Cooximetry

Abstract

There are three distinct methods of determining the oxygen saturation of blood. The results are generally interchangeable in healthy people, but can differ with dyshemoglobinemias. Pulse oximetry utilizes the Beer-Lambert law, which states that light absorbance is proportionate to the concentration of the light attenuating substance. In the arterial blood gas (ABG), the pH and partial pressure of oxygen in the blood are measured, and the saturation is calculated from the standard oxygen dissociation curve. While cooximetry also utilizes the Beer-Lambert law, multiple wavelengths of light are measured. The concentrations of O₂Hb and different Hb species are determined by their different absorptions at various different wavelengths. Fractional SaO₂ = O₂Hb × 100/O₂Hb + HHb + COHb + MetHb. Cooximetry may be indicated if cyanosis or hypoxia measured by pulse oximetry fails to improve with O₂ administration or if there are discrepancies between O₂ sat and PaO₂ by ABG.