An oximeter for continuous absolute estimation of oxygen saturation

Abstract

An oximeter consisting of an earpiece and associated electronic apparatus provides continuous estimation of O2 saturation in human arterial blood. The earpiece, made of Duralumin, contains a light bulb, 2 Se barrier-layer photocells fitted with suitable gelatin filters, and a pressure capsule. An oscillator supplies the light with direct current interrupted at 91 cycles/sec. The A. C. signals from each of the photocells are amplified. The amplified alternating voltages are rectified and each applied to a value voltmeter. With the earpiece mounted on the pinna of the ear, air pressure (300 mm. of Hg ) is applied to the pressure capsule, thus removing blood from the trans-illuminated area. Sufficient positive bias is then applied to each rectifier to set the meters to a fixed point. On release of pressure, blood returns to the ear; the current in one meter (thickness indicator) rises to a finite value depending on the amt. of blood in the illuminated vascular bed; the current in the other meter (saturation indicator) rises to a value depending on both the amt. of blood in the vascular bed and its state of O2 saturation. The "saturation" meter is calibrated to read percentage O2 saturation directly when a variable shunt has been adjusted according to the reading on the "thickness" meter. Instrument accuracy, expressed as standard deviation measured with 3 earpieces, was [plus or minus] 3.8, [plus or minus] 3.3 and [plus or minus] 3.2%, respectively.