Comparison between muscle sympathetic nerve activity and calf vascular resistance with head-up tilting in humans.

The purpose of this study was to determine the relationship between muscle sympathetic nerve activity (MSA) to calf muscle and calf vascular resistance with head-up tilting in humans. In nine healthy volunteers, we performed simultaneous measurements of MSA using microneurography and calf vascular resistance using electrical impedance plethysmography in the same leg. The volunteers were laid on a tilt table and the examinations with head-up tilting (0 degrees, 30 degrees, 60 degrees) were performed. Corresponding to the increase in angles of head-up tilting, MSA and calf vascular resistance significantly increased. A statistically significant positive linear correlation was demonstrated between MSA and calf vascular resistance. These data suggest that impedance plethysmographic measurement of calf vascular resistance can be reliably used to assess the status of MSA with head-up tilting.