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WHAT IS QT INTERVAL
The QT interval is a segment of the electrocardiographic wave that represents cardiac repolarization. The electrocardiogram (ECG), a routine test in cardiology, records the electrical activity of the heart (in the form of an electrocardiographic wave) which is characterized by a series of repetitive complexes representing the activation of the heart with each beat. In addition, these complexes, called “P-QRS-T complexes” can be divided into several segments or waves which have their own specific meaning:
Most cardiac diseases are associated with an anomaly of one of these waves, which makes the ECG particularly helpful to establish the diagnosis.
As the name suggests, the long QT syndrome corresponds to a QT interval on the ECG which is greater than it should be. It reflects an increase in the duration of the electrical relaxation of the heart (called repolarization) above normal levels. This anomaly may be innate (genetic or hereditary) or acquired (caused by certain medications, drugs or metabolic disorders, for instance). It is linked to a dysfunctioning of one of the electrical channels that regulate the heart beats. As the heart rate is generated by a set of specialized cells, regular contractions are linked to a set of ion currents traveling through the membranes of the heart “electrical” cells. The ion currents are regulated by specialized channels, called ion channels. There are different types, all of them controlled by different genes. The main channels are sodium (for sodium ions), potassium (potassium ions) and calcium (calcium ions) channels. These channels may be defective, either because of a structural anomaly (genetic malformation of the channel), or because of a “disruptive element», such as a toxic drug or a metabolic disorder.
There are 2 main causes to the long QT syndrome: acquired and congenital long QTs.
In addition, the lengthening of the QT interval is often found not to be the result of a single element but of the conjunction of several factors (for example: the combination of several drugs likely to prolong the QT and of hypokalemia).
Most patients suffering from long QT syndrome are asymptomatic, so only an electrocardiogram can diagnose an increase in the QT interval. However, the longer the QT interval, the greater risk of developing ventricular rhythm disorders which result in a significant acceleration of the heart rate. The so called “torsades de pointes” can turn into ventricular fibrillation and lead to cardiac arrest and sudden death. Palpitations, unspecific discomfort, dizziness and loss of consciousness are the most often reported symptoms. They depend on the duration and the severity of the tachycardia.
The treatment depends on the aetiology of the LQTS.
The treatment consists in correcting the triggering factor:
Hospitalization is sometimes necessary, according to the severity of the trouble (whether major or symptomatic) until correction. However, the best treatment is preventive: avoiding combining QT- lengthening drugs, detecting and correcting aggravating ionic disorders, and close monitoring for high risk patients seems to be the safest way to prevent acquired LQTS and any complications.
The treatment is the heart rhythm specialist’s responsibility and is based on several elements. The main objectives are: