Can oxygen consumption in blood in vitro be detected by a change in PCO2?

The PO2 measured in a sample of blood can be misleadingly low, owing to consumption of oxygen in vitro in patients with leukemia. The purpose of this study was to determine whether a rise in PCO2 in such blood could be a useful indication of consumption of oxygen in vitro. Because of the non-bocarbonate buffer capacity of blood and the carriage of CO2, mainly as bicarbonate, we reasoned that the rise in PCO2 would be too small to be helpful. Hence the quantitative relationship between the consumption of oxygen and the production of CO2 in blood was determined. Other reactions yielding CO2 (the titration of lactic acid) were monitored. Consumption of oxygen was stimulated in blood of normals in vitro by adding methylene blue (100 mumol/L); in addition, blood from four patients with leukemia was studied without additions. The rate of consumption of oxygen at 22 degrees C was linear over 60 min; the respiratory quotient was close to unity. The rise in PCO2 was small even when the fall in PO2 was 60 mmHg. We conclude that a rise in PCO2 is not a reliable way to diagnose consumption of oxygen in blood in vitro as patients may hyperventilate, making it very difficult to recognize a small rise in PCO2.