QT interval

In medicine, specifically cardiology, 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. The QT interval is dependent on the heart rate in an obvious way (the faster the heart rate, the shorter the QT interval) and has to be adjusted to aid interpretation.

The standard clinical correction is to use Bazett's formula, named after physiologist Henry Cuthbert Bazett, calculating the heartrate-corrected QT interval QTc. The formula is as follows: $$QTc = \frac{QT}{\sqrt {RR} }$$, where QTc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds, often derived from the heart rate (HR) as 60/HR. However, this formula tends to not be accurate, and over-corrects at high heart rates and under-corrects at low heart rates.

In the same year, Fridericia published an alternative adjustment:

$$QT_F = \frac{QT}{RR^{1/3} } $$.

There are several other methods, but a regression based approach is the most accurate according to the current knowledge. An example of the regression-based approach is that developed by Sagie et al., as follows:

$$QT_S = QT + 0.154(1-RR)$$.

Normal values for the QT interval are between 0.30 and 0.44 (0.45 for women) seconds. QT interval can be measured by different methods such as the threshold method in which the end of the T wave is determined by the point at which the component of the T wave merges with the isoelectric baseline or the tangent method in which the end of the T wave is determined by the intersection of a line extrapolated from the isoelectric baseline and the tangent line which touches the terminal part of the T wave at the point of maximium downslope.

Abnormal intervals
If abnormally prolonged or shortened, there is a risk of developing ventricular arrhythmias.

Genetic causes
An abnormal prolonged QT interval could be due to Long QT syndrome, whereas an abnormal shortened QT interval could be due to Short QT syndrome.

Due to adverse drug reactions
Prolongation of the QT interval may be due to an adverse drug reaction. Many drugs such as haloperidol and methadone can prolong the QT interval.