The theoretical effect of carboxyhemoglobin on the pulse oximeter

The relationship between arterial oxygen saturation as measured by the pulse oximeter (SpO2) and the fractional arterial oxygen saturation (SaO2) in the presence and absence of carboxyhemoglobin (COHb) has been derived according to the theory of absorption spectroscopy. We find that our theoretically derived correction equation is similar to that found in the technical literature of Nellcor. However, the correction equations presented by Barker and Tremper and the technical literature of Ohmeda differ substantially from our equation when sufficient quantities of reduced hemoglobin are present and the fractional COHb saturation (SaCO) is high. Our approximated equation, derived from the Lambert-Beer law, is SaO2 = SpO2 (1 - 0.932 SaCO) + 0.032 SaCO. The equation of Barker and Tremper is SaO2 = SpO2 - 0.9 SaCO. The Nellcor equation is SaO2 = SpO2 (1 - SaCO).