A venous blood sample from which site would correlate most reliably with PaO2 and PaCO2?

A venous blood sample from which site would correlate most reliably with PaO2 and PaCO2?

• A. Jugular vein
• B. Subclavian vein
• C. Antecubital vein
• D. Vein on posterior surface of a warmed hand

Answer: (D)

The key idea is that arterial blood gas values (PaO2 and PaCO2) are measured from arterial blood, while venous blood reflects tissue gas exchange. Normally PvO2 is lower and PvCO2 higher than PaO2 and PaCO2, so venous samples don’t reliably match arterial values. The only venous site that can approximate arterial gas tensions is a venous sample from a warmed hand, where warming causes vasodilation and arterial blood to flow through the cutaneous capillaries, making the venous outflow more “arterialized.” This arterialization raises PvO2 and lowers PvCO2 enough to correlate more closely with PaO2 and PaCO2, so the dorsal hand vein on a warmed hand provides the most reliable correlation. By comparison, central veins like the jugular or subclavian and a typical antecubital vein return blood that has already participated in tissue exchange, so their O2 and CO2 tensions differ more markedly from arterial values and don’t correlate as well.

The key idea is that arterial blood gas values (PaO2 and PaCO2) are measured from arterial blood, while venous blood reflects tissue gas exchange. Normally PvO2 is lower and PvCO2 higher than PaO2 and PaCO2, so venous samples don’t reliably match arterial values. The only venous site that can approximate arterial gas tensions is a venous sample from a warmed hand, where warming causes vasodilation and arterial blood to flow through the cutaneous capillaries, making the venous outflow more “arterialized.” This arterialization raises PvO2 and lowers PvCO2 enough to correlate more closely with PaO2 and PaCO2, so the dorsal hand vein on a warmed hand provides the most reliable correlation.

By comparison, central veins like the jugular or subclavian and a typical antecubital vein return blood that has already participated in tissue exchange, so their O2 and CO2 tensions differ more markedly from arterial values and don’t correlate as well.