What exactly does the pulse oximeter measure?
Blood carries oxygen in two forms, the majority is bound to hemoglobin (oxyhemoglobin) and the rest is dissolved in the aqueous phase of blood (the plasma). The pulse oximeter measures the saturation of hemoglobin with oxygen. This is expressed as a percent saturation. Each gram of normal hemoglobin can hold 1.34 milliliters of oxygen. The dissolved fraction is dependent upon the partial pressure of oxygen. As the partial pressure increases, the dissolved fraction of oxygen increases.
