What Does Prolonged QT on ECG mean?
Doctors can measure if the QT interval happens in the correct amount of time. If it takes longer than usual, it is called a prolonged QT interval. The upper limit of a normal QT interval depends on factors such as age, sex, and the regularity and speed of the heart rate.
Long Q-T syndrome (LQTS) is one of the most common inherited syndromes related to sudden death. It has been denominated as a channelopathy or disease of ionic channels. It is known that the cardiac action potential exists since a perfect balance between the ionic depolarization and repolarization currents, which are produced thanks to the correct functioning of the corresponding ion channels. Therefore, a modification in the genes that cipher for these channels is translated into an imbalance in these currents. Normal ventricular repolarization is produced by the balance between the sodium and calcium inward currents and the potassium outward currents (K+). The last ones are many but may be grouped into two, those in charge of repolarization in the initial phase of the action potential, which is called transient outward currents and those dealing with the rest of the repolarization that is known as late rectifier currents. There are many mutations that may influence negatively the ion channels. The consequences of these mutations can be classified into three groups: affection of the channel´s permeability, a variation of the channel´s activation and dysfunction in the channel´s inactivation.
About the Q-T Interval
A normal ventricular repolarization is symbolized by the ST segment and the T wave in an electrocardiogram (ECG). Nevertheless, the electrocardiographic parameter with which doctors have to evaluate the duration of repolarization is the QT interval.
It is vital to remember that the Q-T interval comprises not only to repolarization but also depolarization since it includes the QRS complex. This fact is essential to comprehend why it can be lengthened in the presence of conduction disorders, and not only by alterations in repolarization.
In the present case, the electrocardiogram (ECG) which shows a long Q-T syndrome is characterized by an alteration in repolarization, which is abnormally long. Taking the aforementioned premise of the equilibrium between inward and outward currents, a LQTS can be produced by alterations in the potassium (K+) channels, which consistently decrease the outward current of K+.
It can also occur if alterations of sodium or calcium channel increase in an abnormal way the entrance to the cell of any of these ions. In any case, such prolonged repolarization can result generally favored by certain precipitating factors, in a specific type of polymorphic ventricular tachycardia whose characteristic morphology reflects the torsade of the tissue.
The common denominator of prolonged QT syndromes is the prolongation of this interval. However, there are certain special characteristics of the ST segment and the T wave that allow suspecting the presence of a specific variant. The differential diagnosis of the three main variants can be made through the electrocardiogram, although their results are not exact since the genotype/phenotype relationship is not accurate.
Seven variants of the disease are currently known and the therapy includes pharmacological and non-pharmacological measures.
The syndrome can also respond to secondary causes due to the use of cardiological and non-cardiological drugs and this form of presentation is more frequent than that of genetic origin.
The long Q-T syndrome has offered doctors a unique and valuable understanding of the fundamental mechanisms of cardiac electrical activity. New genes have been discovered and the association between the genetic defect and the clinical phenotype has provided a clearer view of the roles of ion channels in the determination of QT intervals and their regulation. The disease has also offered the clearest example of the importance of obtaining genetic information about patients because this is not only important for the prognosis of the disease but is currently clearly important for individualized therapy. Future research at the clinical and molecular level will undoubtedly increase the number of specific targets for arrhythmia control, not only in this rare disease, but in many other congenital and acquired cardiac arrhythmias.