Perioperative respiratory monitoring of oxygen transport

Oximetry nowadays is understood as the in vitro measurement of O2 saturation (sO2, %) and hemoglobin (Hb) derivatives (%) using 4-7 wavelengths (CO- and Hem-oximeters). Pulse oximeters, using only 2 wavelengths, are designed for the continuous noninvasive measurement of the arterial partial O2 saturation (psO2, %) in vivo. Light-emitting diodes allow light to pass through the peripheral site of measurement with red and infrared light to enable a distinction between oxygenated and deoxygenated hemoglobin during a recorded pulse wave. In case of physiological concentrations of Hb derivatives the determination of psO2 is performed with clinically relevant accuracy of +/- 2-3%. However, at carboxyhemoglobin (COHb) concentrations above normal, under normoxia as well as under hypoxia the accuracy of measurement varies considerably among the instruments from different manufacturers. In the case of elevated methemoglobin (MetHb) concentrations, the situation is completely different. With increasing cMetHb, the psO2 is still the value required, but success depends on the concentration of MetHb: under normoxia psO2 is increasingly underestimated, whereas under hypoxia increasing overestimation must be anticipated. Provided there is a constant Hb concentration, knowledge of the initial sO2, and absence of the derivatives COHb and MetHb as well as of severe perfusion disorders, pulse oximetry is suitable for perioperative respiratory monitoring of oxygen transport.