A multidisciplinary approach to non invasive optical measurements of cellular energetics in the human brain

A major problem in many physiological measurements is that the act of performing the measurement can itself alter the system that is under observation. The challenge for medical physicists is to develop techniques which allow the non invasive assessment of physiological processes. One fundamental physiological process which is of great importance and interest is the way in which cells generate energy for their survival. A key compound involved in using oxygen to generate this cellular energy is an enzyme called cytochrome c oxidase (CCO). As the enzyme accepts and donates oxygen, it changes colour i.e. the wavelength dependence of the amount of near infrared (NIR) light it absorbs changes allowing it to be measured non invasively using spectroscopic techniques. These techniques rely on transmitting deeply penetrating NIR light through the tissue and analysing the change in its spectral characteristics in order to determine the concentrations of specific compounds in the tissue. This type of near infrared spectroscopy (NIRS) has previously been used to measure the wavelength dependence of the optical absorption of blood and therefore its oxygen content (highly oxygenated arterial blood is bright red whilst oxygen depleted venous blood appears purple/blue in colour). The aim of the work described in this proposal is to develop a combination of novel NIRS techniques to measure the oxygenation status of CCO and therefore to provide clinicians and other scientists with a continuous non invasive measure of the well being of cells. This monitor can then be used as a clinical tool to monitor the functioning of organs such as the brain and muscle. An important clinical application of the measurement of CCO is the management and treatment of patients with brain injury in whom the mechanisms of oxygen utilisation and cellular energy generation may be severely compromised and could result in cell death. The CCO signal can also be used to predict whether patients who are at risk of a significant decrease in cerebral oxygenation (e.g. during cardiopulmonary bypass surgery) will have impaired neurological outcome. Although CCO absorbs near infrared light, its concentration in tissue is much lower than that of the dominant tissue absorber, haemoglobin. This makes measurements of CCO much more difficult particularly in the presence of the high degree of light scattering seen in the biological tissue. Additionally, to ensure that the absorption of the enzyme can be fully characterised, it is necessary to make measurements over a much wider range of wavelengths than usually used for the NIR spectroscopic measurements of physiological parameters such as blood oxygen content. We therefore propose to use a novel combination of NIRS instruments which can measure absorption and scattering over a wide range of wavelengths. The system will be used to measure CCO in groups of human volunteers and selected patient groups in whom cellular oxygen utilisation has been shown to be disturbed. This will include patients with a respiratory related sleep disorder, obstructive sleep apnoea, and patients with acute brain injury who are undergoing neurointensive care. We will then use a mathematical model to help us interpret the physiological basis of the CCO signal we are measuring. Clearly CCO is a physiological parameter of significant biological and clinical significance. To measure and interpret it properly will require the combined efforts, expertise and facilities of physicists, engineers, biochemists, clinicians and theoreticians, all of whom are represented as applicants on this proposal. As such we have a group of researchers who have a strong history of multidisciplinary research of this kind and a real interest in finding the best possible way of monitoring this important enzyme.