PSA: don't withhold a QT prolonging drug just because the computer calculated a QTc above 450-460 ms. The computer correction is usually (>50% by the study below) very inaccurate. And it only takes 15 seconds to do it right (or at least better).
PMID: 27317349
#MedEd #tweetorial
PMID: 27317349
#MedEd #tweetorial
Background: QT intervals change depending on HR. To compare them, one can correct for HR using many different formulas:
- Bazett
- Hodges
- Fridericia
- Framingham
- Rautaharju
- Bazett
- Hodges
- Fridericia
- Framingham
- Rautaharju
Here's a graph comparing how each formula compares at different heart rates (technically 1/HR). You want the line to be flat. None are perfect, but you'll notice that one performs nearly as poorly as not correcting at all: Bazett.
The good news is that MDCalc has an easy and free calculator to fix this: just type in the HR and uncorrected QT and choose which formula you want. 15 seconds or less!
Can you actually trust the computer's QT measurement, though? Good point. If the baseline is wonky, maybe not. Have no fear, you can simply manually count the number of small boxes and plug that in on the same calculator on MDCalc. It really does it all.
The computer's QTc is frequently very wrong (>50 ms), particularly for anyone who's tachy or brady. That study I cited up top says the number of patients corrected identified as having prolonged QT can be reduced by 50% using a better formula.
This is important stuff. Patients get meds withheld all the time for QT prolongation: psych meds, first choice abx, antiemetics, pain meds, and beyond. To withhold a drug without at least doing the 15 seconds of legwork to confirm causes real harm.
I don't think many people know this stuff! I sure didn't until an EP attending tipped me off and I did the deep dive. None of my co-residents and few of my attendings knew either. Let's spread the word!
15 seconds could save you from unnecessarily holding important meds.
15 seconds could save you from unnecessarily holding important meds.
Forgot to explicitly point this out: the ECG computer uses the Bazett calculation, the one formula that performs worse than all others. There's debate about which formula is best, but none about which is worst.
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