cardiovascular disorders
Long QT Interval Syndromes
Diagnosis of Long QT Interval Syndromes
Characteristic clinical and electrocardiographic manifestations Sometimes exercise testing Sometimes ambulatory ECG monitoring Sometimes provocative testing using IVepinephrineorisoproterenol Often genetic testing Screening of relatives Diagnosis should be considered in patients with unexplained cardiac arrest or syncope or a family history of such when the affected people do not have structural heart disease. It should also be considered in people who are discovered to have a long QT interval when ECG is done for other reasons. A long QT interval is diagnosed by ECG showing prolongation of the rate-corrected QT interval (QTc). Normal QTc intervals are about 0.40 second for men and 0.41 second for women and are considered prolonged when > 0.47 second for men or > 0.48 second for women. However, given the multiplicity of factors affecting the QTc, a normal QTc does not exclude the diagnosis. Nevertheless, at the moment of torsade de pointes VT, the QTc is essentially always prolonged. When a patient has a significantly prolonged QT interval and documented Torsades de Pointes Ventricular Tachycardia in the absence of other causes of a prolonged QT interval, the diagnosis of a congenital long QT interval syndrome is established. Patients with borderline QT intervals suspected of having LQTS should have exercise testing, because some abnormalities appear only during exercise. Ambulatory ECG monitoring may also disclose transient ventricular repolarization abnormalities. In patients with a normal QTc interval, provocative testing with IV ). Probability is estimated as low, intermediate, or high based on clinical, ECG, and exercise testing criteria, provided that the patient is not presently exposed to any environmental causes of QT-interval prolongation. The score can be used to establish candidacy for genetic testing, which can be time-consuming and expensive because of the multiple gene variants to be tested for. Patients with a low probability of a congenital LQTS do not need genetic testing, but patients in whom the probability is intermediate or high do. High-probability patients without a detected genetic abnormality may be considered to represent one of the 20 to 25% of patients with an unidentified mutation. Intermediate-probability patients who are gene mutation negative are followed closely with repeated electrocardiographic examinations including ECG, ambulatory cardiac monitoring, and exercise testing ( Diagnosis references ). First-degree family members of the index caseshould have clinical evaluation (ie, to detect symptoms suggestive of arrhythmia) and ECG. Thereafter, the first-degree family members of any newly identified patients undergo similar assessment (cascade screening). Genetic testing is done when the index case has a known mutation. Exercise testing is done when the results could alter the Schwartz score probability result. Some forms of LQTS are more associated with certain triggers than others . LQTS1: physical stress, particularly swimming, or emotional stress LQTS2: sudden loud noises like an alarm clock LQTS3: sleep Some forms of LQTS are also associated with particular ECG patterns LQTS1: wide T waves LQTS2: low voltage, notched T-waves LQTS3: long-ST segment with normal appearing T-waves However, neither the triggers nor the ECG findings are very specific and should not be used to confirm type or to direct genetic testing. 1.Schwartz PJ, Moss AJ, Vincent GM, Crampton RS: Diagnostic criteria for the long QT syndrome: an update.Circulation88:782–784, 1993. doi: 10.1161/01.cir.88.2.782 2.Schwartz PJ, Crotti L: QTc behavior during exercise and genetic testing for the long-QT syndrome.Circulation124:2181–2184, 2011. doi: 10.1161/CIRCULATIONAHA.111.062182 