AN EAR OXIMETER USING D-C. OPERATIONAL AMPLIFIERS

Abstract

An ear oximeter was constructed with d-c. operational amplifiers and a conventional two-channel earpiece. The amplifiers are required to provide the necessary signal level for continuous computation of the current ratio, corresponding to an estimate of percentage oxygen saturation.Calibration was effected by analysis of 69 arterial blood samples from 23 patients, the values ranging from 60 to 99% oxygen saturation. With the assumption of a linear relationship between recorder currents (or ratio of two currents, one in each color channel), a regression equation for a straight line was calculated, the points having a standard deviation of ± 3.9% from the regression line. Simple inspection, however, revealed a curvature in the arrangement of calibration points. Cornwall, Marshall, and Boyes assert that this is due to an error in the oximeter equation and propose that an equation in the form of a power function be substituted. The experimental data were recalculated to show that the calibration points fit a power curve best when the exponent is equal to two.