To the Editor: Controlled studies of the toxicity
of over-the-counter drugs are uncommon, so the study by Dr McBride and colleagues1 of a dietary supplement containing ephedra and caffeine
(DSEC) was an important addition to the literature. However, some additional
data from that study would be helpful.
Dr McBride and colleagues reported that participants taking DSEC had
a significantly increased electrocardiographic (ECG) QTc interval, compared
with placebo, when it was calculated by either the Bazett or Framingham formulas.
Unfortunately, the source article for the Framingham calculation2 quoted
an incorrect formula. One of the participants in the DSEC study had a heart
rate of 120/min while taking DSEC. A healthy participant with a heart rate
of 120/min who had a QTc interval of 380 milliseconds by the correct Framingham
formula would have a QTc interval of 429 milliseconds by the Bazett formula
and 1073 milliseconds by the incorrect Framingham formula.2 The
QTc interval is calculated by formulas that correct the QT interval for changes
in RR interval. Therefore, selection of the appropriate QTc correction formula
is critical in this study since participants’ average RR intervals were
785.7 milliseconds at baseline, 834.2 milliseconds after placebo, and 849.0
milliseconds after DSEC.
Milic M, Rizos D, Ziegler MG. Electrocardiographic Effects of a Dietary Supplement Containing Ephedra
and Caffeine. JAMA. 2005;294(12):1487–1488. doi:10.1001/jama.294.12.1487-a
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