S₃₅: A new parameter in blood gas analysis for monitoring the systemic oxygenation

Abstract

In the estimation of oxygen transport the term oxygen availability is used as the product of cardiac output and the arterial oxygen content (CaO₂). Attempts can be made to modify the concept of oxygen availability by subtracting from the CaO₂ the venous content at a critical PO₂ as measured in mixed venous blood (PvO₂), where oxygen diffusion into tissue becomes compromised and oxygen uptake (V˙O₂) may decrease. The real arterial available oxygen content (CavlO₂) can be calculated by estimating the saturation at the critical PvO₂. For our concept S₃₅ was chosen as such a dynamic baseline. Similar modification of oxygen extraction ratio (ERav) defined as vO₂ divided by the real oxygen availability (O₂av) should give, more than the classic ER, a realistic indices of oxygen availability in relation to oxygen consumption. It can be hypothesized that vO₂ starts to decline when ERav is around 1.0. During isovolemic hemodilution V˙O₂ started to drop when ERav reached 1.08 ± 0.09. The S₃₅ changed from 55.0 ± 2.1 % to 41.5 ± 4.1 %, correlated with changes in PvO₂. A direct correlation was also found between the increase of the classic ER and the change in S₃₅.

We conclude that the S₃₅, the CavlO₂ and the ERav can be of value in monitoring the systemic oxygenation and that the concept also includes the effect of changes in oxyhemoglobin characteristics on oxygen delivery.

Key Words:

• arterial available oxygen content
• ERav
• extraction ratio
• hemodilution
• hypoxia
• isovolemic
• oxygen consumption
• oxyhemoglobin dissociation curve
• P₅₀.