Evaluation of autonomic influences on QT dispersion using the head-up tilttest in healthy subjects

Our objective was to examine the autonomic influence on QT interval dispers ion using the head-up tilt test in healthy subjects. RR and QT intervals, h eart rate variability, and plasma norepinephrine concentration were measure d in the supine position and tilting to 70 degrees for 20 minutes using a f ootboard support in 15 healthy male volunteers (mean age +/- SD: 28.0 +/- 4 .5 years). The rate-corrected QT interval (QTc) was calculated using Bazett 's formula, and QT and QTc dispersions were defined as the maximum minus mi nimum values for the QT and QTc, respectively, from the 12-lead EGG. Spectr al analysis of the heart rate variability generated values for the low- and high-frequency powers (LF and NF) and their ratio (LF/HF). Compared with v alues obtained in the supine position, tilting significantly increased QT ( P < 0.05) and QTc dispersion (P < 0.01), the LF/HF ratio (P < 0.0001), and plasma norepinephrine concentration (P < 0.0001), and significantly decreas ed HF (P < 0.0001). QTc dispersion was positively correlated with the LF/HF ratio and plasma norepinephrine concentration, and negatively correlated w ith HF. These results suggest that head-up tilt testing increases QT disper sion by increasing sympathetic tone and/or decreasing vagal tone in healthy subjects.