ctO₂
Arterial concentration of total oxygen

Definition
ctO₂ is the concentration of the total oxygen in the blood. ctO₂ is the sum of the concentration of hemoglobin-bound oxygen and the concentration of physically dissolved oxygen:

ctO₂ = sO2 × (1 - FCOHb - FMetHb) × ctHb + O₂ × pO₂

It is also called the “O₂ content”. The systematic symbol for arterial blood is ctO₂(a). The analyzer symbol may be tO₂ or ctO₂.     

What does ctO₂ tell you
The oxygen content of the blood is an expression of the oxygen transport status of the blood. Changes in ctO₂ reflect the integrated effects of changes in arterial pO₂, in effective hemoglobin concentration, and in the affinity of hemoglobin for oxygen (as expressed by p50).

Reference ranges
ctO₂(a) reference range (adult) [16]:
male: 8.4-9.9 mmol/L (18.8-22.3 mL/dL)
female: 7.1-8.9 mmol/L (15.8-19.9 mL/dL)
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Clinical interpretation
Normal ctO₂:
Normal ctO₂ indicates an adequate oxygen content of the arterial blood.

High ctO₂:
High ctO₂, despite normal pO₂, can only be caused by high ctHb (i.e., hemoconcentration, polycytemia, or excessive red-cell transfusion). As this may increase the cardiac load excessively, hemodilution may be indicated.

Low ctO₂:
Low ctO₂ may be caused by hypoxemia (low pO₂) or if pO₂ is normal, by a low ctHb and/or dyshemoglobinemia. Rarely, an extreme right shift of the ODC (as indicated by a high p50) can cause low ctO₂, even if effective hemoglobin concentration and pO₂ are normal.

The treatment of low ctO₂ is correction of hypoxemia (see above for pO₂), anemia (i.e., by red-cell transfusion), or dyshemoglobinemia, respectively.

ODC and ctO₂ curve
Low values of ctO₂(a) imply a risk of decreased oxygen delivery to the tissue and thus tissue hypoxia, unless it is compensated for by an increase in cardiac output. Therefore, lactate levels (as an indicator of tissue hypoxia) should be reviewed and monitored in conjunction with the monitoring of low ctO₂.

The hemoglobin oxygen dissociation curve (ODC). Note that this is independent of ctHb.

The oxygen content curve. Note that this also reflects ctHb.

Be aware of the risk of preanalytical errors (air bubbles, storage) on ctO₂ values.

For more information, go to Preanalytical considerations.