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Oesophageal pressure measurements in respiratory function tests are commonly performed using a balloon-catheter system. This study investigates the usefulness of catheter-mounted pressure transducers as an alternative to balloon-catheter systems. Calibration related physical properties of the catheter mounted pressure transducers are evaluated in vitro. The behaviour of these transducers in vivo is evaluated in ten volunteers by relating pressures measured in the oesophagus to airway opening pressures and by comparing these relationships with those sequentially obtained by a balloon-catheter system. The catheter-mounted pressure transducers show no drift after a proper preparation procedure. These catheters, with integrated pressure transducers, are tolerated significantly better by the subjects than are balloon catheters. The catheter-mounted pressure transducers are found to give an equivalent performance compared with the balloon-catheter system, if relative pressures are of interest. However, unpredictable and uncontrollable shifts in offset occur during the in vivo measurements, disturbing absolute pressure readings. Possible explanations for these shifts are the presence of bubbles and adhesion of mucus to the transducers, exerting Van der Waals forces, and contact with the tissue of the oesophageal wall. These shifts are found to be quite stable throughout a period of measurement and therefore of minor disturbance to relative pressure measurements, for instance in assessing the elastic properties of lungs.

In thoracic impedance cardiography (TIC) measurements the neck electrodes are often positioned at the basis of the neck, close to the neck-thorax transition. Theoretically, this neck-thorax transition will cause inhomogeneities in the current density and potential distribution. This was simulated using a 3D finite element method, solely representing the geometrical neck-thorax transition. The specific conductivity was 7 10(-3) (omega cm)-1 and the injected current was 1 mA. As expected, the model generated inhomogeneities in the current distribution at the neck-thorax transition, which reached as far as 5 cm into the neck and 20 cm into the thorax. These results are supported by in vivo measurements performed in 10 young male subjects, in which the position of the neck electrodes was varied. A two-way ANOVA revealed that the stroke volume of the lowest neck position was significantly different from the other positions. Small shifts in the position of the neck electrode resulted in large changes in impedance and stroke volume (127 to 82 ml for the Kubicek equation). To standardise the electrode position, the authors strongly recommend placement of the neck electrodes at least 6 cm above the clavicula.

A method is presented to measure transcellular fluid shifts during haemodialysis based on a simplified model of the electrical admittance of biological tissues. It allows for the measurement of intracellular and extracellular conductivities and their ratios. The method is noninvasive, clean and harmless, and can be easily computerised in order to be performed continuously. A typical example is given of a recording during haemodialysis.

A method to measure intra- and extracellular conductivity is evaluated. In vitro experiments show that these two variables can be measured separately. The conductivity appears to depend on the concentration and fluid volume of the compartment concerned. In vitro variation of the intracellular volume of blood by dilution and by shrinking and swelling of blood cells is detectable by this method. In vivo too the method seems to detect intra- and extracellular fluid volumes separately. The course of these two variables during different strategies of haemodialysis is determined.

A method for measuring the susceptibility of a patient to develop decubitus ulcers is described and initially evaluated. It is based on an indirect, noninvasive measurement of the transient regional blood flow response after a test pressure load which simulates the external stimulus for pressure-sore formation. This method was developed to determine the individual risk of a patient and to study the subfactors which contribute to the susceptibility. This would also offer the possibility of evaluating the effect of preventive treatment aimed at reducing the susceptibility. The method was found to discriminate between preselected elderly patients at risk on the one hand, and non-risk patients and healthy young adults on the other hand. No differences in blood flow responses were found between the non-risk elderly patients and the healthy young adults. This suggests that age per se is not a factor in the formation of pressure sores. In the risk group the recovery time after pressure relief was found to be three times as long as the duration of the pressure exercise. This indicates that the recovery time after pressure exercise may be as important as the period of pressure exercise in deducing the risk of developing decubitus ulcers.