PERIOPERATIVE RESPIRATORY MONITORING OF OXYGEN-TRANSPORT

Oxymetry nowadays is understood as the in vitro measurement of O2 satu ration (sO2, %) and hemoglobin (Hb) derivatives (%) using 4-7 waveleng ths (CO- and Hem-oxymeters). Pulse oxymeters, using only 2 wavelengths , are designed for the continuous noninvasive measurement of the arter ial partial O2 saturation (psO2, %) in vivo. Light-emitting diodes all ow light to pass through the peripheral site of measurement with red a nd infrared light to enable a distinction between oxygenated and deoxy genated hemoglobin during a recorded pulse wave. In case of physiologi cal concentrations of Hb derivatives the determination of psO2 is perf ormed with clinically relevant accuracy of +/- 2-3%. However, at carbo xyhemoglobin (COHb) concentrations above normal, under normoxia as wel l as under hypoxia the accuracy of measurement varies considerably amo ng the instruments from different manufacturers. In the case of elevat ed methemoglobin (MetHb) concentrations, the situation is completely d ifferent. With increasing cMetHb, the psO2 is still the value required , but success depends on the concentration of MetHb: under normoxia ps O2 is increasingly underestimated, whereas under hypoxia increasing ov erestimation must be anticipated. Provided there is a constant Hb conc entration, knowledge of the initial sO2, and absence of the derivative s COHb and MetHb as well as of severe perfusion disorders, pulse oxyme try is suitable for perioperative respiratory monitoring of oxygen tra nsport.