Tibia Bone Microvascular Flow Dynamics as Compared to Anterior Tibial Artery Flow During Body Tilt

BACKGROUND: We compared microvascular and macrovascular blood flows of the tibia and anterior tibial artery during graded whole-body tilt. We hypothesized equal responses for bone microvascular and macrovascular blood flows during varying angles of tilt. METHODS: There were 18 volunteers who were randomly positioned in the following postures: supine, 15 degrees head-up tilt, 6 degrees head-up tilt, 6 degrees head-down tilt, and 15 degrees head-down tilt using an inversion table with reference to seated posture (baseline control). Ultrasonography quantified anterior tibial arterial diameter and peak systolic velocity, enabling calculation of macrovascular blood flow to the tibia. Tibial bone microvascular blood flow was measured noninvasively using photoplethysmography in the same leg. RESULTS: Transitioning from a seated position to a supine position, macrovascular blood flow did not change significantly (1.81 +/- 1.18 to 2.80 +/- 1.74cm(3) . s(-1)). However, bone microvascular flow increased significantly (0.36 +/- 0.23 to 1.11 +/- 0.79 V) from the seated to the supine position. Transitioning from a seated posture to 15 degrees head-down tilt, both arterial macrovascular and bone microvascular flows increased significantly (1.81 +/- 1.18 to 3.32 +/- 2.08 cm(3) . s(-1) and 0.36 +/- 0.23 V to 2.99 +/- 2.71 V, respectively). The normalized flow for microvascular blood flow as a function of body tilt was significantly greater than that for macrovascular blood flow at 6 degrees and 15 degrees head-down tilt. DISCUSSION: These data do not support our hypothesis that bone microvascular flow and arterial macrovascular flow share equal responses to altered body tilt. Therefore, for a given decrease in local blood pressure in the leg with head-down body tilt, the magnitude of increase in blood flow is greater in the microcirculation as compared to the feeding artery.