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).
ctO2(a) reference range (adult) :
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)
Normal ctO2 indicates an adequate oxygen content of the arterial blood.
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 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.