QT interval
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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 of the heart rate and has to be corrected.
The standard clinical correction is to use Bazett's formula1 calculating the heart rate corrected QT interval QTc. Bazett's formula is 
,
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. However, this formula tends to not be accurate, and over-corrects at high heart rates and under-corrects at low heart rates.
A more accurate method to correct the QT interval for the rate was developed by Rautaharju et al.2, who developed the formula 
.
This method is not widely used by clinicians. There are several other methods, but a regression based approach is the most accurate according to the current knowledge.
Normal values for the QT interval are between 0.30 and 0.44 (0.45 for women) seconds. If abnormally prolonged or shortened, there is a risk of developing ventricular arrhythmias. 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.
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[edit] References
1. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart 1920; 7:353-370
2. Rautaharju PM, Warren JW, Calhoun HP. Estimation of QT prolongation. A persistent, avoidable error in computer electrocardiography. J Electrocardiol. 1990;23 Suppl:111-7. PMID 2090728.
