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

Abstract

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 (V˙O₂) may decrease. The relation between V˙O₂ 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 (SvO₂) 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 (PvO₂) was also observed. The authors conclude that besides PvO₂, the S₃₅, the CavlO₂ and especially the ERav are of value in monitoring the systemic oxygenation during hypothermic ECC.

Key Words:

• blood gases
• coronary bypass
• ERav
• extraction ratio
• hypoxia
• mixed venous PO₂
• mixed venous saturation
• oxygen consumption
• oxyhemoglobin dissociation curve
• pump flow.