Sören Weyer

Bioelectrical impedance spectroscopy as a fluid management system in heart failure

Episodes of hospitalization for heart failure patients are frequent and are often accompanied by fluid accumulations. The change of the body impedance, measured by bioimpendace spectroscopy, is an indicator of the water content. The hypothesis was that it is possible to detect edema from the impedance data. First, a finite integration technique was applied to test the feasibility and allowed a theoretical analysis of current flows through the body. Based on the results of the simulations, a clinical study was designed and conducted. The segmental impedances of 25 patients suffering from heart failure were monitored over their recompensation process. The mean age of the patients was 73.8 and their mean body mass index was 28.6. From these raw data the model parameters from the Cole model were deduced by an automatic fitting algorithm. These model data were used to classify the edema status of the patient. The baseline values of the regression lines of the extra- and intracellular resistance from the transthoracic measurement and the baseline value of the regression line of the extracellular resistance from the foot-to-foot measurement were identified as important parameters for the detection of peripheral edema. The rate of change of the imaginary impedance at the characteristic frequency and the mean intracellular resistance from the foot-to-foot measurement were identified as important parameters for the detection of pulmonary edema. To classify the data, two decision trees were considered: One should detect pulmonary edema (n(pulmonary) = 13, n(none) = 12) and the other peripheral edema (n(peripheral) = 12, n(none) = 13). Peripheral edema could be detected with a sensitivity of 100% and a specificity of 90%. The detection of pulmonary edema showed a sensitivity of 92.31% and a specificity of 100%. The leave-one-out cross-validation-error for the peripheral edema detection was 12% and 8% for the detection of pulmonary edema. This enables the application of BIS as an early warning system for cardiac decompensation with the potential to optimize patient care.

A differential capacitive electrical field sensor array for contactless measurement of respiratory rate.

Although respiratory rate is an important vital sign for early detection of deterioration, on general wards it is not routinely monitored. Since patients may not tolerate cables attached to their chest, we developed an unobtrusive and contactless measurement method which can be placed under a mattress. The sensor array uses the Maxwell-Wagner relaxation effect by capacitive injection of a high-frequency voltage into the torso and subsequent measurement of respiratory-induced phase shift. Simulations of the entire measurement scenario indicate an improved signal-to-noise ratio if a differential method is applied with specific positioning of the electrodes. A prototype was designed and the simulations were confirmed with measurements on a phantom and in a human self-experiment by the authors using self-constructed hardware. Movement artifacts were detected using an artifact detection algorithm which allows reliable estimation of the respiratory rate.

Development of a real-time, semi-capacitive impedance phlebography device

Abstract Chronic venous insufficiency of the lower limbs is a disease which is caused by an increased blood pressure inside the veins of the leg and the resulting increase of the contained blood volume. This work focuses on developing a device which uses impedance plethysmography, also known as impedance phlebography, to obtain information about the blood volume in the lower leg and provides the possibility to measure the impedance semi contact-less, e.g. through compression stockings. Furthermore a real-time beat-to-beat interval detection algorithm was implemented. Finally, the function of the developed impedance measuring system and the whole system was verified by comparing it with a gold standard. In comparison to the conductive coupling, the system performed similarly. The analysis showed that the developed system is suitable for semi-capacitive IPG. The algorithm was implemented conservatively since it provided a good false-positive rate of 0 %, but only a moderate sensitivity of about 68 %. Reliable and continuous measurement of the pulse signal was only possible in periods of immobility.

A physiological model for extracorporeal oxygenation controller design

Long term extracorporeal membrane oxygenation can be used in cases of severe lung failure to maintain sufficient gas exchange without the need to apply higher ventilation pressures which damage the lung additionally. The use of cardiopulmonary bypass devices is well established inside the operating room. The usage of such devices as long-term support in the intensive care unit is still experimental and limited to few cases. This is because neither machine architecture nor staff situation provides for the long term application scenario. In the joint research Project “smart ECLA” we target an advanced ECMO device featuring an automation system capable of maintaining gas concentrations automatically. One key requirement for systematic controller design is the availability of a process model, which will be presented in this article.

Closed-Loop Control of Humidification for Artifact Reduction in Capacitive ECG Measurements

Recording biosignals without the need for direct skin contact offers new opportunities for ubiquitous health monitoring. Electrodes with capacitive coupling have been shown to be suitable for the monitoring of electrical potentials on the body surface, in particular ECG. However, due to triboelectric charge generation and motion artifacts, signal and thus diagnostic quality is inferior to galvanic coupling. Active closed-loop humidification of capacitive electrodes is proposed in this work as a new concept to improve signal quality. A capacitive ECG recording system integrated into a common car seat is presented. It can regulate the micro climate at the interface of electrode and patient by actively dispensing water vapour and monitoring humidity in a closed-loop approach. As a regenerative water reservoir, silica gel is used. The system was evaluated with respect to subjective and objective ECG signal quality. Active humidification was found to have a significant positive effect in case of previously poor quality. Also, it had no diminishing effect in case of already good signal quality.

RheoStim: Development of an Adaptive Multi-Sensor to Prevent Venous Stasis

Chronic venous insufficiency of the lower limbs is often underestimated and, in the absence of therapy, results in increasingly severe complications, including therapy-resistant tissue defects. Therefore, early diagnosis and adequate therapy is of particular importance. External counter pulsation (ECP) therapy is a method used to assist the venous system. The main principle of ECP is to squeeze the inner leg vessels by muscle contractions, which are evoked by functional electrical stimulation. A new adaptive trigger method is proposed, which improves and supplements the current therapeutic options by means of pulse synchronous electro-stimulation of the muscle pump. For this purpose, blood flow is determined by multi-sensor plethysmography. The hardware design and signal processing of this novel multi-sensor plethysmography device are introduced. The merged signal is used to determine the phase of the cardiac cycle, to ensure stimulation of the muscle pump during the filling phase of the heart. The pulse detection of the system is validated against a gold standard and provides a sensitivity of 98% and a false-negative rate of 2% after physical exertion. Furthermore, flow enhancement of the system has been validated by duplex ultrasonography. The results show a highly increased blood flow in the popliteal vein at the knee.

Periodic funnel-based control for peak inspiratory pressure

Intensive care unit patients generally require a mechanical ventilator with the aim to improve gas exchange. Hereby, the quality of artificial ventilation significantly depends on the mechanical ventilator in terms of its stability, accuracy and transient response. In this article, we demonstrate a non-identifier based control algorithm, called funnel-based control of peak inspiratory pressure (PIP). In pressure-controlled ventilation based on our tailor-made mechanical ventilation system, PIP can be periodically regulated to the pre-defined target reference value using an output feedback structure, so that airway pressure can be delivered into the front-end respiratory system. The goal of the control system is to guarantee a fast response with minimal steady state error on a breath-by-breath basis and to suppress the overshooting of the response, in order to avoid alveoli overinflation and possible lung injury. The control performance is assessed with distinctive results from a number of experiments with a plastic lung, based on the hyperbolic and exponential funnel performance boundaries.

Development of a wearable multi-frequency impedance cardiography device

Abstract Cardiovascular diseases as well as pulmonary oedema can be early diagnosed using vital signs and thoracic bio-impedance. By recording the electrocardiogram (ECG) and the impedance cardiogram (ICG), vital parameters are captured continuously. The aim of this study is the continuous monitoring of ECG and multi-frequency ICG by a mobile system. A mobile measuring system, based on ‘low-power’ ECG, ICG and an included radio transmission is described. Due to the high component integration, a board size of only 6.5 cm × 5 cm could be realized. The measured data can be transmitted via Bluetooth and visualized on a portable monitor. By using energy-efficient hardware, the system can operate for up to 18 hs with a 3 V battery, continuously sending data via Bluetooth. Longer operating times can be realized by decreased transfer rates. The relative error of the impedance measurement was less than 1%. The ECG and ICG measurements allow an approximate calculation of the heart stroke volume. The ECG and the measured impedance showed a high correlation to commercial devices (r = 0.83, p < 0.05). In addition to commercial devices, the developed system allows a multi-frequency measurement of the thoracic impedance between 5–150 kHz.

Feasibility of Bioelectrical Impedance Spectroscopy Measurement before and after Thoracentesis

Background. Bioelectrical impedance spectroscopy is applied to measure changes in tissue composition. The aim of this study was to evaluate its feasibility in measuring the fluid shift after thoracentesis in patients with pleural effusion. Methods. 45 participants (21 with pleural effusion and 24 healthy subjects) were included. Bioelectrical impedance was analyzed for “Transthoracic,” “Foot to Foot,” “Foot to Hand,” and “Hand to Hand” vectors in low and high frequency domain before and after thoracentesis. Healthy subjects were measured at a single time point. Results. The mean volume of removed pleural effusion was 1169 ± 513 mL. The “Foot to Foot,” “Hand to Hand,” and “Foot to Hand” vector indicated a trend for increased bioelectrical impedance after thoracentesis. Values for the low frequency domain in the “Transthoracic” vector increased significantly (P < 0.001). A moderate correlation was observed between the amount of removed fluid and impedance change in the low frequency domain using the “Foot to Hand” vector (r = −0.7). Conclusion. Bioelectrical impedance changes in correlation with the thoracic fluid level. It was feasible to monitor significant fluid shifts and loss after thoracentesis in the “Transthoracic” vector by means of bioelectrical impedance spectroscopy. The trial is registered with Registration Numbers IRB EK206/11 and NCT01778270.

A model-based approach for analysis of intracellular resistance variations due to body posture on bioimpedance measurements

Bioimpedance spectroscopy is a known option for measuring body fluid volume. However, it is prone to a variety of influence factors which prevent a wider use. One of these influencing factors is the body posture. It could be shown that the average intracellular resistance percentage changes when the subject changes position from lying to standing. Most authors explain this phenomenon by fluid shifts. Another possible reason is the stray capacitance between the body and the ground, because if a certain fraction of the injected current follows other paths than between the potential electrodes, the result will be wrong. This paper analyses the influence of different body postures on the measured intracellular resistance and the posture depending capacity. For this purpose, FEM simulations are used. Subsequently, an electrical equivalent model with capacitances was developed. With this model, it is possible to correct the measured impedance and to neglect the influence of the stray capacitance.