Can central venous blood replace mixed venous blood samples?

To estimate the value of central venous blood as representative of real changes in pulmonary shunt (Qsp/Qt), mixed venous oxygen saturation and arteriovenous oxygen content difference [C(a-v)O2] during active phases of adult intensive care therapy, 86 blood samples were withdrawn from 42 patients as quadruple simultaneous collections from systemic artery, pulmonary artery (PA), superior caval vein (CV) and right atrium (RA). We found a significant positive correlation of the measured variables and especially of the subsequent changes of these variables in individual patients between PA blood samples and both CV and RA blood samples (p less than 0.001). We, therefore, conclude that a central venous catheter can replace the PA catheter to collect blood representative of mixed venous blood samples for the above purposes. However, the exact numerical value of mixed venous blood samples can only be measured from blood collected from the PA itself.