Microvascular fluid filtration capacity (Kf) assessed with cumulative small venous pressure steps and with various degrees of tilt.

Orthostatic dysregulation is a frequent phenomenon in pilots experiencing extreme G forces and after prolonged exposures to microgravity in cosmonauts. We used non-invasive venous congestion plethysmography (VCP) to study microcirculatory changes and the ability to activate peripheral and centrally mediated protective mechanisms during various degrees of tilt, which we used as an orthostatic challenge.The study, which was approved by the local ethical committee of the Institute of Biomedical Problems, was performed on six healthy 20 - 26 years old male volunteers. We applied 6 - 8 cumulative small venous congestion pressure steps (8 mmHg) to the thigh and determined the fluid filtration capacity (Kf), the linear relationship between cuff pressure (Pcuff) and measured fluid filtration (Jv). We then measured the fluid filtration (Jv) response to varying cumulative degrees of tilt, starting at 0 degrees followed by head down -8 degrees -15 degrees, -30 degrees -15 degrees, -8 degrees, 0 degrees, and then head up 15 degrees, 30 degrees, 70 degrees, 30 degrees, 15 degrees and 0 degrees. Each tilt stage was sustained for 15 minutes. The change in hydrostatic load, at the level of the strain gauge, was determined by measuring the difference in vertical height between the right atrium and mid calf at the level of the gauge. Limb arterial blood flow was measured and lung function tests were performed before and after the small cumulative pressure step protocol, as well as at the end of each tilt step.No significant changes in blood pressure (BP) and heart rate (HR) occurred during the cumulative pressure step protocol. However, an increase in HR was observed during the tilt, but only reached significance at 30 degrees and 70 degrees. The mean Kf value measured with small cumulative pressure steps was 3.25 +/- 0.5 (10(-3) ml.100 ml tissue(-1) mmHg(-1) = KfU), which was significantly (p < 0.005) higher than the value obtained using tilt to increase the hydrostatic load (0.98 +/- 0.2 KfU). One subject had an unchanged Kf value and experienced vaso - vagal syncope following the imposition of 70 degrees tilt, with a heart rate < 35 / min and a systolic blood pressure of 60 mmHg. The measurement of blood flow in the limb showed no significant change during the tilt procedure. The lung function measurements revealed, that only expiratory reserve volume (ERV) did significantly increase during 35 degrees and 70 degrees head up tilt. All other parameters were unchanged. We suggest that the differences in these Kf values can be explained by the activation of both central and peripheral mediated changes in pre- capillary resistance in response to the tilt. We suspect, that the vaso - vagal syncope, experienced by one subject, reflected his inability to activate these protective reflex mechanisms, a situation that could be exacerbated by sub-clinical venous insufficiency. We believe that these results show that VCP is a useful tool for the determination of intolerance to orthostatic stress.