Pulse Oximetry and Plethysmography

Stokes (1) reported that the colored substance in blood carries oxygen. This was followed by Hoppe-Seyler (2), who first crystallized this substance and coined the term hemoglobin. Additionally, it was shown that the pattern of light absorption changes when shaken with air (2). Hertzman (3) described using photoelectric plethysmography of fingers and toes as a dynamic analysis of the peripheral circulation. The device consisted of a beam of light directed from an ordinary automobile headlight bulb on the finger or toe placed above a shielded photoelectric cell of the photoemissive type, purchased from the radio trade. The photoelectric oscillations with variations in the blood content of the digit were recorded by a string galvanometer or suitable oscillograph after amplification. Movements of the arm were minimized as transmission to the finger would compromise the reading; a comfortable saddle or sling was necessary to secure the arm to achieve the desired muscle relaxation that affects finger volume. This method was also used over the nasal septum, and the values were compared. Millikan et al. (4) first coined the term oximeter and described a method for the continuous measurement of arterial saturation. A small unit placed over the shell of the ear contained a lamp, two color filters, and two barrier-type, lightsensitive cells, with which the transmission of either green or red light was measured. The green reading was dependent on how much total hemoglobin was between the lamp and the photocell, and was used to measure the degree of vasodilation, or “blood thickness” in the ear. This enabled one to choose the correct direct reading calibration scale for the estimation of arterial oxygen saturation, as measured in the red reading. This method has an accuracy of 5% in the top half of its range and 8% in the bottom half. In 1942, Goldie (5) developed a device for the continuous measurement of oxygen saturation of circulating blood in humans. These devices led to Wood and Geraei (6), who improved upon these to develop a method for photoelectric determination of arterial oxygen saturation in humans. Prior instruments were required to be preset to known arterial saturation values, and could not be conveniently used in patients who had arterial hypoxia, nor could they be used for the actual determination of arterial oxygen saturation. The older devices could only be used for qualitative changes in saturation. As a result of these shortcomings, Wood and Geraei (6) developed a device that could measure, and follow continuously, the absolute value of arterial oxygen saturation from a pickup unit attached to the pinna of the human ear. This new design consisted of a photoelectric earpiece that allowed simultaneous measurement of the transmission of red and near-infrared light through either the normal heat-flushed ear or the bloodless ear. Then, by calculation, the light transmission of the blood alone in these spectral regions could be determined, and in turn the percentage of oxygen saturation of this blood content could be derived. While this device was used in clinical physiologic laboratories, its use did not spread. After this promising beginning, oximetry research went dormant until 1972, when Aoyagi (7) began his work. Aoyagi and his group wanted to build upon the theories and success of the Wood oximeter, and created a dye densitometry method in which two wavelengths of light were used; the ratio of the two optical densities was calculated to obtain a dye curve. This curve was expected to correspond to dye concentrations in blood. It was during this series of experiments that the importance of the pulsatile variations was first reported. After investigating the effect of this pulsatile component, using mathematical analysis of the Beer-Lambert law, it was concluded that calculating the ratio of two optical densities compensates for the pulsations. It was at this point that Aoyagi derived three main conclusions: