• Most oxygen carried in the blood is bound to hemoglobin and is referred to as oxygen saturation. A small percentage also is dissolved in the plasma. The total blood oxygen content is determined by adding the amount of oxygen bound to hemoglobin to that dissolved in the plasma. Oxygen content can be calculated for the arterial blood (CaO₂) and the venous blood (CO₂). By calculating the oxygen contents for arterial and venous blood and subtracting them, a rough estimate of oxygen use can be made. This value is either expressed as mL/dL or as vol %. A normal arterial-venous oxygen content difference (a-vDO₂) is 5 vol % (range, 4 to 6 vol %). In general, because the contribution of dissolved oxygen is slight, it is not used clinically to calculate CaO₂.3,4 It is essential to remember that hemoglobin is one of the most important variables in oxygenation status.

• The partial pressure of mixed venous oxygen pressure (PO₂) and mixed venous oxygen saturation (SO₂) reflects tissue oxygenation under most conditions. When blood flow does not increase to meet higher tissue oxygen demands (as in hypodynamic conditions [e.g., shock]), more oxygen is extracted from the arterial blood, and the PO₂ and SO₂ decrease. The gradient between CaO₂ and CO₂ widens. Conversely, when blood flow is increased (e.g., hyperdynamic flow, as in sepsis), less oxygen is extracted from the arterial blood; PO₂ and SO₂ increase, and a-vDO₂ decreases.3,4

• A major clinical goal of positive-pressure ventilation (PPV) and positive end-expiratory pressure (PEEP) is improved oxygenation. One potential complication of these therapies is hypotension from the effect of increased intrathoracic pressures on venous return.1,2

• Calculation of a-vDO₂ may reflect tissue oxygenation in some cases; however, it is not a direct measurement and can be used only for approximation. The measurement of lactic acid is thought to be a more accurate assessment of tissue hypoxia, but the formation of lactic acid occurs late in the clinical course and is often irreversible.² It slowly clears on improvement of the patient’s condition.

• In addition to hemoglobin, one of the most important variables affecting oxygen use is cardiac output. When cardiac output is inadequate, more oxygen is extracted from the arterial blood, which lowers the CO₂.

• The product of cardiac output and CaO₂ is oxygen delivery. Oxygen consumption may be calculated by determining the product of cardiac output and a-vDO₂.1–4

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