When you have a Long QT syndrome, your heart beats at an irregular pace because the electrical waves that run through it are not coordinated and therefore the cardiac muscle loses efficiency and effectivity. Your heart will not only beat at an irregular pace, but also have trouble pumping the blood through your body. The lower half of your heart, which is the part that pumps the blood back to the arteries, takes a bit too long to respond with every beat, so your blood stays inside your heart for longer each time.
This disturbs the way your heart functions and disrupts the cycle of beats. People with Long QT syndrome can experience palpitations as a symptom, as their heart struggles to pump the blood. The palpitations are often felt on the chest, though there are other sections of the body where arteries are closer to the surface and therefore the palpitations can be felt, including the neck and throat.