Correspondence

Hypokalemia

N Engl J Med 2004; 350:1156March 11, 2004

Article

To the Editor:

In an Image in Clinical Medicine, Nosworthy (Nov. 27 issue)1 shows an electrocardiogram from a patient with hypokalemia and describes a prolonged QT interval. However, the corrected QT interval (QTc) of the electrocardiogram shown is approximately 0.416 second, which is normal. The QTc is defined as the QT interval measured from the start of the Q wave to the end of the T wave, divided by the square root of the RR interval. The normal QTc should be 0.38 to 0.46 second. I suspect that the author mistakenly measured the QU interval, rather than the QT interval, which is a common error in patients with hypokalemia because of T-wave flattening. In the electrocardiogram shown, the genuine QT interval is best appreciated in lead I. It should also be pointed out that, unlike hypocalcemia and hypomagnesemia, neither hypokalemia nor hyponatremia is usually associated with a prolonged QT interval. Although many drugs are associated with a prolonged QT interval, the only cause, to my knowledge, of a prolonged QT interval and a U wave together is quinidine toxicity.

Edward Jones, M.B., B.S.
Hull Royal Infirmary, Hull HU3 2JZ, United Kingdom
ej@doctors.org.uk

1 References