Medical Applications of In-line Laser Absorption Spectroscopy

This work forms part of an ongoing collaboration between the Ritchie group in Physical Chemistry and the Robbins group in the Department of Physiology and Genetics. Recent work has developed a completely new technique of in-airway molecular sensing using laser absorption spectroscopy. This has changed the experimental precision with which mass balance can be tracked during rebreathing of various molecular species from 1:20 to 1:500 molecules, and has allowed the first detailed measurements of oxygen consumption in patients undergoing surgery, and most recently, those receiving critical care. In collaboration with the Department of Computer Science the researchers have also developed a low order model of the lung's heterogeneity - the log-normal lung - where it is possible to recover the parameters underpinning the heterogeneity of the lung from measurements made using their high fidelity, in-airway molecular flow sensing.
This project will build upon this successful collaboration, with early work aiming to increase the accuracy of the current measurements of oxygen, carbon dioxide and water vapour and then to use this improved instrument in clinical trials. The improved instrument will have an impact in a range of clinical settings. For example, in critical care, accurate measurement of oxygen consumption for patients in shock could be used to give feedback on the success of treatment in real time. In the field of respiratory medicine, measurement of lung function and heterogeneity will be applied to following the progression of diseases such as COPD, cystic fibrosis and interstitial lung disease, as well as informing medical practitioners on a patient's response to therapy. Collaborations in all of these have been formed and clinical trials in at least two of these areas are scheduled to start during the first year of study.
Further advances in physiological phenotyping will then be made by developing new technology to follow exogenous tracer gases, in order to measure aspects of lung function such as lung diffusing capacity. The method relies upon determining the uptake of foreign gases delivered in low quantities to the patient, and analysed in real time by laser absorption techniques within a ventilation tube. In particular, this involves the sensitive analysis of blood soluble and blood insoluble gases (for example, carbon monoxide and methane) with fast time resolution of 10ms. Such studies will allow the current "log-normal" lung model to be extended. In addition, the project has the potential to deliver a non-invasive instrument for measuring cardiac output in anaesthetised and critically ill patients. The validity of currently used invasive procedures, such as pulmonary artery catheterization, is questionable, with several studies suggesting that the associated risks might outweigh the clinical benefits, and the growing consensus is that non-invasiveness should be a paramount feature of the next-generation cardiac output monitors. Foreign gas uptake has been established as a satisfactory method for assessing cardiac output, but the methods of gas analysis have been cumbersome or insensitive, and unsuitable for use with ventilated patients. The instrument that will be developed will enable very low concentrations of these gases to be delivered to the patient and will result in minimal disruption to the ventilation procedures, with the whole optical instrumentation being able to connect with a standard 20 mm diameter ventilation tube with no disturbance to the gas flow.
This project falls firmly within the Clinical Technologies, Sensors and Instrumentation and Optical Devices and Subsystems research areas of the EPSRC. This work should offer clinicians a quick and simple alternative to the invasive methods with which cardiac output is currently measured in surgical and intensive care settings.