QT Interval

The QT interval consists of ventricular depolarization (QRS segment) through ventricular repolarization (T wave). The main evaluation of the full QT interval itself is its duration. Its components (QRS, ST segment, and T waves) are evaluated individually. The QT interval is dependent on the heart rate and is inversely proportional to it. At faster heart rates, the QT interval shortens; at slower heart rates, the QT interval lengthens. This is because the time of the refractory period of the conduction system is somewhat adjustable and attempts to shorten with quicker heart rates in order to be able to properly allow for full depolarization. However, there are certain pathologies that do not allow for significant adjustment and predispose patients to ventricular arrhythmias; these can sometimes be a cause of sudden cardiac death.

The QT interval is typically measured in lead II; it is measured from the very beginning of the QRS to the end of the last wave. Although this is typically the end of the T wave, the end of the U wave should be measured if they are present. Normal QT values depend on sex of the patient. For men, the QT should be between 350-440ms. For women, it should be between 350-460ms. Patients are at risk of ventricular arrhythmias if less than that or >500ms. One rule of thumb is that the QT interval should be less than 1/2 of the RR interval.

The QT interval can be compared across patients when it is corrected ('QTc'). There are multiple calculations for this, though the most common is Bazett's formula. The QTc=QT divided by the square root of the RR interval (RR interval in seconds). This allows for some correction to a heart rate of 60bpm for standardized comparisons.