Accurate in vivo monitoring of glucose concentration would be a valuable asset, particularly for management of diabetes and preterm infants during critical care. In vivo glucose monitoring devices can be divided into two categories: implanted and non-invasive. Extensive research into in vivo glucose monitoring over recent decades has not resulted in the widespread use of clinically reliable monitoring systems. For implanted devices, poor biocompatibility of the materials used for fabrication remains a major challenge, whilst progress in the commercial development of non-invasive devices is hampered by the problem of multiple interference between the detected signals and the biological components. In this review, the methods available for in in-vivo glucose monitoring are described and the associated problems are discussed.

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Cyclical fluctuations of cerebral blood flow velocity have been reported previously using Doppler ultrasound. The same phenomena was detected during investigations of changes in cerebral blood volume using near infrared spectroscopy. Rhythmic fluctuations of cerebral blood volume at a frequency of 3-5 cycles/minute is reported here.

The effects of intermittent positive airway and continuous negative extrathoracic pressure ventilation on cerebral blood volume in preterm infants were studied using near infrared spectroscopy. In 12 infants continuous negative extrathoracic pressure caused a median decrease in cerebral blood volume of 0.14 ml/100 ml brain (95% confidence intervals (CI) 0.035-0.280) compared with no respiratory support. Oxygenated and deoxygenated haemoglobin also decreased, implying increased venous drainage as the main effect. In 17 infants intermittent positive pressure ventilation also caused a median reduction in cerebral blood volume of 0.06 ml/100 ml brain (95% CI 0.010-0.115) compared with endotracheal positive airway pressure. Deoxygenated haemoglobin increased by 0.07 ml/100 ml brain (95% CI 0.010-0.100) while oxygenated haemoglobin decreased by 0.10 ml/100 ml brain (95% CI 0.005-0.175). The increase in deoxygenated haemoglobin implies decreased venous drainage and the decrease in oxygenated haemoglobin implies that other factors may also be significant. Heart rate, blood pressure and oxygen saturation were monitored continuously and remained stable.