It’s common for people to avoid key first line medications and choose less ideal alternatives when the computer’s QTc reads out 490 or 510...
But when there is a LBBB, RBBB, or paced rhythm, the vast majority of the time this is not necessary. Why?
We measure the QT because impaired REpolarization creates a risk for Torsades, a dangerous ventricular arrhythmia.
But if you think about it, QT interval includes both DEpolarization (QRS) and REpolarization (ST segment + TW, together referred to as the JT interval).
This hasn’t really stuck, perhaps because of intertia as well as established standards and med effect data based on the QT.
So then back to the question by @RebeccaEBerger , how do you interpret/account for this to see if there’s underlying DEpolarization impairment too?
So if your DEpolarization is prolonged (BBB or paced), your QT will be prolonged, but that doesn’t necessarily reflect an impairment in REpolarization, nor a risk for TdP.
This is why some people have advocated abandoning the QT in favor of the JT:
There are different formulas, but the simplest concept that I think is enough for most is this:
1. How much is the QRS prolonged by?
2. Subtract that from the QT (take away the irrelevant prolongation due to DEpolarization)
3. Then correct for heart rate as always.
For instance, this study compared different methods of correction and proposed their own. But they’re close, and will generally lead to same clinical judgment as the “intuitive” approach above. Same with the other study/method @RebeccaEBerger linked.
3. Now correct for heart rate. 360 / sqrt (0.64) = 450.
Repolarization is not significantly impaired.
Take-homes: - QT includes QRS; when BBB or V-pacing, standard QT way overestimates risk - By how much is QRS prolonged? Subtract that from QT. Correct for HR. This gives sense of “true” QTc. - Bradycardia is a risk for TdP
Would appreciate more tips/expertise from #medtwitter !
And linking two more great threads on QTc prolongation, one by @seanlena
While we correct for heart rate to estimate degree of repol impairment, the risk of Torsades is actually ~ to the absolute QT, not QTc. Thus, tachycardia is protective, and bradycardia a risk. Take this into account when making close calls on meds.
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