Bioimpedance spectroscopy in haemodynamic analysis

Abstract

Venous insufficiency is estimated to affect between 10% and 35% of the population in the US with symptoms ranging from mild discomfort to chronic ulceration which may reduce quality of life. Early diagnosis is the key to pre-emptive treatment. We have previously reported [5] an impedance technique for measurement of calf muscle pump function although it was noted that the results could be confounded by change in limb geometry during the exercise protocol. We report here a modified protocol to account for change in limb geometry. Impedance of a 20 cm segment of the calf was continuously recorded using an SFB7 bioimpedance spectrometer whilst subjects performed a sequence of manoeuvres: a) supine with leg raised (for 10 min); b) standing (4 s); c) plantar flexion (tiptoe, 4 s); d) standing elevated on one leg removing tension on the measured leg with the foot horizontal (4 s); e) with the leg relaxed (4 s) and then the sequence repeated. The impedance ratio, k = (e-d)/(c-b), was the proportion of the impedance change, occurring during calf muscle pumping, due primarily to change in limb shape only, i.e. independent of the impedance change due to ejection of blood as a function of calf muscle pump action. Thus (1-k) can be used to correct the ejection fraction (%) calculated as (c-b)/(a-b)*100 for the confounding effect of change in limb geometry. Ejection fractions calculated by this method in 10 control subjects were 51.5 ± 30.1% with no values greater than 100% as found previously.