QT interval in right and left bundle-branch block

The QT interval of the electrocardiogram is prolonged in right and left bundle-branch block. Since the QT interval is not prolonged outside the normal range due to myocardial disease alone, this lengthening of the QT interval is due to the conduction defect and is probably due to delayed depolarization (and thus repolarization) of the heart. There is, however, a significant difference in the QT interval in left and right bundle-branch block (t = 386; P< ooi), since the QT was shorter in right than in left bundle-branch block. This may be due to the completion of normal repolarization in the left ventricle, so that only in some cases of right bundle-branch block does repolarization end in the right ventricle. Correction of the actual QTc for the conduction defect of left bundle-branch block can be made by sub-tracting theprolongation of the QRS due to the blockfrom the actual QTc. This will allow the QTC (corrected for conduction defect and rate) to be directly compared with normal (o042 sec). Correction for right bundle-branch block can be made by subtracting 0-20 sec less from the actual QTc. The QT interval is prolonged in hypokalaemia and hypocalcaemia,and shortened in hypercalcaemia. It is used to evaluate such abnormalities (Grant, I970) and may also reflect cellular levels of these electrolytes