Description and validation of a novel liquid metal-free device for venous congestion plethysmography

We present a newly developed electromechanical sensor with automated calibration for strain-gauge plethysmography (filtrass) and compare it to a conventional mercury-in-Silastic strain-gauge plethysmograph (MSG). Fluid filtration capacity (K(f)) and isovolumetric venous pressure (Piv) of the limb were assessed noninvasively with both devices in 29 healthy volunteers. We found significantly higher K(f) and Piv values with MSG [4.6 +/- 2.0 x 10(-3) ml. min(-1). mmHg(-1). 100 ml tissue(-1) (K(f) units; K(f)U) and 21.2 +/- 8.1 mmHg for Pvi], than with filtrass, giving values of 3.1 +/- 0.8 K(f)U and 15.1 +/- 7.1 mmHg. Because K(f) and Piv are profoundly influenced by the calibration, we investigated the quality of the calibration signal and its impact on the obtained values. We could show that the reproducibility of repeated calibrations was higher with filtrass (58% lower mean +/- SD). The data were grouped according to the quality of calibration, and we found no significant difference in K(f) and Piv between filtrass (3.0 +/- 0.7 K(f)U and 15.9 +/- 6.9 mmHg, respectively) and MSG with good calibration signal (3.3 +/- 0. 8 K(f)U and 18.6 +/- 7.1 mmHg, respectively; no significant difference). However, we obtained significantly higher MSG values (5. 6 +/- 2.0 K(f)U and 23.1 +/- 8.4 mmHg, respectively; P < 0.001) in the group with a bad calibration signal. We suggest that the filtrass sensor, which performs an automatic, standardized calibration procedure and shows a linear signal response to stretch, gives highly reproducible and reliable results and thus is more suitable for routine application.