Final answer:
To detect a patent ductus arteriosus, the PO2 from a newborn's right brachial artery should be compared to the mixed venous blood PO2. This comparison helps identify abnormal mixing of oxygenated and deoxygenated blood due to a patent ductus arteriosus.
Step-by-step explanation:
To rule out patent ductus arteriosus in a newborn, arterial blood is extracted from the right brachial artery. To assess for the presence of this condition, the PO2 from this arterial sample should be compared to mixed venous blood PO2. In the normal closure of the ductus arteriosus, oxygenated blood from the lungs is kept separate from the deoxygenated blood returning to the heart. However, if a patent ductus arteriosus is present, there will be a mixing of the high-oxygen blood from the aorta with the low-oxygen blood in the pulmonary artery. This would be indicated by a higher than normal PO2 in the mixed venous blood sample.
Mixed venous blood, as its name indicates, is a mixture of venous blood from various locations, reflecting the overall oxygen saturation of the blood returning to the heart before oxygenation in the lungs. By comparing arterial PO2 to mixed venous blood PO2, the presence of abnormal shunt flow can be detected. The other options, such as comparing to PvO2 (option A) which is the venous PO2 from the same site, or the umbilical blood gases (option B and C), are not as appropriate in this situation.