A new biomedical sensor for measuring PCO2

Carbon Dioxide Sensing-Biomedical Applications to Human Subjects

Carbon dioxide (CO2) monitoring in human subjects is of crucial importance in medical practice. Transcutaneous monitors based on the Stow-Severinghaus electrode make a good alternative to the painful and risky arterial "blood gases" sampling. Yet, such monitors are not only expensive, but also bulky and continuously drifting, requiring frequent recalibrations by trained medical staff. Aiming at finding alternatives, the full panel of CO2 measurement techniques is thoroughly reviewed. The physicochemical working principle of each sensing technique is given, as well as some typical merit criteria, advantages, and drawbacks. An overview of the main CO2 monitoring methods and sites routinely used in clinical practice is also provided, revealing their constraints and specificities. The reviewed CO2 sensing techniques are then evaluated in view of the latter clinical constraints and transcutaneous sensing coupled to a dye-based fluorescence CO2 sensing seems to offer the best potential for the development of a future non-invasive clinical CO2 monitor.

Tissue gas tensions and tissue metabolites for detection of organ hypoperfusion and ischemia

BACKGROUND

The aim of this study was to evaluate how tissue gas tensions and tissue metabolites measured in situ can detect hypoperfusion and differentiate between aerobic and anaerobic conditions during hemorrhagic shock. We hypothesized that tissue PCO(2) (PtCO(2)) would detect hypoperfusion also under aerobic conditions and detect anaerobic metabolism concomitantly with or earlier than other markers.

METHODS

Prospective experimental animal study with eight anesthetized pigs subjected to a continuous blood loss ∼8% of total blood volume per hour until death. We measured cardiac index, organ blood flows, and tissue levels of PO(2), PCO(2), glucose, pyruvate, lactate, and glycerol in intestine, liver, kidney, and skeletal muscle.

RESULTS

With reduction in blood flow to the organs under aerobic conditions, PtCO(2) increased ∼1-4 kPa from baseline. With the onset of tissue hypoxia there was a pronounced increase of PtCO(2), lactate, lactate-pyruvate (LP) ratio, and glycerol. Tissue pH and bicarbonate decreased significantly, indicating that metabolic acid was buffered by bicarbonate to generate CO(2).

CONCLUSION

Moderate tissue hypoperfusion under aerobic conditions is associated with increased PtCO(2), in contrast to metabolic parameters of ischemia (lactate, LP ratio, and glycerol) which remain low. From the onset of ischemia there is a much more rapid and pronounced increase in PtCO(2), lactate, and LP ratio. PtCO(2) can be used as a marker of hypoperfusion under both aerobic and anaerobic conditions; it gives an earlier warning of hypoperfusion than metabolic markers and increases concomitantly with or earlier than other markers at the onset of tissue anaerobiosis.

Cole parameter estimation from electrical bioconductance spectroscopy measurements

Several applications of Electrical Bioimpedance (EBI) make use of Cole parameters as base of their analysis, therefore Cole parameters estimation has become a very common practice within Multifrequency- and EBI spectroscopy. EBI measurements are very often contaminated with the influence of parasitic capacitances, which contributes to cause a hook-alike measurement artifact at high frequencies in the EBI obtained data. Such measurement artifacts might cause wrong estimations of the Cole parameters, contaminating the whole analysis process and leading to wrong conclusions. In this work, a new approach to estimate the Cole parameters from the real part of the admittance, i.e. the conductance, is presented and its performance is compared with the results produced with the traditional fitting of complex impedance to a depressed semi-circle. The obtained results prove that is feasible to obtain the full Cole equation from only the conductance data and also that the estimation process is safe from the influence capacitive leakage.

Electrical impedance of stainless steel needle electrodes

We present experimental findings regarding variability and stability of the electrical impedance properties of medical grade stainless steel needle electrodes in vitro. Monopolar impedance spectra (1 Hz to 1 MHz) were measured and scanning electron microscope images were obtained for five needle types with active electrode area from 0.28 to 0.7 mm(2). A saline tank (0.9% NaCl) was used as tissue model. Measurements were done before and after electrolytic treatment with anodic and cathodic DC currents of 1 muA. With active electrode areas below 1 mm(2), high influence from electrode polarization impedance (EPI) was expected at low frequencies (LF). For higher frequencies (HF) the EPI decreases and the impedance of the surrounding tissue is more pronounced. The hypothesis tested was that the EPI at LF would depend upon contact area, alloy composition, surface structure, and treatment of the active electrode, and at HF upon the electrode area geometry, and the specific resistivity of saline. Our results show large differences in electrical properties between needle types. After electrolytic treatment the EPI decreased. After 5-48 h of saline exposure the EPI increased, both for treated and untreated needles. Cathodic treatment gave lower impedance and drift than anodic or no treatment.

Aspects of nonlinear dielectric spectroscopy of biological cell suspensions

In this paper the technique of nonlinear dielectric spectroscopy is employed to examine the nonlinear response of a suspension of the yeast S. cerevisiae to a low frequency perturbating ac electric field. Metabolically active and resting yeast states, as well as the electrolyte medium are considered, and experimental time-course spectral data are presented. Conductivity is found to increase in the active case, resulting in variations in magnitude of the applied field. An empirical model is fitted to the experimental data at discrete points over time, enabling simulation and resulting in a software-based method to compensate for these variations in effective field strength.

Miniaturization of a biomedical gas sensor

In a previous study, we concluded that a conductivity based PCO2 sensor is an attractive solution for early detection of ischemia and presented two design geometries. For organ surface measurements, the planar design was suitable but it was difficult to insert the sensor into the tissue. A cylindrical design solution was favored for insertion due to the large membrane contact area and easy placement in a medical catheter. Since the previous cylindrical prototype was large and could damage the tissue, a more miniaturized sensor was needed. In the current paper, we present a miniaturized sensor with an outer diameter of 1 mm. The applied technology for manufacturing the sensor was a combination of mechanical turning, excimer laser drilling and conventional molding technique. The materials applied were PEEK (polyetherether ketone), PI (polyimide) with gold layers and polysiloxane. The membrane had to be gas permeable while acting as a barrier for ion transport, and was made of polysiloxane and had a thickness of 100-150 microm. The miniaturized sensor was tested for calibration, response time, drifting and pressure sensitivity. The results show that the miniaturized PCO2 sensor is capable of rapid and stable measurements both in vitro and ex vivo. The result from this study will be applied for the industrial manufacturing of such a biomedical sensor as a clinical product.