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Abstract: A review of the theory of phosphorescence quenching is given, and its particular application to the sensing of oxygen is outlined. The advantages of measuring phosphorescence lifetime as opposed to phosphorescence intensity are reviewed. The advantages of using the metalloporphyrins as such sensors are identified and in particular the characteristics of palladium coproporphyrin are discussed. The exceptionally long room temperature lifetime of this material makes it possible to use relatively simple PC-based instrumentation to measure lifetimes, with a xenon flashlamp light source. The design of such a system is given and its performance in measuring phosphorescence lifetime in aqueous solutions is demonstrated.

A gaseous oxygen sensor is described based on the measurement of phosphorescence lifetime of the dye palladium coproporphyrin, immobilised in a range of membrane materials including polyvinyl chloride, polymethylmethacrylate, polystyrene and silicone rubber. The lifetime of the dye in these materials is described, measured using a simple system consisting of a pulsed xenon flash lamp light source and a PC-based data acquisition system. The linearity of the sensing membranes over the 0–100 per cent oxygen range is presented together with data on temperature sensitivity, the effects of CO2, N2O, H2O and halothane, and the effects of ageing over a 90 day period. The sensors based upon polystyrene immobilised dye appear to provide the optimum characteristics for a gaseous oxygen sensor.

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Abstract: A design for the manufacture of a flexible blood gas catheter and connecting cannula for use with a mass spectrometer is outlined. The design minimises gas diffusion through the wall of the catheter or cannula by enclosing it in another flexible plastic tube. The annular space between the concentrically arranged tubes is evacuated, thus preventing gas diffusing through the outer tube wall from reaching the inner tubing. Results are presented showing that with this configuration the wall diffusion signal can be reduced to a level similar to that obtained with metal walled catheters and cannulae.

Abstract: Images of the electrical current distribution in an intact piglet head, measured by MRI, are presented for the first time. Remarkable differences in the distribution of the electrical current between live and post mortem studies are found. After death, there is a decrease of 62% in the current reaching the brain, compared with the situation in the living animal. This reduction is associated with the increase in the brain impedance after death, which agrees with previous in vivo studies.

Abstract: A thermal clearance blood flow sensor has been designed which incorporates four separate sensing elements, permitting flow changes at four differing depths to be measured. Static and dynamic laboratory tests have confirmed the differing flow depth response of the sensor and demonstrated a linear response to flow over the physiological skin blood flow range. Flow depth response has been shown to be dependent upon sensor geometry. Response should also in theory be dependent on the thermal conductivity of the sensor body material. However, using conventional encapsulation materials, it does not seem possible to obtain a depth sensitivity of less than 02mm.

Abstract: The technique of near-infra-red spectroscopy allows safe continuous monitoring of changes in blood and tissue oxygenation on an intact organ. This is made possible by observing spectral changes in the tissues caused by oxygenated haemoglobin [HbO2], deoxygenated haemoglobin [Hb] and cytochrome aa3 [Cyt aa3]. The paper describes the design and performance of an instrument that has been developed to apply this technique to the monitoring of the brain in newborn infants. The instrument monitors optical transmission changes across a newborn infant's brain at four wavelengths. A standard deviation in error of 1 per cent (001 optical density OD) is achieved on measurements of transmission changes at 20s intervals. This performance is obtained at a mean attenuation of 10 OD, the approximate attenuation across a term infant's head. Long-term monitoring is possible as instrumental drift is less than 0004 OD per hour.

Abstract: A technique for monitoring the oxygen tension of blood in a bypass loop is described. The technique employed is to separate the blood from a thin electrolyte layer by means of an oxygenpermeable membrane. The membrane is sufficiently permeable to oxygen that the electrolyte layer rapidly equilibrates with the oxygen tension in the blood. A ‘Clark type’ polarographic oxygen electrode is then employed to measure the oxygen tension in the thin electrolyte layer. The oxygen consumption of the polarographic sensor is sufficiently low that the equilibrated oxygen tension is not disturbed by the measuring electrode. The technique has the advantage that the sensor can be removed from the measurement cell (for calibration checks etc.) without leakage of blood from the loop.

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