Short QT syndrome
Short QT syndrome is one the cardiac channelopathies. It is characterised by an abnormally short QT interval and a propensity to develop atrial and ventricular arrhythmias. Short QT syndrome was first described by Gussak and associates in 2000 and has a risk of sudden cardiac death (SCD) like its counterpart, the long QT syndrome. Gaita and associates have highlighted the familial nature of short QT syndrome by describing six patients from two unrelated families with a strong family history of sudden cardiac death in association with short QT intervals. Using a gaussian distribution of corrected QT intervals (QTc), values less than 350 milliseconds in males and 360 milliseconds in females have been considered short QT interval. But it should be remembered that Bazett formula for calculating QTc underestimates QTc during bradycardia and overestimates QTc with tachycardia. This can lead to branding of apparently normal individuals as having short QT if their ECGs are recorded during sinus bradycardia. Alternate method of calculating a predicted QT interval (QTp) has been proposed by Rautaharju and associates. Using this method, a QTp of less than 360 milliseconds at a heart rate of 60 per minute can be taken as suggesting short QT.
Before considering a diagnosis of short QT syndrome, secondary causes for a short QT interval like hyperkalemia, acidosis, digitalis effect, catecholamine effect and hyperthermia should be excluded. A rare differential diagnosis is paradoxical shortening of QT interval in deceleration dependent shortening of QT interval due to activation of the KACh current caused by strong parasympathetic stimuli.