From the Guidelines
The answer is b. Total arterial oxygen content. In this patient with severe anemia (hemoglobin 6 g/dL), the total arterial oxygen content would be reduced because oxygen in the blood is primarily carried by hemoglobin. With significantly decreased hemoglobin levels, there are fewer molecules available to bind and transport oxygen, directly reducing the total oxygen content in arterial blood. The other parameters would remain relatively normal:
- arterial PO2 (partial pressure of oxygen) reflects the amount of dissolved oxygen in plasma and is not directly affected by hemoglobin levels 1;
- dissolved oxygen content is also independent of hemoglobin concentration;
- percent O2 saturation refers to the proportion of available hemoglobin that is bound to oxygen and can remain normal even with low hemoglobin;
- and oxygen extraction (the amount of oxygen removed from blood as it passes through tissues) may actually increase to compensate for the reduced oxygen delivery. This patient's symptoms of fatigue and cold extremities are classic manifestations of anemia resulting from this reduced oxygen-carrying capacity. Guidelines for oxygen use, such as those outlined in the BTS guideline for oxygen use in adults 1, emphasize the importance of maintaining adequate oxygen saturation, but in this case, the primary issue is the reduced oxygen-carrying capacity due to severe anemia, rather than an issue with oxygen saturation itself.
From the Research
Anemia and Oxygen Transport
- In a patient with anemia, the total arterial oxygen content would be reduced 2.
- This is because the hemoglobin concentration is low (6 g/dL), which impairs the blood's ability to carry oxygen to the tissues.
- According to the study, adequate oxygen delivery to the tissues can usually be assured until the hemoglobin concentration falls below 7-8 g/dL in patients with chronic anemia 2.
- In this case, the patient's hemoglobin concentration is below this threshold, indicating that oxygen transport is impeded.
Effects of Anemia on Oxygen Parameters
- The following parameters would not be directly reduced:
- Arterial PO2: This measures the partial pressure of oxygen in the arterial blood, which is not directly affected by the hemoglobin concentration.
- Dissolved oxygen content: This is a small component of the total oxygen content and is not significantly affected by anemia.
- Percent O2 saturation in the arterial blood: This measures the percentage of hemoglobin that is saturated with oxygen, which may not be directly reduced in anemia.
- Oxygen extraction: This refers to the amount of oxygen extracted by the tissues from the blood, which may not be directly reduced in anemia.
Clinical Implications
- The patient's symptoms, such as increasing fatigue and cold extremities, are consistent with anemia and impaired oxygen transport 3, 4.
- The patient's low hemoglobin concentration and symptoms suggest that oxygen transport is impeded, and transfusion may be necessary to avoid jeopardizing the patient 2.