Written by Clay Smith
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The “Half-RR-rule” to screen for long QT works well if the heart rate (HR) is 60 or more. If <60, any uncorrected QT >485 msec was considered prolonged.
Why does this matter?
Most of us use the “half the R-R” as a rough estimate that the QT interval isn’t too long. But prior study has shown this method isn’t as accurate as it needs to be compared with other QT formulas. Is there a better, quick way to do this?
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They compared a number of QT and HR pairs (Half-RR, Nomogram, Bazett, Fridericia, Framingham, and Hodges) and found that at normal and elevated heart rates, the half-R-R-rule was actually more conservative than the QT formulas, making it a good screening tool. But with bradycardia, it performed poorly. As a fix, any uncorrected QT of >485 msec in bradycardic patients made the half-RR-rule more sensitive for detecting long QT.
Source
Screening for QT Prolongation in the Emergency Department: Is There a Better “Rule of Thumb?” West J Emerg Med. 2020 Feb 21;21(2):226-232. doi: 10.5811/westjem.2019.10.40381.
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