Long QT syndrome is often a genetic condition, although it can also be caused by medications used for other conditions (such as some antibiotics and antidepressants). It affects the proteins that make up ion channels in the myocytes (heart muscle cells). As ion channels are a vital part of how electrical messages are transmitted through the heart, when they don’t work properly, or there are too few of them, it causes problems with the electrical activity of the heart.
How blood flows through the heart as it beats is referred to as the cardiac cycle, and the beating of the heart (when it contracts and relaxes) is controlled by electrical messages sent through the heart muscle. These electrical messages, when recorded on an EEG, form a typical pattern of five waves. These are referred to as P, Q, R, S and T waves, and each corresponds to a particular part of the cardiac cycle. The waves from Q to T corresponds to the electrical messages as they move through the ventricles. In long QT syndrome the length of time from the Q to the T waves are, and the name suggests, longer than normal. This means that the ventricles take longer to recover or ‘reset’ between contractions.
Some people don’t experience any symptoms from Long QT syndrome. However, it can cause loss of consciousness and seizures, when the heart’s rhythm is affected and it cannot get enough blood to the brain, which causes it to be temporarily starved of oxygen. Once the heart rhythm returns to normal, blood flows back to the brain. In some cases, abnormal heart rhythms are persistent, and can cause ventricular fibrillation. In this case, the heart’s rhythm needs to be restored with cardioversion (an electric shock).
Although there isn’t always a trigger for the change in the heart rhythm, it can be triggered by stress, sudden or loud noises or strenuous exercise. Avoiding triggers and eating a potassium-rich diet helps to manage the symptoms of Long QT syndrome. Treatment with medications will also usually be necessary, and a pacemaker or ICD may be recommended to treat any dangerous heart rhythms.
For more about QT see cardiac complex