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:
- The P wave: corresponds to the electrical activation (= depolarization) of the atria
- The PQ space: corresponds to electrical conduction between the atria and the ventricles
- The QRS complex: corresponds to the electrical activation (= depolarization) of the ventricles
- The T wave and the QT space: correspond to the electrical relaxation (= repolarization) of the ventricles.
Most cardiac diseases are associated with an anomaly of one of these waves, which makes the ECG particularly helpful to establish the diagnosis.
WHAT DOES THE LONG QT SYNDROME DESCRIBE?
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.
WHAT CAUSES THE LONG QT SYNDROME?
There are 2 main causes to the long QT syndrome: acquired and congenital long QTs.
- Acquired LQTs are reversible causes of the prolongation of the QT segment. They may come from different origin such as a toxic drug intake, or a metabolic disorder.
- Among the metabolic disorders, a decrease in potassium (hypokalemia), magnesium (hypomagnesemia) and calcium (hypocalcemia) is most often involved.
- A great many drugs may be responsible for the lengthening of the QT interval. The list is available and regularly updated on the crediblemeds.org website. Some of the most common ones are:
- Some anti arrhythmics, (quinidine, amiodarone, sotalol)
- Some antibiotics (macrolides : azithromycin, erythromycin ; quinolones)
- Some antimalarials : quinine and derivatives
- Some psychotropic drugs (tricyclic antidepressants)
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).
- Congenital long QT syndromes are genetic, hereditary diseases which are linked to a mutation in one or more genes regulating certain ion channels involved in repolarization (potassium channels or calcium channels most often). However, they are rare diseases and require specialized care (rhythmologist).
HOW DOES LONG QT SYNDROME MANIFEST? WHAT ARE THE RISKS?
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.
WHAT IS THE TREATMENT FOR LONG QT SYNDROME?
The treatment depends on the aetiology of the LQTS.
- In case of acquired LQTS
The treatment consists in correcting the triggering factor:
- Stopping the medication (s) responsible
- Correction of a metabolic disorder
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.
- In case of congenital long QT syndrome
The treatment is the heart rhythm specialist’s responsibility and is based on several elements. The main objectives are:
- On the one hand, the suppression of aggravating factors:
- Contraindication to drugs that increase the QT interval : an exhaustive list, available on crediblemeds.org., must be given to the patient.
- Early detection and treatment of aggravating metabolic disorders (potassium, magnesium, calcium)
- Avoid stress and intensive sport sessions
- On the other hand, genetic screening which consists in looking for the causal mutation. This allows for better monitoring, rhythmic risk assessment and the detection of the mutation found in the patient’s relatives for early diagnosis and treatment. A simple blood sample is needed for the test. However, it can take months, even years, to get the results.
- Finally, a specific treatment:
- Drug treatment with adrenergic beta-blockers : recommended in all patients with a congenital long QT Syndrom. As their benefits in terms of rhythmic risk reduction have been demonstrated, beta-blockers are the key to LQTS therapy. The most widely used betablocker is nadolol.
- Other treatments (but more rarely necessary) :
- Implantation of an internal cardiac defibrillator
- Surgical sympathetic denervation of the heart