Why anemia doesn't affect PaO2 and SaO2?

Oxygen Content Equation

All physicians know that hemoglobin carries oxygen and that anemia can lead to severe hypoxemia. However, the partial pressure of oxygen and the concentration (content) of O2 requires knowledge of the oxygen content equation in order to be understood.

CaO2 = (SaO2 x Hb x 1.34) + .003(PaO2)

Oxygen is a gas and its molecules do exert a pressure but, like any other substance, oxygen also has a finite content in the blood, in units of ml O2/dl blood. Tissues have to have a certain amount of oxygen per minute in order to live, a need met by oxygen content, not oxygen pressure. (Patients can and do live with very low PaO2 values, as long as their oxygen content and cardiac output are adequate.)

The oxygen carrying capacity of one gram of hemoglobin is 1.34 ml.

Given normal pulmonary gas exchange (i.e., a normal respiratory system), factors that lower oxygen content - such as anemia, carbon monoxide poisoning, methemoglobinemia, shifts of the oxygen dissociation curve - do not affect PaO2. PaO2 is a measurement of pressure exerted by uncombined oxygen molecules dissolved in plasma; once oxygen molecules chemically bind to hemoglobin they no longer exert any pressure.

PaO2 affects oxygen content by determining, along with other factors such as pH and temperature, the oxygen saturation of hemoglobin (SaO2).

When hemoglobin content is adequate, patients can have a reduced PaO2 (defect in gas transfer) and still have sufficient oxygen content for the tissues (e.g., hemoglobin 15 grams%, PaO2 55 mm Hg, SaO2 88%, CaO2 17.8 ml O2/dl blood). Conversely, patients can have a normal PaO2 and be profoundly hypoxemic by virtue of a reduced CaO2. This paradox - normal PaO2 and hypoxemia - generally occurs one of two ways: 1) anemia, or 2) altered affinity of hemoglobin for binding oxygen.

A common misconception is that anemia affects PaO2 and/or SaO2; if the respiratory system is normal, anemia affects neither value.

Obviously, however, the lower the hemoglobin content the lower the oxygen content. It is not unusual to see priority placed on improving a chronically hypoxemic patient's low PaO2 when a blood transfusion would be far more beneficial.

Anemia can also confound the clinical suspicion of hypoxemia since anemic patients do not generally manifest cyanosis even when PaO2 is very low. Cyanosis requires a minimum quantity of de-oxygenated hemoglobin to be manifest - approximately 5 grams% in the capillaries. A patient whose hemoglobin content is 15 grams% would not generate this much reduced hemoglobin in the capillaries until the SaO2 reached 78% (PaO2 44 mm Hg); when hemoglobin is 9 grams% the threshold SaO2 for cyanosis is lowered to 65% (PaO2 34 mm Hg).

Altered hemoglobin affinity may occur from shifts of the oxygen dissociation curve (e.g., acidosis, hyperthermia), from alteration of the oxidation state of iron in the hemoglobin (methemoglobinemia), or from carbon monoxide poisoning.

From: http://scuba-doc.com/overview.html