Robustness of focused and global impedance estimates of bladder volumes against uncertainty of urine conductivity

Abstract

Bioimpedance measurements are currently used to monitor various biological processes and are potentially useful for studies of urodynamics. Global impedance (GI) and focused impedance measurements (FIM) can be used to monitor bladder volumes, but these are subject to varying conductivity of urine. To address this, we emulated a human bladder using an agar phantom filled with saline solutions of varying conductivities and estimated volumes using a modified FIM-based approach. Using this novel strategy, electrical potentials did not change significantly with constant liquid volumes, even when the conductivity of the saline solutions was varied between 1.027 to 1.877 and 2.610 S/m. Conversely, GI and classic FIM measurements of constant liquid volumes varied with conductivity. These observations suggest that the proposed FIM approach is suitable for bladder volume estimation due to its robustness against uncertainties of conductivity. The bioimpedance hardware used in our experiments comprised 8 electrodes and a a small and low cost impedance measurement system based on an AFE4300 direct impedance measurement device.

Noninvasive Bladder Volume Monitoring Using Bioimpedance

“…The study of lower urinary tract function is called urodynamics, which consists of monitoring the filling and emptying of the bladder for the diagnosis and management of lower urinary tract dysfunction. This dysfunction occurs in elderly people or those with neural disorders, such as spinal cord diseases, which generate urinary incontinence (UI), detrusor hyperactivity, and prostatic hyperplasia, pathologies that cause urinary tract infections [7,8].…”

“…This measurement reflects the kinetics of detrusor contraction. This method cannot be used as the only alternative for the diagnosis of urinary tract pathologies [7,8,10].…”

“…In bladder volume monitoring, EIT presents many advances that have made it possible to establish a correlation between urine conductivity and bladder volume using the Global Impedance Index (GI) [4], which is calculated from the pixels of the EIT image reconstruction. On the other hand, there are indices based on bioimpedance measurements that do not require image reconstruction algorithms to estimate bladder volumes, such as the IRM (Impedance Ratio Method) [6], VCR (Voltage Change Ratio) [13], and MVCR (Modified Voltage Change Ratio) [7], which, like GI, present a correlation between bioimpedance measurements and bladder volume. The aforementioned methods are highly sensitive to the change in urine conductivity, which occurs due to infections or the patient's diet, so volume estimation with these methods cannot be applied in the long term.…”

“…For this reason, a method based on global focused impedance (FIM) is proposed in Ref. [7] that allows the estimation of bladder volume independently of urine conductivity, which allows long-term monitoring.…”

“…The mathematical background of the EIT is explained in Ref. [7]; where Maxwell equations and the Faraday and Ampere laws are formulated in the differential form to define the relationship between the admittivity (γ) and the potential on the electrodes (φ) (Eq. 1).…”