GB1325039A - Oximeter and method for in vivo determination of oxygen saturatiion in blood - Google Patents

Abstract

1325039 Blood oximeters R F SHAW 7 Oct 1970 47787/70 Heading G1A In a device for measuring the oxy-haemoglobin content of blood in a portion of human tissue, e. g. an ear 15, and in which the tissue contains N-1 other components which also have absorption coefficients which vary with wavelength, light passing through or scattered from the tissue is arranged to produce N+1 output signals each corresponding to absorption at one wavelength by one component, and these signals are combined to give an absolute measure of the amount of oxyhaemoglobin in the tissue. As shown, three electro-luminescent diodes 9, 11, 13 illuminate an ear 15 containing two absorbing species oxy-haemoglobin and (reduced) haemoglobin. Each diode emits a different wavelength, two of which are strongly absorbed by the two respective absorbing species, modulated at different frequencies. Light transmitted through the ear is received by three photo-cells 47-51, e.g. photo-multipliers or photo-diodes, each supplying a demodulator 53-57 tuned to one of the light source frequencies. The three demodulator outputs therefore represent the light absorption at the three incident wavelengths and are supplied to log gain amplifiers 23-27. The amplifier outputs feed a network of resistors 59-69 whose values are chosen empirically so that the output on the meter 43 represents the ratio of oxy-haemoglobin to haemoglobin, i. e. the oxygen content of the blood. The instrument must first be calibrated by introducing a number of blood samples of known different oxygen content into a suitable absorption cell, and choosing the resistors 59-69 to give "correct" readings on the meter in all cases. The demodulators may be dispensed with by providing each photo-cell with a different wavelength filter 52-56. Alternatively, a single detector may have its response to the different wavelengths separated out by synchronous demodulation at the three different light source frequencies, or by time sharing sequential operation of the sources. A single broadspectrum light source may be used with three filtered detectors, or with a single detector having associated therewith a filter wheel containing three different filters, the detecting cycles of the detector being synchronized with the wheel rotation. In this latter case, light guides convey light both from the source to the ear and from the ear to the detector. In a further embodiment a number of sources (91) in a circular configuration, Figs.3ab (not shown) irradiate a body portion and the light scattered from within the tissue is received by a number of detectors (47-51). A circular aperture (95) placed between the detectors and the tissue ensures that light both direct from the sources and also merely reflected from the tissue surface is not received by the detectors.