Utility of a simplified lidocaine and potassium infusion in diagnosing long QT syndrome among patients with borderline QTc interval prolongation

Background: Congenital long QT syndrome (LQTS) is caused by mutations in the cardiac Na+ or K+ channels that result in a prolonged QTc interval and increased QT dispersion. Na+ channel blockers and K+ can reverse the repolarization abnormalities in the Na+ channel variant (LQT3) and K+ channel variant (LQT1, LQT2), respectively. The phenotype of LQTS can be difficult to recognize, especially when the QTc interval is mildly prolonged. Additional noninvasive testing methods are needed to enhance the diagnosis of LQTS. This study compared the response of the QTc interval and QT dispersion to a sequential lidocaine/K+ infusion in LQTS patients with borderline QTc interval prolongation and control patients as a means of diagnosing LQTS. Methods: In this study, eight LQTS patients with borderline QTc, defined as QTc < 470 ms, and 10 healthy controls received sequential lidocaine/K+ infusion. Results: At baseline, LQTS patients had a longer QTc (446 +/- 29 vs 416 +/- 28 ms, P < 0.05) but similar QT dispersion (43 +/- 14 vs 29 +/- 10 ms) compared to controls. After lidocaine administration, baseline QTc and QT dispersion did not change in either LQTS or controls. One LQTS patient had a 54 ms (12%) reduction in his QTc but no change in QT dispersion. Following K+ infusion, baseline QTc and QT dispersion decreased by 9% (P < 0.005) and 45% (P < 0.005), respectively in LQTS. No effect was seen in control patients, where QTc and QT dispersion shortened by 1% (5 +/- 14 ms) and 20% (6 +/- 7 ms), respectively, compared to baseline. The combined lidocaine/K+ infusion had a sensitivity, specificity, and accuracy of 88%, 100%, and 94%, respectively, in diagnosing LQTS. Conclusions: A simplified sequential lidocaine/K+ challenge is accurate in diagnosing LQTS among patients with borderline QTc prolongation.