Methods: An IRB-approved, retrospective analysis of 200 patients (116 females, average age 21 ± 15 years) who were referred to our LQTS Clinic and who had an ECG performed on two consecutive days. These patients either i) had genetically proven LQT1 (N = 74), LQT2 (N = 50), or LQT3 (N = 24) or ii) were dismissed as genotype negative normal (N = 52). Blinded to their genotype, both the computer-derived and manually calculated QTc, using Bazett’s formula, were recorded.
Results: Expectantly, the QTc was far greater among patients with LQTS (479 + 44 ms) than those patients dismissed as otherwise normal, i.e. “hyper-controls” (432 + 27 ms, p < 0.0001). The day-to-day QTc change was 21 ± 22 ms (LQT1), 25 ± 19 ms (LQT2), 33 ± 53 ms (LQT3), and 16 ± 16 ms for those dismissed as normal (p = NS at all levels except between LQT3 vs normal with p = 0.01). The QTc variability was not influenced by age, gender, genotype, or the time of day the ECG was obtained. Importantly, however, 37/148 LQTS patients (25%) and 6/52 hyper-controls (11.5%, p < 0.04) had a QTc > 460 ms on one day but < 460 ms on the other day. In addition, 5 LQTS patients (2 LQT1, 1 LQT2, and 2 LQT3) had a QTc > 480 ms on one day but < 440 ms on the other day with the greatest difference being a 16-year-old LQT1 female with a QTc of 420 ms on day 1 and 492 ms on the next day.
Conclusions: For patients referred for an evaluation for LQTS, serial ECGs on consecutive days is an inexpensive and potentially invaluable tool to catch LQTS. As shown here, a single 12-lead ECG may be insufficient to diagnose LQTS as 25% of patients with LQTS might have been missed had their ECG been performed on the wrong day.