How should the QT interval react to changes in heart rate?
Normally, as the heart rate increases, the QT interval shortens in a predictable and proportional manner.
There are lots of different formulas for correcting the QT. Most machines still use Bazett (QTcB) but the gold standard is now considered to be QTcF. Ultimately they all have their flaws and inaccuracies though.
A 'Stand Up' test can be more revealing than a resting Ecg. Here, as the heart rate increases when the patient stands up, the QT interval reduces. When their heart rate returns back to a resting rate, the QT increases again. This ensures efficient heart function and blood flow.
However, in people with Long QT Syndrome, while the QT interval does change with heart rate, it does not shorten as effectively or appropriately as it should. The time it takes for the heart muscle to reset actually gets longer as the heart gets quicker. It has barely any time before the next beat starts. When the heart rate slows back down, it takes even longer to relax after a beat.
I love an analogy. This is kind of what happens to your heart. If you suddenly increase the speed on a treadmill, you need to adapt to keep up. If you don't you'll fall off the back. Similarly, if you suddenly slow the treadmill down while sprinting... but keep sprinting... you're going to fall.
Dysfunctional ion channels cause a lack of proper adaptation to heart rate increase which leads to prolonged QT intervals and an increased risk of arrhythmia.