Reproducibility of heart rate and rate variability responses to incremental head-up tilt

Recently, the authors have developed a protocol that invokes gradual autonomic changes by incremental tilt. For each tilt angle, heart rate (HR) and the normalized low-frequency (0.07-0.14 Hz) heart rate variability power (LF) are computed. Linear regression of LF on HR allows assessment of the sympathovagal transition (SVT). After 5 to 8 months, the authors repeated the measurements in young healthy males (n=17), and assessed the reproducibility of the supine HR and LF (HR/sub supine/ and LF/sub supine/), HR and LF at 50/spl deg/ tilt (HR/sub tilt/ and LF/sub tilt/), and of the range and position of the sympathovagal transition (SVT/sub range/, SVT/sub position/) by computing the coefficient of variation (CV), the relative error (RE), and the reliability coefficient (RC). HR/sub supine/ reproduced best (CV=4%, RE=8%, and RC=96%), followed by SVT/sub position/, HR/sub tilt/, LF/sub supine/, LF/sub tilt/, and SVT/sub range/ (CV=35%, RE=25%, and RC=48%). It is concluded that HR and SVT/sub position/ reproduce much better than LF. The remarkable lack of reproducibility of SVT/sub range/ suggests that the size of the sympathovagal transition range is a dynamic autonomic feature within subjects. >