) interval is often used to compensate for this 3.
- Several correction formulas have been suggested, with Bazett's formula being the most widely used, but it overcorrects at a heart rate above 80 beats per minute 3.
- Fridericia's formula is considered more appropriate to use in cases with a heart rate above 80 beats per minute 3.
QT Interval Prolongation
- A QT interval >500 ms markedly increases the risk for torsades de pointes and sudden cardiac death (SCD), and should lead to discontinuation of the offending drug and correction of underlying electrolyte disturbances 3, 4.
- QTc prolongation is considered ≥470 milliseconds (males) and ≥480 milliseconds (females), with >500 milliseconds considered severe 5.
- The risk of torsades de pointes should be assessed in patients who are about to begin taking a QT-prolonging medication, although there is no established threshold below which prolongation of the QT interval is considered free of proarrhythmic risk 4.
Clinical Implications
- QT interval prolongation is frequent among patients admitted to acute medical units, with 7% of patients having a prolonged QTc interval upon admission 5.
- QT interval-prolonging drugs are commonly prescribed to patients presenting with prolonged QTc, but whether this affects clinical outcomes is uncertain 5.
- Antipsychotic drugs can prolong the QT interval, usually by blocking the potassium IKr current, and careful evaluation of QTc prolonging properties of these drugs is needed 6.