From which location should a mixed venous sample for PvO2 measurement be taken?

To measure mixed venous oxygen tension (PvO2), the most appropriate location is at the distal end of a pulmonary artery catheter. This site is selected because the distal end allows for the sampling of blood that has mixed from the systemic venous return prior to entering the pulmonary artery. Blood from this site reflects the overall oxygen content of the blood returning from the entire body, as it includes deoxygenated blood from both the superior and inferior vena cavae, which have received blood from various organs and tissues.

Taking a blood sample from the right atrium or any other area that does not capture the mixed venous return would provide an inaccurate representation of the body's overall oxygen status. The right atrium could be influenced by varying inflows and might not represent the true mixed venous saturation of blood that has passed through the systemic circulation. The proximal end of the pulmonary artery catheter typically samples blood that is still substantially oxygenated due to its closeness to the heart's outflow from the right ventricle, thus it does not provide the true mixed venous oxygen tension.

In summary, the distal end of the pulmonary artery catheter is deemed the best location for sampling mixed venous blood to assess PvO2, as it accurately captures the oxygen content