Pulse Oximetry – Carboxyhemoglobin
Standard pulse oximetry aims to provide a noninvasive, in vivo, and continuous assessment of functional arterial oxygen saturation, SaO2. Estimates of SaO2 based on pulse oximetry are denoted as SpO2.
A pulse oximeter will measure red and infrared light transmitted through and reflected by the tissue bed. Any light is usable since all light frequencies will transmit through and be reflected by the blood and tissue. The most commonly used wavelengths of light are red at 660 nm and infrared at 940 nm light. At 660 nm, deoxyhemoglobin has greater light absorption than oxyhemoglobin. At 940 nm, oxyhemoglobin has greater light absorption than deoxyhemoglobin. Additionally, it is technically possible to produce these wavelengths using an LED.
The ratio of light absorption at these wavelengths is empirically related to oxygen saturation based on a calibration curve internal to each pulse oximeter. Each manufacturer develops its calibration curve by having volunteers breathe hypoxic gas mixtures to create a range of SaO2 values between 70% and 100%.
Pulse oximetry has been an integral component of intraoperative anesthetic management as a minimum monitoring standard since the American Society of Anesthesiologists adopted it in 1986. It is also part of the WHO Safe Surgery checklist.
Normal blood has four hemoglobin species: oxyhemoglobin, reduced (deoxy-) hemoglobin, methemoglobin, and carboxyhemoglobin. Each species has a distinct light absorption profile, as shown in the graph below.
Light Absorption with Pulse Oximetry
As the graph shows, the light profiles in one of the chosen areas (red = 660 nm) overlap considerably for carboxyhemoglobin and oxyhemoglobin (as it does for methemoglobin and deoxyhemoglobin). This means the pulse oximeter will see the reflected light for carboxyhemoglobin and oxyhemoglobin similarly when measuring in the red light spectrum, thereby measuring a combined reflection. Carboxyhgb is not reflected/absorbed in the infrared spectrum and, therefore, does not add to this part of the ratio. Remember, the ratio between these measurements is checked against the pulse oximeter database. This ratio will correspond with the displayed SpO2.
When the pulse oximeter reads the oxygen concentration as a combination of the non-oxygen-carrying-caboxyhemoglobin and oxyhemoglobin, the result will be a false high reading. The higher the carboxyhemoglobin level, the more significant the discrepancy.