SUMMARY1. We have used non-invasive mercury in a silastic strain gauge system to assess the effect of pressure step size, on the time course of the rapid volume response (RVR) to occlusion pressure. We also obtained values for hydraulic conductance (Kf), isovolumnetric venous pressure (Pvi) and venous pressure (Pv) in thirty-five studies on the legs of twenty-three supine control subjects.2. The initial rapid volume response to small (953 + 045 mmHg, mean +S.E.M.) stepped increases in venous pressure, the rapid volume response, could be described by a single exponential of time constant 15-54 + 114 s.3. Increasing the size of the pressure step, to 49 8+ 1+ mmHg, gave a larger value for the RVR time constant (mean 77-3 + 11P6 s).4. We propose that the pressure-dependent difference in the duration of the rapid volume response, in these two situations, might be due to a vascular smooth musclebased mechanism, e.g. the veni-arteriolar reflex.5. The mean (+ S.E.M.) values for Kf, Pvi and Pv were 427+018 (units, ml min-(100 g)-1 mmHg-1 x 10-3), 21 50+0 81 (units, mmHg) and 9-11 +0-94 (units, mmHg), respectively.6. During simultaneous assessment of these parameters in arms and legs, it was found that they did not differ significantly from one another.7. We propose that the mercury strain gauge system offers a useful, non-invasive means of studying the mechanisms governing fluid filtration in human limbs.

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Venous congestion strain‐gauge plethysmography enables the non‐invasive assessment of arterial blood flow, fluid filtration capacity (Kf), venous pressure (Pv)and isovolumetric venous pressure (Pvi)in man. One of the major assumptions of this technique, that cuff pressure (Peuff) applied to the limb equals Pv at the level of the strain gauge, was tested in this study. In nine healthy male volunteers (mean age, 29.3 ± 1.2 years) the saphenous vein was cannulated with an 18‐gauge catheter proximal to the medial malleolus. The subjects were supine and Pv was continuously measured during the application of small step (8–10 mmHg) increases in congestion Pcuff (up to 70 mm Hg). Pcuff, changes in limb circumference and Pv were recorded by computer for off‐line analysis. Since the determination of Pv is influenced by the changes in plasma oncotic pressure, venous blood samples were obtained at the start of the study, when P& was raised to 30 mmHg and again to 65 mmHg and 4 min after deflation of the cuff. The relationship between Pv and Pcuff was linear over the range of 10–70 mm Hg (n= 9, 69 measurements, slope 0.9.1, r= 0.97, P. <<0.001). The non‐invasively measured calf Pv based on the intercept of the relationship between the vascular compliance component (Vv) and Pcuff, was 8.0±0.4mmHg, which was not significantly different from the corrected invasively measured Pv value of 8.8±0.3 mmHg P= 0.08). Venous blood lactate and haemoglobin concentrations, as well as colloid osmotic pressure, total protein and albumin concentrations were unchanged throughout the protocol, whereas significant decreases in Po2 and blood glucose concentration were observed when Pcuff reached 65 mmHg. Assuming a constant oxygen consumption, this may suggest a reduction in tissue perfusion. This study demonstrates the close correlation between Pcuff and Pvin the saphenous vein. Since the small congestion Pcuff step protocol does not cause significant increase in plasma oncotic pressure, we conclude that Pv as well as Kf can be accurately determined with this venous congestion plethysmography protocol.