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_supine and LF_supine), HR and LF at 50° tilt (HR_tilt and LF_tilt), and of the range and position of the sympathovagal transition (SVT_range, SVT_position ) by computing the coefficient of variation (CV), the relative error (RE), and the reliability coefficient (RC). HR_supine reproduced best (CV=4%, RE=8%, and RC=96%), followed by SVT_position , HR_tilt, LF_supine, LF_tilt, and SVT_range (CV=35%, RE=25%, and RC=48%). It is concluded that HR and SVT_position reproduce much better than LF. The remarkable lack of reproducibility of SVT_range suggests that the size of the sympathovagal transition range is a dynamic autonomic feature within subjects