Motivation: In the past few months, the most common EKG abnormality I have observed has been "nonspecific" ST-T wave changes. Second to that has been mildly prolonged QT interval. I often don't quite know what to do with a QT interval of 470 msec. Does this mean that the patient does not get ondansetron? Or, should this QT interval lead to the reflex of K>4 and Mg>2? The feared consequence of prolonged QTc is Torsades de Pointes. What is the association between the degree of QTc prolongation and Torsades de Pointes? The issue is complicated further by the fact that there is no good consensus on the definition of "normal." Conventionally, upper limit of normal for QTc is 450 msec for men and 470 msec for women.
This question turned out to be a lot harder to answer than anticipated. There are not great prospective studies, but there have been reviews which have compiled cases of Torsades de Pointes and associated QTc intervals.
Paper: Bednar, M.M. et. al. "The QT Interval" Progress in Cardiovascular Diseases. 43 (2001): 1-45 (supplement)
Methods: Data collected from 202 reports of Torsades de Pointes and prolonged QT interval. Corrected QT interval was by Bazett formula.
Results:
QTc (ms) TdP Cases (% of total cases)
<500 9 (7.8)
500-549 13 (11.2)
550-599 24 (20.7)
600-649 36 (31.0)
650-699 21 (18.1)
>700 13 (11.2)
Discussion: The results illustrate that while Torsades de Pointes from prolonged QTc is uncommon with QTc less than 500, the risk is not zero with mildly prolonged QTc (<500 msec). The most commonly associated QTc interval with TdP is between 600-649 msec. The patients who experienced QTc in the lowest QTc interval (<500 msec) may have had some inherited propensity for ventricular ectopy that was enhanced with mildly prolonged QTc. Another explanation could be that the Bazett formula probably did not adequately correct the QT interval. The Bazett formula overcorrects at faster heart rates and undercorrects at low heart rates. Interestingly, Bazett proposed the formula after only examining 39 healthy patients. An alternative way to calculate QTc is using an empirically derived formula from the Framingham study which used a much larger number of subjects (Pubmed ID: 1519533). Anyway, in the future, I will think a little bit about risk of TdP in patients with QTc<500 and a lot more when QTc>500.
This question turned out to be a lot harder to answer than anticipated. There are not great prospective studies, but there have been reviews which have compiled cases of Torsades de Pointes and associated QTc intervals.
Paper: Bednar, M.M. et. al. "The QT Interval" Progress in Cardiovascular Diseases. 43 (2001): 1-45 (supplement)
Methods: Data collected from 202 reports of Torsades de Pointes and prolonged QT interval. Corrected QT interval was by Bazett formula.
Results:
QTc (ms) TdP Cases (% of total cases)
<500 9 (7.8)
500-549 13 (11.2)
550-599 24 (20.7)
600-649 36 (31.0)
650-699 21 (18.1)
>700 13 (11.2)
Discussion: The results illustrate that while Torsades de Pointes from prolonged QTc is uncommon with QTc less than 500, the risk is not zero with mildly prolonged QTc (<500 msec). The most commonly associated QTc interval with TdP is between 600-649 msec. The patients who experienced QTc in the lowest QTc interval (<500 msec) may have had some inherited propensity for ventricular ectopy that was enhanced with mildly prolonged QTc. Another explanation could be that the Bazett formula probably did not adequately correct the QT interval. The Bazett formula overcorrects at faster heart rates and undercorrects at low heart rates. Interestingly, Bazett proposed the formula after only examining 39 healthy patients. An alternative way to calculate QTc is using an empirically derived formula from the Framingham study which used a much larger number of subjects (Pubmed ID: 1519533). Anyway, in the future, I will think a little bit about risk of TdP in patients with QTc<500 and a lot more when QTc>500.
Now we need a study to show us what to do with "non-specific ST element" changes. The most useful thing to do is still to have old ECGs. Anything thats old is less worrisome.
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