From the Guidelines
PaO2 remains normal in anemia because it measures the partial pressure of oxygen dissolved in plasma, not the total oxygen content in blood. In anemia, there is a reduction in hemoglobin concentration, which decreases the oxygen-carrying capacity of blood, but this doesn't affect the amount of oxygen dissolved in plasma, as explained in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1. The lungs continue to function normally, maintaining proper gas exchange and keeping the partial pressure of oxygen in arterial blood within normal range. This is why patients with anemia can have normal PaO2 values on arterial blood gas analysis despite having reduced total blood oxygen content.
Some key points to consider:
- The amount of oxygen carried in the blood is often expressed in terms of how saturated circulating haemoglobin is with oxygen (SO2), which is not directly related to PaO2 1.
- The body compensates for anemia through increased cardiac output and respiratory rate to maintain tissue oxygenation, but these compensatory mechanisms don't alter the PaO2, as the heart responds to low oxygen levels by increasing its output in seconds 1.
- It's essential to understand that while PaO2 is normal, oxygen content (CaO2) and oxygen saturation may be affected in severe anemia, which is why symptoms of tissue hypoxia can still occur despite normal PaO2 readings.
- Oxygen therapy is less effective in conditions like anemia where there is a low carrying capacity, since oxygen availability is not the limiting feature in these conditions, as discussed in the guideline 1.
From the Research
Normal PaO2 in Anemia
- PaO2, or partial pressure of arterial oxygen, can remain normal in anemic patients due to several factors 2, 3, 4.
- In anemia, the body compensates for the decreased oxygen-carrying capacity of the blood by increasing red cell 2,3-DPG, which shifts the oxygen-hemoglobin curve to the right, allowing for more oxygen to be released to the tissues 2.
- Additionally, an increase in P50, which is the partial pressure of oxygen at which hemoglobin is 50% saturated, can also contribute to maintaining normal PaO2 levels in anemic patients 2.
- Studies have shown that arterial oxygen content, which is a more accurate measure of oxygen delivery to the tissues, is precisely maintained by graded erythrocytotic responses in settings of high/normal serum iron levels, and predicts exercise capacity in hypoxemic patients with pulmonary arteriovenous malformations 4.
- Furthermore, research has demonstrated that hemoglobin and oxygen measurements in isolation do not indicate the more physiologically relevant oxygen content per unit blood volume, and that serum iron concentrations, not ferritin, seem to predict more successful polycythaemic responses 4.
Relationship Between PaO2 and Anemia
- The relationship between PaO2 and anemia is complex, and PaO2 can remain normal in anemic patients due to compensatory mechanisms 3.
- However, anemia can still have significant effects on oxygen delivery to the tissues, and arterial oxygen content should be considered in addition to PaO2 when evaluating oxygenation in anemic patients 3, 4.
- Studies have shown that hemoglobin and oxygen content are more important determinants of exercise capacity than SaO2, and that modest changes in hemoglobin can substantially modify oxygen content 3, 4.