S35 and derived parameters during extracorporeal circulation together with hemodilution and hypothermia in humans

A new concept in monitoring systemic oxygenation that includes the effect of changes in oxyhemoglobin dissociation curve (ODC) has been introduced. Using the S35 (saturation of hemoglobin at PO2 = 35 mmHg), real arterial available oxygen content (CavlO2) can be calculated being the maximum amount of oxygen that can be extracted from hemoglobin before oxygen diffusion into tissue becomes compromised and oxygen uptake (VO2) may decrease. The relation between VO2 and CavlO2 expressed by the extraction ratio of the arterial available oxygen content (ERav) gives a realistic indices of oxygen supply in relation to oxygen consumption. In the present study, during extracorporeal circulation (ECC), a severe shift to the left of the ODC could be observed. THe classic parameters for monitoring systemic oxygenation as mixed venous saturation (Sv-O2) and extraction ratio (ER) did not change. The S35 increased because of the shift to the left of the ODC with consequent decrease in CavlO2. The ERav reached critical values during ECC together with hemodilution and hypothermia. A severe decrease in mixed venous PO2 (Pv-O2) was also observed. The authors conclude that besides Pv-O2, the S35, the CavlO2 and especially the ERav are of value in monitoring the systemic oxygenation during hypothermic ECC.