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In a previous paper, a new dielectric technique was used to estimate hematocrit (HTC) in extracorporeal blood circulation systems, independently of plasma conductivity or osmolarity. Although many impedance techniques have been formerly proposed in the literature, none has been evaluated against plasma conductivity and osmolarity. Herein, we estimate HTC based on permittivity changes and also with other four techniques found in the literature. Besides, the error incurred in each is also studied when plasma conductivity and osmolarity changed as much as 1 mS/cm and 50 mOsm/kg, respectively. The dielectric (permittivity) technique has an error close to 5.4%, while the others showed both tendencies, i.e., lower error (2.5%, two of them) and higher error (8.6% and 16.3%, the other two). The dielectric technique, even though did not produce the lowest error, provides a well-described physical model along with simple instrumentation.

The conductance catheter has gained momentum since its introduction in cardiovascular dynamics back in 1980. However, measuring errors are still blurring its clinical acceptance. The main objective here was to study the effects of the injected saline concentration and-temperature on the evaluation of the parallel conductance, G/sub p/, and thus, on the correction volume V/sub p/. That conductance, G/sub p/, and its associated volume, V/sub p/, were computed using 167 saline dilution curves obtained with boluses at different concentrations and temperatures, injected in seven anesthetized closed-chest dogs. The excursion of the total conductance relative to the steady-state value during a saline manoeuvre showed good correlation with the injected concentration at both studied temperatures. The reference parallel volume (one reference per dog) was defined as the average value obtained with three successive manoeuvres, at 6-M concentration and at body temperature; therefore, the method acted as its own reference. The variation of V/sub p/ relative to the reference value was clearly dependent on the injected concentration and on its temperature; dispersion was greater at 22/spl deg/C than at 40/spl deg/C. The variability would recognize also other causes, such as uncertainty of the extrapolation procedure and the thoracic redistribution of electrical field lines. As a conclusion, it is recommended to characterize each manoeuvre by its concentration and temperature. Body temperature and 6-M concentration appear as the most recommendable combination for the injectate in most animals. Finally, these results intend to characterize the V/sub p/ estimation procedure in order to minimize errors. The variability of V/sub p/, in different experimental conditions, demonstrated that both concentration and temperature are additional parameters that may modify the G/sub p/ estimate.

Classic impedance microbiology (CIM) is based on the measurement of the impedance components that appear between a pair of electrodes submerged in a cell containing inoculated broth. Either a bipolar or a tetrapolar technique can be applied, requiring about 1/spl times/10/sup 3/ to 3/spl times/10/sup 7/ cells/ml to produce detectable changes in the impedance curves. Theoretical analysis of the electrode-electrolyte interface during bacterial growth is lacking, with no generally accepted measuring standards. Besides, there is considerable disagreement. The authors separated out the interface and medium components using the frequency variation technique (FVT) and also analyzed the interface reactance-resistance diagram, both before and after bacterial growth. Medium resistance R/sub m/, interface reactance X/sub i/, and interface resistance R/sub i/, were quantified as time functions growth curves, from the complex bipolar impedance seen between two electrodes. The authors took into account the electrical current density, the temperature and the associated circuitry, also explaining the theoretical and experimental bases that justify the proposed dissecting procedure. It was found that, within the working frequency range, R/sub m/, R/sub i/, and X/sub i/ percental growth curves are frequency-independent, i.e., neither R/sub m/(J), nor N/sub i/(f) nor R/sub i/(f) changed their slopes before, during and after bacterial growth. Besides, no /spl alpha/-dispersion effect in R/sub m/ curves was detected. It is concluded that impedance microbiology could become a fertile area for interdisciplinary knowledge; its development might offer new avenues for basic and applied research.