I can tell you why there is a difference.
The calculators online are using the old Bazett formula for calculating QTc. What those formulas do, is trying to determine "what the QT interval (which shortens at higher heart rates and prolongs at lower) would be if your heart rate was 60". The Bazett formula is (in-)famous for over-correcting the QTc at high heart rates (making it longer than it would be at heart rate 60) which is why your EKG machine is using the newer Fridericia formula which doesn't over-correct QTc at higher heart rates.
I have also experienced that my QTc is "prolonged" when I had panic attacks when EKG was registered and my heart rate was high. I was just telling the doctor "ah, you're using Bazett" when my QTc was in the 460s and my heart rate was 130. Bazett doesn't work as well at heart rates above 80.
Preferrably, you should register an EKG when your heart rate is slow (closer to 60) to find your "true" QT interval. The formulas used are just theoretical approaches to what your QT interval likely would be at slower heart rates. Every person have their unique relationship between QT interval and heart rate. In some people, QT interval narrows much with increased heart rate and in some people it doesn't.
is something wrong is a excellent member of med help heart rhythm. always answering and for the most part online.
On the subject ( hope this is ok BBECK3 ), would you be able to tell me why the q t time varies on different days?
so may people i have spoken to on here, and from my own experience, we have had ecg's multiple times and sometimes the q t interval in quite largely different. once mine was around 420 calculated and most of the time now its around 390 calculated ?