How long is a normal QT interval?

As a medical professional with expertise in cardiology, I can provide you with information on the normal range for the QT interval. The QT interval is a measure of the time between the start of the Q wave and the end of the T wave in the heart's electrical cycle, as seen on an electrocardiogram (ECG). It represents the duration of both the depolarization and repolarization phases of the ventricles. In adults, a normal QT interval is typically between 320 to 440 milliseconds (ms) for men and between 360 to 460 ms for women. However, it's important to note that the QT interval can vary based on several factors, including age, gender, and heart rate. The QT interval is also influenced by certain medications and conditions that affect the heart's electrical activity. To adjust for heart rate variability, the corrected QT interval (QTc) is often used. The QTc is calculated using a formula that takes into account the heart rate (HR), ensuring that the QT interval is adjusted to a standard HR of 60 beats per minute (bpm). There are several formulas to calculate QTc, but one of the most commonly used is the Bazett's formula: \[ QTc = \frac{QT}{\sqrt{RR}} \] Where QT is the measured QT interval, and RR is the interval between two consecutive R waves (representing the time between heartbeats) in the ECG. Prolonged QT intervals can be associated with an increased risk of potentially life-threatening heart rhythm disturbances known as torsades de pointes. It's crucial for healthcare providers to be aware of the normal ranges and factors that can influence the QT interval to ensure appropriate diagnosis and treatment.

For example, if the QT interval measures 0.44 second and the R-R interval measures 0.86 second, then the QTc is 0.47. The normal QT interval varies depending on age and gender, but it's usually 0.36 to 0.44 second (see QT interval ranges).