GB777651A - Improvements in or relating to devices for determining the oxygen saturation of the blood - Google Patents

Abstract

777,651. Determining oxygen content of blood. MINISTER OF NATIONAL DEFENCE OF CANADA. June 28, 1955 [Oct. 13, 1954], No. 18695/55. Class 81(2) [Also in Group XL(b)] In a device for determining the oxygen saturation of the blood in which a bifurcated frame holds a light source on one side of the pinna and a photo-electric detector on the other, a light scattering device is positioned in the light path between the ear and the photo-electric device and is mounted on the same side of the frame as the photo-electric device. As shown, the bifurcated frame 2 of the instrument carries a lamp 3 mounted in a socket 4 in a lamp holder 5. A flange 6 onholder 5 enters ring 8 and is clamped there by a screw 9. Telescopic member 12 carrying a window 14 is spring urged forwardly and, after adjustment to allow the pinna of the ear to enter the space between it and the light receiving side of frame 2 is clamped by nut 18. A membrane 20 of polyvinyl acetate film is stretched over the window and is inflated by air introduced into the space between it and the ear through a tube (not shown) to remove most of the blood from the area under test. The other arm of the frame 2 comprises a block 27 of acrylic resin such as "Lucite" acting as a light pipe to direct the light received from the source 3 through the ear on to photo-cells 28 combined with filters at the bottom of the block. The block is rectangular in section and a flat face engages the pinna while a polished and frosted sloping face 27b directs the light downwardly. The effect of the block is to ensure mixing of light received from various parts of the ear. The photo-cell (Fig. 7) is in four sections 29a, to 29d each associated with a filter component 28a to 28d. The filters are such that cells 29a, and 29d have a minimum sensitivity at 650 m,u, cells 29b, 29c at 800 mÁ. Transmitted light in the former region is attenuated by oxygen in the blood while that in the latter is not. The lamp 3 is fed from the output of an oscillator 34 at 30 cycles/sec. The output of cells 29a, 29d are combined and amplified in a conventional amplifier 36 while that of cells 29b, 29c is amplified in conventional amplifier 35. The outputs of both amplifiers are indicated on valve voltmeters 44, 39. Initially, with the pressure in the membrane both meter readings are brought to zero by bias controls 40, 45. The pressure is then released and since the reading of voltmeter 39 is independent of oxygen saturation a variable resistor shunting meter 44 is adjusted in accordance with the reading of meter 39 so that meter 44 indicates oxygen content directly.