Shortening of the QT interval
It is generally accepted that the QT interval on the ECG reflects the duration of the electric systole of the ventricles. The duration of the QT interval is influenced by many factors, but mainly by the heart rate. It is believed that the QT interval is within normal limits if its duration does not exceed ± 15% of the value calculated for the corresponding heart rate. As a rule, sudden changes in heart rate are not accompanied by immediate changes in the duration of the QT interval. From the other factors influencing on the duration of the QT interval, it is necessary to distinguish age, sex (for men and children, the QT interval is somewhat shorter than for women), as well as the size of the heart chambers, changes in the position of the body, and some others. For many decades, the greatest clinical significance is attributed to the syndrome of the extented QT interval due to an unfavorable prognosis. A much more rare phenomenon is the shortening of the QT interval. A short QT interval is observed in the early phase of repolarization, i.e. in a situation where the T wave begins immediately after the QRS complex, and the ST segment itself is often absent. The interval QT interval can also be shortened on the background of treatment with digitalis preparations, the effect of which is already noticeable in therapeutic doses and becomes more pronounced in case of intoxication. Hypercalcemia and hyperkalemia, hyperthermia, acidosis, decreased thyroid function, and an increase in the tone of the vagus nerve may be accompanied by a shortening of the QT interval. There is evidence that the QT interval in patients with idiopathic atrial fibrillation (AF) is shorter than in healthy individuals. There is a short QT syndrome, which is considered a genetic anomaly with a high risk of developing a family form of AF and / or sudden death or fainting. Electrocardiographically and electrophysiologically, it is characterized by a shortening of the QT interval less than 320 ms, a shortening of the effective refractory period of the atria and ventricles and a high probability of ventricular tachyarrhythmia with programmed stimulation.