ST prolongation as a cause of QT interval prolongation is less common than other mechanisms of QT prolongation. In most cases it is a wide T wave or a combination of T-U which causes QT prolongation. QRS widening can also contribute to QT prolongation, though it is seldom a cause of QT prolongation beyond the upper limit of normal by itself. ST segment roughly corresponds to the phase 2 of myocardial action potential and ST prolongation without QRS widening or T wave widening is classical of hypocalcemia. This type of QT prolongation is less likely to cause the life threatening ventricular arrhythmia of torsades des pointes which is more common with the other varieties of QT prolongation which affect mostly phase 3 of myocardial action potential.
Severe hypocalcemia due to hypoparathyroidism has been reported to cause cardiomegaly and congestive heart failure . An interesting case of reversible left ventricular dysfunction in an infant with vitamin D deficiency and hypocalcemia has been documented recently . QT interval prolongation due to ST segment prolongation was noted in this baby.