The response of the QT interval to the brief tachycardia provoked by standing: a bedside test for diagnosing long QT syndrome.

OBJECTIVES: This study was undertaken to determine whether the short-lived sinus tachycardia that occurs during standing will expose changes in the QT interval that are of diagnostic value. BACKGROUND: The QT interval shortens during heart rate acceleration, but this response is not instantaneous. We tested whether the transient, sudden sinus tachycardia that occurs during standing would expose abnormal QT interval prolongation in patients with long QT syndrome (LQTS). METHODS: Patients (68 with LQTS [LQT1 46%, LQT2 41%, LQT3 4%, not genotyped 9%] and 82 control subjects) underwent a baseline electrocardiogram (ECG) while resting in the supine position and were then asked to get up quickly and stand still during continuous ECG recording. The QT interval was studied at baseline and during maximal sinus tachycardia, maximal QT interval prolongation, and maximal QT interval stretching. RESULTS: In response to brisk standing, patients and control subjects responded with similar heart rate acceleration of 28 +/- 10 beats/min (p = 0.261). However, the response of the QT interval to this tachycardia differed: on average, the QT interval of controls shortened by 21 +/- 19 ms whereas the QT interval of LQTS patients increased by 4 +/- 34 ms (p < 0.001). Since the RR interval shortened more than the QT interval, during maximal tachycardia the corrected QT interval increased by 50 +/- 30 ms in the control group and by 89 +/- 47 ms in the LQTS group (p < 0.001). Receiver-operating characteristic curves showed that the test adds diagnostic value. The response of the QT interval to brisk standing was particularly impaired in patients with LQT2. CONCLUSIONS: Evaluation of the response of the QT interval to the brisk tachycardia induced by standing provides important information that aids in the diagnosis of LQTS.

J-point elevation in survivors of primary ventricular fibrillation and matched control subjects: incidence and clinical significance.

OBJECTIVES: The purpose of this study was to determine whether J-point elevation is a marker of arrhythmic risk. BACKGROUND: J-point elevation has been considered an innocent finding among healthy young individuals (the "early repolarization" pattern). However, this electrocardiogram (ECG) finding is increasingly being associated with idiopathic ventricular fibrillation (VF). METHODS: In a case-control study, the ECG of 45 patients with idiopathic VF were compared with those of 124 age- and gender-matched control subjects and with those of 121 young athletes. We measured the height of J-point and ST-segment elevation and counted the presence of slurring in the terminal portion of the R-wave. RESULTS: J-point elevation was more common among patients with idiopathic VF than among matched control subjects (42% vs. 13%, p = 0.001). This was true for J-point elevation in the inferior leads (27% vs. 8%, p = 0.006) and for J-point elevation in leads I to aVL (13% vs. 1%, p = 0.009). J-point elevation in V(4) to V(6) occurred with equal frequency among patients and matched control subjects (6.7% vs. 7.3%, p = 0.86). Male subjects had J-point elevation more often than female subjects and young athletes had J-point elevation more often than healthy adults but less often than patients with idiopathic VF. The presence of ST-segment elevation or QRS slurring did not add diagnostic value to the presence of J-point elevation. CONCLUSIONS: J-point elevation is found more frequently among patients with idiopathic VF than among healthy control subjects. The frequency of J-point elevation among young athletes is intermediate (higher than among healthy adults but lower than among patients with idiopathic VF).