Non-invasive haemoglobin monitoring by an LED based optical sensor system

During the perioperative period, which includes the period before surgery (preoperative), during surgery (intraoperative) and after surgery (postoperative), it is essential to measure diagnostic parameters such as: blood oxygen saturation; haemoglobin (Hb) concentration; and pulse rate. The Hb concentration in human blood is an important parameter to evaluate the physiological condition of an individual, as Hb is the oxygen carrying component of red blood cells. By determining the Hb concentration, it is possible, for example, to observe intraoperative or postoperative bleeding, and use this information as a trigger for autologous/ allogenic blood transfusions. The primary objective of this research has been the development of a non-invasive, real-time, Haemoglobin monitor based on the photo-plethysmography (PPG) method and using LEDs as the frequency sources. The system developed operates in the spectral range between 600nm and 1400nm. In an analogous way to pulse oximeters, the Hb sensor is clipped on a fingertip and light is transmitted through the fingertip. The system provides continuous real-time monitoring of the Hb concentration without the need to take blood samples. This thesis discusses the theory needed to understand and design such a device, e.g.: the transport of light in biological tissue and turbid media; the PPG method; and a model for non-invasive Hb measurement is derived from first principles. In order to empirically validate the theory of the Hb system, a prototype Hb sensor system and an artificial blood flow model were constructed, which simulate the human circulatory system. The blood flow model allowed a controlled variation of the oxygen saturation and haemoglobin concentration. Results obtained from spectrometer measurements and from the non-invasive Hb sensor system are presented in this thesis. Furthermore, an in vivo validation was undertaken, where the non-invasive method was compared with an invasive method and an excellent correlation were observed.