It’s a question I see a lot and a very common misconception that beta blockers reduce the QT. Beta blockers are preventative medicine for Long QT Syndrome, they are not a cure or a fix for the condition. Beta Blockers can prevent the corrected QT interval going as long by numbing the adrenaline effect on the heart. They essentially put a limiter on your heart rate. As they suppress the heart rate, they also suppress the phenomenon that occurs in Long QT patients where the QT interval gets longer when the heart rate increases suddenly. By doing this, beta blockers do in a way reduce the QTc, but that is because they keep the heart rate and QT interval under control rather than actually reduce the QT.
In Long QT 3 patients, propranolol has been found to reduce the QT slightly as Long QT 3 is a sodium channel malfunction and propranolol blocks the wild sodium channel.
The Long Cutie 