Which parameter is the best measure of oxygen delivery to tissues?

Which parameter is the best measure of oxygen delivery to tissues?

• A. CaO2
• B. PaO2
• C. SaO2
• D. PaCO2

Answer: (A)

Oxygen delivery to tissues depends on how much oxygen is carried in the arterial blood, not just how much is dissolved in plasma. The quantity that reflects this best is arterial oxygen content (CaO2). Delivery of oxygen to tissues is the product of how much oxygen is delivered per unit time, which equals cardiac output times CaO2 (with a simple conversion factor). CaO2 accounts for both the oxygen bound to hemoglobin and the small amount dissolved in plasma: CaO2 = (Hb × 1.34 × SaO2) + (0.003 × PaO2). The bound oxygen term is far larger than the dissolved portion under typical conditions, so CaO2 mainly tracks how much oxygen is actually available to tissues. PaO2 measures only dissolved oxygen in plasma and can be normal or high even when total oxygen content is low if hemoglobin is reduced. SaO2 indicates the percentage of hemoglobin carrying O2 but does not tell you how much hemoglobin is present to carry it. PaCO2 is a ventilation and gas-exchange parameter, not a direct measure of oxygen delivery. Therefore CaO2 best reflects the capacity of arterial blood to deliver oxygen to tissues.

Oxygen delivery to tissues depends on how much oxygen is carried in the arterial blood, not just how much is dissolved in plasma. The quantity that reflects this best is arterial oxygen content (CaO2). Delivery of oxygen to tissues is the product of how much oxygen is delivered per unit time, which equals cardiac output times CaO2 (with a simple conversion factor). CaO2 accounts for both the oxygen bound to hemoglobin and the small amount dissolved in plasma: CaO2 = (Hb × 1.34 × SaO2) + (0.003 × PaO2). The bound oxygen term is far larger than the dissolved portion under typical conditions, so CaO2 mainly tracks how much oxygen is actually available to tissues.

PaO2 measures only dissolved oxygen in plasma and can be normal or high even when total oxygen content is low if hemoglobin is reduced. SaO2 indicates the percentage of hemoglobin carrying O2 but does not tell you how much hemoglobin is present to carry it. PaCO2 is a ventilation and gas-exchange parameter, not a direct measure of oxygen delivery. Therefore CaO2 best reflects the capacity of arterial blood to deliver oxygen to tissues.