The effect of passive tilting on microvascular parameters in the human calf: a strain gauge plethysmography study.

1. Cumulative small steps in venous congestion pressure were used to study the effect of passive tilt on vascular parameters in dependent tissues. Using this protocol we have non-invasively assessed venous pressure (Pv,est), isovolumetric cuff pressure (Pv,i), which is the congestion cuff pressure (Pcuff) that has to be exceeded to induce fluid filtration. We have also assessed microvascular filtration capacity (Kf), which is the linear relationship between filtration rate (Jv) and Pcuff, when Pcuff > Pv,i, and is the product of the available exchange vessel surface area and wall conductance. 2. Subjects were passively tilted to increase the venous pressure at the level of the calf by 47.4 +/- 2.4 mmHg (mean +/- S.E.M.). The value of Pv,i increased from 20.6 +/- 1.8 to 48.5 +/- 3.8 mmHg after the imposition of the tilt. This change may reflect the increased colloid osmotic pressure at the microvascular interface that is known to occur in response to this manoeuvre. 3. The pre-tilt value of Kf did not change after the imposition of the passive tilt, the values being 3.2 +/- 0.4 x 10(-3) and 3.6 +/- 0.4 x 10(-3) ml min-1 (100 ml-1) mmHg-1, respectively, (n = 13). 4. These results support the notion that passive postural change alters the pre-capillary resistance, thereby altering the pressure and flow characteristics within the exchange vessels, but does not alter the surface area available for fluid exchange in the calf, contrary to previous findings in the dependent human foot using a single-step venous occlusion protocol.