The ECG demonstrates prolongation of the QT segment as demonstrated by a QT interval of 0.6 seconds, with a calculated QTc of 0.61 seconds (Figure 2).
A QT interval greater than 0.44 seconds is generally considered to be prolonged, but established formulas provide validated values. The QT interval is affected by the heart rate, and a corrected value referred to as the QTc is calculated with the following formula (the Bazett formula): QTc = QT/square root of the R-R interval. In this case, with an R-R interval of 0.96 seconds, the QTc is equal to 0.61 seconds. The QTc value must then be compared with the maximal QT interval allowed for the heart rate and gender of the patient. Reference values for the maximal allowed QT interval can be found by using established tables, but in this case, for a heart rate of 65 bpm in a woman, the maximal normal QT interval is 0.42 seconds. Because the QTc is greater than the maximal allowed QT interval, the interval is prolonged. This patient's presentation of two episodes of syncope without any obvious triggers, a similar history in an immediate family member, and no history of medications that can prolong the QT interval is suggestive of congenital long QT syndrome (LQTS). In the case of an irregular ventricular rate due to atrial fibrillation, the average of 10 QT intervals may be used in the QTc calculation.
The diagnosis of LQTS has been increasingly recognized as a cause of unexplained dizziness, syncope, and sudden cardiac death in otherwise healthy, young individuals. The prevalence is difficult to estimate, but rough estimates place the occurrence at 1 in 10,000 individuals. This number is difficult to ascertain because 10%-15% of patients with LQTS genetic defects have a normal QTc duration at various times. Most patients with congenital forms of the disease develop symptoms in childhood or adolescence. The age of first presentation is somewhat dependent on the specific genotype inherited. The possibility of this diagnosis should be considered in any patient with a history similar to the one in this case.
Congenital LQTS is now considered to be a heritable abnormality in one of the cardiac myocyte membrane sodium and potassium channels. Several specific genotypes have been identified, with different mutations. Twelve different types of LQTS have been identified, with types 1, 2, and 3 accounting for most cases (45%, 45%, and 7%, respectively). In both LQT1 and LQT2, the potassium ion current is affected. However, in LQT3, the sodium ion current is affected. Other notable elements of the most common forms are include the following:
LQT1: Swimming or strenuous exercise can trigger malignant arrhythmias in this type.
LQT2: Sudden emotional stress can trigger arrhythmias in this type. Postpartum women with LQT2 are susceptible.
LQT3: Malignant arrhythmias occur during rest.
The QT interval reflects the duration of activation and recovery of the ventricular myocardium. Prolonged recovery from electrical excitation raises the chance for dispersing refractoriness, when some parts of myocardium may be refractory to depolarization. From a physiologic standpoint, dispersion occurs with repolarization between three layers of the heart. Also, the repolarization phase is often prolonged in the mid-myocardium. Thus, the T wave is normally wide; the interval from Tpeak to Tend (Tp-e) indicates the transmural dispersion of repolarization (TDR). In LQTS, TDR increases and creates a functional substrate for transmural reentry.
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