Abstract 98 Cardiology: Signaling Mechanisms and Cardiovascular Function Platform, Sunday, 5/2

The detection of Long QT syndrome (LQTS) is important as it can cause a lethal arrhythmia in an otherwise normal child and is easily controlled with beta-blockers. Pediatricians may encounter this rare condition (incidence 1: 5000-10,000) when assessing children presenting with syncope or unexplained seizures. The traditional criteria for diagnosis- a QTc >0.44 on a routine 12-lead ECG- proved not to be a very specific test for this condition and thus a cut-off of 0.45 for males and 0.46 for females was suggested. However, in studies of large kindreds with LQTS these criteria are found to be absent in as many as 20-30%. One criticism of such studies is that the diagnosis of LQTS was made on clinical grounds alone and could not be established with certainty. With the recent discovery of the cardiac ion channel genes that cause LQTS re-evaluation of QTc criteria are possible. The database at the LDS Hospital in Salt Lake City contains clinical and ECG information on 27 large families with LQT1, LQT2 and LQT3 genotypes. From the database we assessed the QTc in 117 genotype-positive children <16 years of age and their 135 unaffected siblings <16 years old. We also assessed the QTc in genotype positive parents of these children. The QT was measured in lead II or V5 and the QTc was calculated from three similar intervals using Bazett's formula. A QTc of 0.47 or greater was present in 71% (83/117) of genotype positive children. The QTc of their genotype negative siblings was 0.47 or greater in 0/135. In order to assess the usefulness of sampling parental QTc in an attempt to improve the ECG detection of this condition when the child's QTc is <0.47 we measured QTc in the group of parents whose genotype positive children had a QTc of 0.41-0.46. Of 23 genotype positive parents, 15 (65%) had QTc of 0.47 or greater despite their child's QTc being < 0.47. Thus, a QTc of 0.47 or greater in children <16 is highly suspicious for LQTS. However, 29% of genotype positive children will have borderline or normal QTc and in the setting of syncope or seizure related to exercise or fright will need further testing, including serial ECG's (QTc may be prolonged on other ECGs), exercise electrocardiography and holter monitoring. Evaluation of the parents' ECGs may also be helpful in this autosomal dominant condition. A QTc of 0.47 in a parent does not establish the child's diagnosis but increases the probability of LQTS. In contrast, finding a QTc < 0.47 in the child and the parents would make LQTS unlikely.