Abstract
Background & Aims: Patients with Long QT Syndrome (LQTS) often display paradoxical prolongation of the QT interval in response to an increase in heart rate. Studies in adults have shown that the tachycardia induced by standing can be a useful aid in the investigation and diagnosis of LQTS. However, data on the response of the QT interval to standing in the paediatric LTQS population are limited. This study, therefore, aimed to observe the changes in the QT interval invoked by the action of standing in children with LQTS. Methods: 64 consecutive children from 56 families with LQTS (aged <18 years; median 12.8 years [interquartile range 7.4-14.9]; 55% female), followed up in a specialist referral centre for paediatric inherited cardiovascular disease underwent supine 12-lead ECG followed by a second ECG immediately after standing. Corrected QT interval (QTc) was measured in lead II using Bazett's and Fridericia's formula. 48 patients from 43 families (75%) had undergone genetic testing (LQT1 n=14, 29%; LQT2 n=8, 17%, LQT3 n=2, 4%, variants of uncertain significance n= 7, 15%). Results: The corrected QT interval (QTc) was 429ms supine vs. 452ms standing (mean increase of 23ms [+/−51ms]) in lead II. When subdivided by genotype, impairment of the QT response was most pronounced in LQT1 (416ms supine vs. 463ms standing; mean increase of 38ms [+/−54ms]) compared to LQT2 patients who showed appropriate QTc shortening (447ms supine vs. 430ms standing; mean decrease 16ms [+/−30ms] p=0.006 compared to LQT2 patients). In patients with a diagnosis of LQTS but a normal QTc at rest, QTc was 406ms supine vs. 439ms standing (mean increase of 33ms [+/−49ms]). Conclusions: This study shows that the QTc increases in children with LQTS on standing, particularly in children with LQT1. This may improve diagnostic accuracy in children in whom a diagnosis of LQTS is suspected.
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