Consequences of static and pulsatile pressure on transmembrane exchanges during in vitro microdialysis: implication for studies in cardiac physiology.

Microdialysis is an established technique for measuring the kinetics of various neurotransmitters within the extracellular space in the field of neurochemistry. Recently, its use has been extended to sampling in other tissues, including liver, kidney and the heart. A persistent problem in cardiac microdialysis concerns two parameters related to myocardial function: pressure and frequency (heart rate). The aim of the study is to evaluate the consequences of pressure and frequency on transmembrane exchanges. Linear flexible microdialysis probes (membrane length: 12 mm, outside diameter: 390 microns, MWCO 50,000 Daltons) were designed in our laboratory. The probes, perfused at 2 microL/min with sterile water, were placed in a system filled with a glucose solution (2 g/L) and able to generate either static: 0 to 400 mmHg (0 to 53.31 kPa) or pulsatile pressure: 0-100; 0-200; 0-300 mmHg (0-13.32; 0-26.65; 0-39.98 kPa) at different frequencies: 1, 2 and 3 Hz. At 2 mu litre min-1 perfusion rate, the pressure inside the probe is estimated to be 80 mmHg (10.66 kPa). Under static pressure conditions, the glucose recovery rate can be expressed as an exponential function, and the outflow rate can be expressed as a linear function of the external pressure level. Under dynamic conditions, the external mean pressure must be accounted for. When external mean pressure exceeds 80 mmHg (10.66 kPa) (pressure generated by the flow rate of perfusion inside the probe), the recovery rate increases with frequency. Conversely, if the outer mean pressure is lower than 80 mmHg (10.66 kPa), the recovery rate decreases with frequency. Theoretical and experimental modelling results in a nomogram that can be used to estimate in vivo recovery. In conclusion, mass transfer across a microdialysis membrane is dependent on the direction of the transmembrane pressure gradient and increases with heart rate. These findings must be taken into account when in vivo recovery rates during cardiac microdialysis are determined.

[Extreme results in electrolyte determination]. / Extreme Ergebnisse bei Elektrolytbestimmungen.

Besides statistical quality control, quality control based on patient specimens is an important tool for quality enhancement and thus for an increased diagnostic certainty in laboratory medicine. One of three possibilities of plausibility judgement is the control of extreme results, that is alert and absurd value check. The aim of our study was to look for extremely high or low findings of the most frequently examined clinical-chemical parameters, to scrutinize their validity according to clearly defined criteria and to find out the underlying actual clinical situations and diseases. In this publication only the results for the electrolytes are discussed. Retrospectively the most extreme values of all results for serum sodium, potassium and chloride concentrations of a 21-month interval were extracted in a large university hospital. The clinical situation was then evaluated by reading the medical reports of these patients. The validity of the findings was judged by previously defined criteria and rated as confirmed, questionable and not confirmed. In all cases the survival time was determined. The most extreme confirmed results were for sodium 191 and 100 mmol/l, for potassium 9.0 and 1.3 mmol/l and for chloride 138 and 65 mmol/l. All these findings were compatible with life, at least for several hours. Even if it is probably impossible to give generally valid extreme ranges. Nevertheless our results should certainly have practical importance in absurd and alert value check.

Evaluation of a new automatic device for taking and recording blood pressures.

A new instrument capable of recording systolic and diastolic blood pressure automatically and repetitively is described, and its accuracy is shown to be equal to that obtained by a carefully trained observer among 53 patients with different levels of blood pressure. The instrument (BosoMat) is also shown to be more accurate than two others available, namely, the Arteriosonde Model 1213 and the SphygmoStat Model B-300. The standard deviation among three consecutive blood pressures on the same patient as determined by the new instrument or by the nurse using traditional methods is shown to be approximately 4 mm. Hg.