At high heart rates, Bazett's formula will really often give high results results (close to 500 msec (at stress
tests, etc.). When my mom had a stress test
done, I asked her cardiologist about this specificly.
At heart rate 180 her QTc (Bazett) was close to 500 msec, the QT was 280 msec. If the Frederica formula is used (which it often is) the QTcF would be 404 msec.