Influence of head-down and lateral decubitus neck flexion on heart rate variability

The purpose of this study was to examine the response of heart rate variabi lity (HRV), a noninvasive index of autonomic control, to head-down neck fle xion (HDNF), which engages both otoliths and neck muscle afferents, and to lateral decubitus neck flexion (LNF), in which neck afferents are activated , whereas otolith afferent input is not. HRV and forearm blood flow were ev aluated in participants lying prone, during HDNF, lying in the lateral decu bitus position, and during LNF. Compared with the prone position, HDNF resu lted in lower high-frequency (46.9 +/- 7.1 vs. 62.3 +/- 6.2) and higher low -frequency (53.1 +/- 7.1 vs. 37.7 +/- 6.2) power, expressed as normalized u nits, along with higher low-frequency-to-high-frequency ratio (1.65 +/- 0.3 vs. 0.78 +/- 0.2), whereas LNF resulted in no alterations in HRV indexes. Furthermore, there were no significant differences in forearm blood flow or vascular resistance among any of the positions. Our data suggest that otol ith organs influence autonomic modulation of the heart, supporting previous studies reporting that HDNF elicits increased sympathetic outflow. These d ata further suggest that HDNF results in a parasympathetic withdrawal from the heart in addition to sympathetic activation.