Objectives: The aim of this study was to develop a diagnostic technique to detect latent long QT syndrome. Background: Asymptomatic patients with genetically diagnosed long QT syndrome(LQTS)may have a normal resting QT interval, yet remain at risk for cardiac events. We hypothesized that the QT response during a novel burst exercise protocol simulating clinical events might distinguish patients with “latent LQTS”from healthy subjects. Methods: A burst bicycle protocol was performed on 31 healthy subjects and 31 patients with LQTS(13 LQT2, 3 LQT1, 15 unknown genotype). The bicycle exercise protocol involved sudden maximal exertion against a fixed workload(200W)for 1 minute. Digitized 12-lead eletrocardiograms were acquired every 10 seconds at baseline for 1 minute, during 1 minute of burst exercise, and for 5 minutes during recovery. Patients with LQTS were segregated according to whether the baseline QTc was normal(≤440 milliseconds, n=13)or abnormal( >440 milliseconds, n=18). Results: During exercise, the QTc increa sed to a greater extent in the group with latent LQTS(ΔQTc 98±36 milliseconds)in comparison with controls(ΔQTc 65±19 milliseconds, P< .01)and those with baseline QTc prolongation(ΔQTc 17±70 milliseconds, P< .01). In patients with a normal baseline QTc, a ΔQTc >85 milliseconds had a sensitivity of 85%and a specificity of 86%for LQTS(P=.0004). Conclusion: Marked QTc prolongation during burst bicycle ergometry provides potentially diagnostic information for patients with normal baseline QTc and suspected LQTS. Objectives: The aim of this study was to develop a diagnostic technique to detect latent long QT syndrome. Background: Asymptomatic patients with genetically diagnosed long QT syndrome (LQTS) may have a normal resting QT interval, yet yet at risk for cardiac events. hypothesized that the QT response during a novel burst exercise protocol simulating clinical events might distinguish patients with “latent LQTS” from healthy subjects. Methods: A burst bicycle protocol was performed on 31 healthy subjects and 31 patients with LQTS (13 LQT2, 3 LQT1, 15 unknown genotype). The bicycle exercise protocol involves sudden maximal exertion against a fixed workload (200W) for 1 minute. Digitized 12-lead eletrocardiograms were acquired every 10 seconds at baseline for 1 minute, during 1 minute of burst exercise, and for 5 minutes during recovery. Patients with LQTS were segregated according to whether the baseline QTc was normal (≤ 440 milliseconds, n = 13) or abnormal (> 440 milliseconds, n = 18) sults: During exercise, the QTc increa sed to a greater extent in the group with latent LQTS (ΔQTc 98 ± 36 milliseconds) in comparison with controls (ΔQTc 65 ± 19 milliseconds, P <.01) and those with baseline QTc prolongation (ΔQTc 17 ± 70 milliseconds, P <.01). In patients with a normal baseline QTc, a ΔQTc> 85 milliseconds had a sensitivity of 85% and a specificity of 86% for LQTS (P = .0004). Conclusion: Marked QTc prolongation during burst bicycle ergometry provides potentially diagnostic information for patients with normal baseline QTc and suspected LQTS.