A 32-year-old woman comes to the office with worsening shortness of breath. She has the following arterial blood results: Which of the following is the most likely cause of these results?

Question

A 32-year-old woman comes to the office with worsening shortness of breath.  She has the following arterial blood results:  Which of the following is the most likely cause of these results?

Quizplus · Accepted Answer

11ec160b_3cdf_bd5f_9249_a7fc69cee69b_MD0001_00 Oxygen is carried in the blood both as a dissolved gas and in combination with hemoglobin. The total oxygen content of the blood is determined primarily by the amount of hemoglobin in the blood and its percentage of oxygen saturation (SaO₂). Dissolved oxygen is not bound to hemoglobin and accounts for a very small proportion of the total oxygen content of blood due to low solubility in plasma. Changes in arterial oxygen tension (PaO₂) affect both the SaO₂ and the amount of oxygen dissolved in the plasma; however, the change in SaO₂ is far more influential on the total oxygen content of the blood. This patient has arterial pO₂ (PaO₂) and SaO₂ within the normal range, but decreased total blood oxygen content (CaO₂). If the PaO₂ and SaO₂ are both normal, the lower total blood oxygen content is most likely secondary to a lower hemoglobin concentration. A common cause of anemia in premenopausal women is chronic blood loss secondary to menstruation. (Choice B) Cyanide inhibits cellular oxidative phosphorylation by inhibiting Fe³⁺ in cytochrome c oxidase, lowering peripheral tissue oxygen consumption. Arterial PaO₂, SaO₂, and CaO₂ remain unchanged, but venous oxygen content rises and the arterial-venous oxygen gradient falls. Cyanide may also bind to the ferrous (Fe²⁺) iron of hemoglobin to form cyanohemoglobin, but only in small amounts that do not appreciably change arterial oxygen content. (Choice C) Reduced alveolar oxygen tension (PAO₂) (eg, at high altitude) or disruption of gas exchange at the alveolar-capillary interface can reduce the PaO₂ and SaO₂. In addition, factors that reduce the affinity of hemoglobin for oxygen (eg, acidosis) can lower SaO₂ (Bohr-Haldane effect). (Choice D) Obesity causes alveolar hypoventilation due to reduced chest wall compliance. Obese individuals also have increased lower lobe perfusion causing ventilation perfusion mismatch. These factors, in combination with other complex mechanisms, lead to hypoxemia (low PaO₂), low SaO₂, and hypercapnia. (Choice E) In tetralogy of Fallot, mixing of deoxygenated and oxygenated blood occurs due to ventricular septal defect, lowering PaO₂ of the left ventricle. The aorta overrides the right and left ventricles, delivering blood with low arterial PaO₂ and SaO₂ to the periphery. Educational objective: The 3 variables that affect the total oxygen content of blood are hemoglobin concentration, oxygen saturation of hemoglobin (SaO₂), and the partial pressure of oxygen dissolved in blood (PaO₂). Anemia is characterized by decreased hemoglobin concentration in the setting of normal SaO₂ and PaO₂.

A 32-Year-Old Woman Comes to the Office with Worsening Shortness