Introduction: “Don’t just measure the QT interval, look at it!” This dictum is based on the fact that specific T-wave patterns in the resting ECG aid in diagnosing long QT syndrome (LQTS) and in identifying the specific genotype. However, provocation tests (like exercise test or epinephrine infusion) are often required to establish a diagnosis when the QT interval is borderline at rest. We recently showed that “quick standing,” a simple bedside test, easily exposes LQTS through the phenomenon of “QT stretching.” Here we attempted to determine if the T-wave morphology changes provoked by standing aid in the diagnosis of LQTS and determining the genotype.
Methods: A quick standing test was performed by 100 LQTS patients (40 LQT1, 42 LQT2 and 18 LQT3) and 100 controls. QT, QTc and T-wave morphology were evaluated at baseline and during maximal “QT stretching” (the time when, in response to standing, the end of the T-wave gets nearest to the next P wave due to R-R–interval shortening without sufficient QT-interval shortening). We used logistic regression to determine if T-wave morphology changes provoked by standing add to the already established diagnostic value of QTc-stretching for identifying LQTS.
Results: During maximal QT stretching, the T-wave morphologies that best discriminated LQTS from controls included: “notched,” “late-onset” and “biphasic” T-waves. These 3 categories were grouped into a category named “abnormal T-wave response to standing”. During quick standing, a QTc-stretched ≥ 490 msec increased the odds for correctly identifying LQTS by 21 (95% CI 7-58, p<0.001) and these odds were further increased to 145 (95% CI 44-480, p<0.001) with the combination of QTc stretching ≥ 490 plus abnormal “T wave morphology response to standing.” T-wave morphology changes in response to standing were most helpful for identifying LQT1, less helpful for LQT2 and least helpful for LQT3.
Conclusion: The sudden heart rate acceleration produced by abrupt standing, not only increases the QTc but also exposes abnormal T-waves that are valuable for diagnosing LQTS.
