Development of a noninvasive multi-detector tissue reflectance oximeter

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(1) A hybrid reflectance sensor consisting of light emitting diode (LED) and photodiode (PD) for measurement of hemoglobin content (Hb) and oxygen saturation (SOィイD22ィエD2) of whole blood was designed using the 3-dimensional photon diffusion theory. Wavelengths selected included 660, 730 and 830nm and separation distance between the LED and PD was 2.1mm to optimize the linearity in hematocrit measurement at 830nm. The sensor was evaluated using the fresh bovine blood. For Hb measurement, the reflectance at 830nm yielded the standard error of 3.37%. As for SOィイD22ィエD2 accuracy over 30-100% range, the combination of 730/830 wavelength yielded better results than 660/830 wavelength pair, with the standard error of 4.24%.(2) Secondly, the 3-dimensional photon diffusion theory was utilized to design a reflectance pulse oximeter sensor for improved accuracy over a wider SOィイD22ィエD2 range. In order to attain this goal, selection of wavelength pair and sensor geometry were examined theoreticall … More y. A new sensor incorporated wavelengths of 730 and 880nm and 10 LED chips for each wavelength were placed at equal distance of 7mm around the photodiode. The separation distance of 7mm was selected to maximize the pulsatile signal to background signal level. Initially, the sensor was evaluated in dogs ; the arterial SOィイD22ィエD2 was varied through controlling the oxygen content of the air breathed. The reflectance sensor was attached to the internal lining of the mounth and its signals were compared against the arterial blood sample data analyzed separately. The 730/880nm sensor showed better accuracy in comparison with the 660/910nm sensor with the standard error of 2.69%. In 30 human volunteers, reflectance measurements were made from finger-tip, forehead and chest wall. The pulsatile signal level from the forehead was approximately 30% of the finger-tip and that of the chest was about 10%. The pulsatile signal level at 730nm was largest in comparison to other wavelengths at any location. Clinical evaluation of the sensor was carried out in 6 volunteers. The reflectance measurements from the forehead were compared against the arterial blood samples obtained from the radial artery during controlled ventilation. The 730/880nm sensor showed the standard error of 4.2% in comparison to 6.3% of 660/910nm sensor. Continuous monitoring of the patients with 730/880 sensor in the operating room revealed stable and reliable performance than 660/910 sensor over 6-8 hour duration.(3) Thirdly, two layer tissue model was constructed with the lower layer being diluted whole blood (hematocrit 14%) and upper layer being non-blood tissue element. The thickness of the non-blood tissue element was varied from 0 to 6mm, while the reflectance from the compound tissue model was measured using a multi-array photodiode sensor consisting of 46 elements with 1mm pitch. The separation distance between the light source and detector in the multi-array sensor was adjustable from 12.5 to 60mm. The same wavelengths used in whole blood study were also employed in this study and the SOィイD22ィエD2 was derived from the ratio of the two wavelengths using a linear regression equation. The effect of non-blood element thickness resulted in decrease of sensitivity to SOィイD22ィエD2 variation. With the increase in tissue thickness, the larger separation distance sensor resulted in better measurement sensitivity. Provided that the tissue thickness is know, the detection of deeper layer SOィイD22ィエD2 may be derived from the surface of the tissue model. It is suggested that through proper design of sensor geometry 3-dimensional detection of oxygen content inside the tissue will become possible. Less