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Available descriptors of irregularities of ventricular repolarization are of limited clinical value. We studied the effect of autonomic variations on several new descriptors of the three-dimensional T loop. Twelve-lead digital ECGs were recorded continuously in 40 healthy subjects at baseline in the supine position, during postural changes (supine → sitting → standing → supine → standing), and during Valsalva maneuver performed three times in the supine and three times in the standing positions. A minimum dimensional space was constructed from the 12-lead ECG, using singular value decomposition, on the basis of median ECG beats constructed from 10-second consecutive ECG recordings. Temporal variations (TLA and PL, which measure the T loop area, and LD, the interlead relationship during repolarization) and wavefront direction descriptors (TCRT, the deviation between the QRS and T vectors) were calculated and expressed as normalized values. Values of TLA, PL, and TCRT were significantly lower in the sitting than in the supine position (-38139 ± 9099 vs 47133 ± 7511, -0.017 ± 0.005 vs 0.033 ± 0.005 and -0.032 ± 0.019 vs 0.071 ± 0.015, respectively, P < 0.001 for all) and decreased further in the standing position (-88288 ± 14468, -0.067 ± 0.013, -0.198 ± 0.025, respectively, P < 0.001 for all). LD increased from supine to sitting (98.7 ± 29.4 vs -87.5 ± 15.2, P < 0.001) and increased further, though nonsignificantly in the standing position (118.3 ± 35.2). TLA, PL, and TCRT decreased from baseline during Valsalva in the supine (-34118 ± 11424 vs 62234 ± 12215, -0.038 ± 0.014 vs 0.065 ± 0.010, -0.08 ± 0.03 vs 0.10 ± 0.02, respectively, P < 0.001 for all). LD was significantly increased by Valsalva in the supine position (13 ± 46 vs -153 ± 30, P < 0.001) and nonsignificantly in the standing position (99 ± 50 vs 86 ± 30, P = NS). There were significant correlations among TLA, PL, and LD, and no significant correlation between TCRT and any of the temporal variation descriptors. These new temporal and wavefront direction descriptors are sensitive and rapid detectors of autonomic effects on ventricular repolarization. [ABSTRACT FROM AUTHOR]