Renal function monitoring

Electrical Bioimpedance measurements have been used for many years to study the electrical properties of biologic tissue and to measure physiological events, being applied in several clinical areas, including body composition. The assessment of oedema formation from EBI measurements is based on the dependency of the electrical properties of tissue on its structure and intrinsic constituents, i.e. alterations in the tissue structure produce a modification of its electrical properties.

When the renal function is impaired in the kidneys, an immediate physiological consequence is an excessive accumulation of liquid in the rest of the body, and consequently oedema is formed. Oedema is considered by the physician a common indicator of renal failure, and therefore visual inspection by the physician is a regular practice to detect such kidney function impairment. Such an inspection is usually performed targeting the limbs, legs and arms, targeting the inspection for peripheral oedema.

There are several methods of assessing extracellular swelling, and the EBI measurement approach is one of the more comfortable ones, due to the fact that the electrical properties of tissue can be measured non-invasively and without tightening the skin. Some studies about the excess of fluid in chronic HD patients, using Bioimpedance spectroscopy measurements, where performed. The experimental results suggested that HD patients keep their excess fluid volume primarily in the extracellular compartment (interstitial fluid). Bioimpedance spectroscopy together with a stable measurement of lean tissue can determine the degree of relative excess hydration. Due to the capacitance effects of the cell membrane, the tissue impedance depends on the measurement frequency. As a consequence, the accuracy of measurements by means of multiple frequency Bioimpedance spectroscopy analysis is superior to the accuracy of measurements based on a single frequency for the prediction of extracellular water. Some authors suggest that the best frequency range to assess extra-cellular fluid is up to 10 kHz and the range between 50-100 kHz is a suitable measurement range for a successful assessment of extra- and intra-cellular fluid.

Concluding, the monitoring principle of the EBI measurement system implemented in this thesis work lays as follows: during renal failure the amount of interstitial fluid in the limbs increases, causing extracellular oedema. The consequent interstitial swelling modifies the electrical properties of the tissue, and by means of the combination of non- invasive EBI measurements with skin-surface electrodes and EBI spectroscopy analysis, the ongoing swelling can be detected. Therefore changes in the EBI of the limbs may be used as an efficient indicator for early detection of renal failure.