Hypoxaemia, which is defined as insufficient oxygenation of arterial blood, is a very common problem amongst different patient groups. These patients may require supplemental oxygen to maintain an adequate level of blood oxygenation, but too much oxygen is toxic to the body and is as inappropriate as too little. Therefore, a monitoring system should be in place that can accurately determine a patient's pulmonary oxygen exchange performance, i.e. the relationship between the levels of oxygen inspired by a patient and the resulting levels of oxygen in their arterial blood. Such a monitoring system would also be useful in predicting a patient's response to changes in their environment, such as whether they require any supplemental oxygen for air travel.

For my M.Eng project at the University of Cambridge, I implemented a program that can determine a patient's pulmonary oxygen exchange performance by model-fitting. More specifically, the program is based on a pre-existing algorithm in which a two-parameter model of the lungs is fitted over measurements of the patient's arterial oxygen levels at different inspired oxygen levels. These measurements can estimated non-invasively using pulse oximeters, while inspired oxygen levels can be controlled by the clinician. The program can also fit the data points over a four-parameter model of the lungs in which blood oxygen saturation is controlled by:

• The ventilation-perfusion ratio, i.e. the ratio of air leaving the lungs and blood being oxygenated.
• The shunt, i.e. the fraction of the total blood flow that is not oxygenated at the lungs.
• The fraction of air flowing through one of the lungs (e.g. left).
• The fraction of blood being oxygenated at one of the lungs (e.g. left).