A 32-year-old woman is brought to the emergency department due to worsening shortness of breath. Pulse oximetry shows 99% on room air. Results from analysis of an arterial blood sample are as follows:
| Partial pressure of oxygen (PaO2) | normal |
| Oxygen saturation (SaO2) | normal |
| Total arterial oxygen content (CaO2) | low |
Which of the following is the most likely cause of this patient's laboratory findings?
PaO2 | SaO2 | Oxygen content | |
CO poisoning | Normal | ↓* | ↓ |
Cyanide poisoning | Normal | Normal | Normal |
Anemia (↓ Hb) | Normal | Normal | ↓ |
Polycythemia (↑ Hb) | Normal | Normal | ↑ |
High altitude | ↓ | ↓ | ↓ |
*Detected as normal using standard probes. CO = carbon monoxide; Hb = hemoglobin; PaO2 = partial pressure of oxygen; SaO2 = arterial oxygen saturation. | |||
Oxygen (O2) is carried in the blood, both as a dissolved gas and in combination with hemoglobin. The total O2 content of the blood (CaO2) is determined primarily by the amount of hemoglobin in the blood and its percentage of O2 saturation (SaO2), which is directly influenced by changes in arterial O2 tension (PaO2) of the dissolved O2 according to the oxygen-hemoglobin dissociation curve. Freely dissolved O2 normally accounts for only a very small proportion of the total O2 content of blood due to its low solubility in plasma.
This patient's low CaO2 in the arterial blood but normal PaO2 and SaO2 are suggestive of low hemoglobin (ie, anemia), which reduces the total O2 content in the blood despite a normal partial pressure and percentage of O2 saturation. A common cause of anemia in this patient's age group includes chronic blood loss (eg, heavy menses).
(Choice B) Cyanide inhibits oxidative phosphorylation by inhibiting ferric iron (Fe3+) in cytochrome c oxidase, lowering O2 consumption in peripheral tissue. Arterial PaO2, SaO2, and CaO2 remain unchanged, but venous O2 content rises and the arterial-venous O2 gradient falls. Cyanide may also bind to the ferrous (Fe2+) iron of hemoglobin to form cyanhemoglobin but typically only in small amounts that do not appreciably change arterial O2 content.
(Choice C) Primary polycythemia (polycythemia vera) is due to a genetic mutation causing increased red blood cell (RBC) production. The hemoglobin within RBCs functions normally (normal SaO2), but there is a greater total amount of hemoglobin (increased CaO2). There are no effects on dissolved oxygen (normal PaO2).
(Choice D) The O2 tension in high-altitude settings is relatively low compared to that at sea level. Recent exposure to a high-altitude environment would result in reduced alveolar O2 tension, leading to reduced PaO2 and SaO2, with a corresponding decrease in CaO2.
(Choice E) Right to left intrapulmonary shunting occurs when venous blood flows through inadequately ventilated alveoli (eg, pulmonary edema). This blood remains deoxygenated as it reenters the systemic circulation. It has a low PaO2 and SaO2, with a corresponding decrease in CaO2.
Educational objective:
The total oxygen content of blood (CaO2) is affected primarily by 2 variables: hemoglobin concentration and its percentage of oxygen saturation (SaO2), which is directly affected by the arterial oxygen tension (PaO2). Anemia is characterized by decreased hemoglobin concentration in the setting of normal SaO2 and PaO2.