Mercury in silastic strain gauge plethysmography for the clinical assessment of the microcirculation

We have used the mercury in silastic strain gauge plethysmography (MSG) system, which is wholly non-invasive, to obtain values (mean +/- s.e.m.) for the following parameters in the legs of supine control subjects: venous pressure (Pv) 9.89 +/- 0.88 mmHg, vascular compliance (Comp) 3.50 +/- 0.29 ml 100 g-1 mmHg-1 x 10(-2), microvascular hydraulic conductivity (Kf) 4.10 +/- 0.16 ml min-1 100 g-1 mmHg-1 x 10(-3) and isovolumetric venous pressure (Pvi) 22.12 +/- 0.82 mmHg. On passive foot-down tilting of control subjects through about 45 degrees, both Pv and Pvi rose to 28.00 +/- 2.33 and 36.58 +/- 2.29 mmHg respectively. The increases in both parameters were highly significant, P < 0.0001. We found that the time course of the vascular compliance component was short if small pressure steps were used. The value increased markedly if large steps were used. We also observed that none of the microvascular parameters studied in the arms differed from those in the legs of the same subjects. We observed that the mean value Kf in the arms of 12 young, non-neuropathic diabetics (9.08 +/- 6.12 x 10(-3)) was significantly greater than that of age and sex-matched controls (P < 0.001). Studies on 14 venous ulceration patients indicated that the values for Kf and Pv were unchanged, whilst those of vascular compliance (5.60 +/- 0.58 x 10(-2) ml 100 g-1 mmHg-1) and Pvi (37.30 +/- 2.25 mmHg) were significantly greater than those found in age-matched controls (P < 0.002 and 0.001, respectively). We feel that Pvi is a useful index of microvascular perfusion. We attribute the pressure-dependent difference, in the time course of the vascular compliance response, to the veni-arteriolar reflex. We feel that the pressure dependency of this response might form the basis for the assessment of peripheral autonomic neuropathy. We believe that the MSG system offers a means of monitoring the development of pathologies resulting from microvascular under-perfusion and, of course, the improvement in response to therapeutic interventions.