Question 1 · Hard
A 27-year-old man is referred because pulse oximetry reads 86% at rest. He is not breathless, his chest X-ray and echocardiogram are normal, and he and his mother have had bluish lips since childhood. He takes no medicines. Arterial blood on room air: pO2 12.9 kPa (97 mm Hg), pCO2 5.1 kPa (38 mm Hg), pH 7.41. Co-oximetry: oxyhemoglobin 85%, methemoglobin 0.8% (reference below 1.5%), carboxyhemoglobin 0.6%. Hemoglobin 122 g/L (12.2 g/dL) (reference 130-170 g/L), reticulocytes 1.1% (reference 0.5-2.5%), bilirubin and LD normal. Measured P50 is 6.1 kPa (46 mm Hg) (reference 3.3-3.9 kPa; 25-29 mm Hg). The most likely cause is:
- AA low-oxygen-affinity hemoglobin variant
- BA right-to-left cardiac shunt
- CPyruvate kinase deficiency
- DCongenital methemoglobinemia
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Answer: A · A low-oxygen-affinity hemoglobin variant
The arterial pO2 is normal, so the low saturation is not caused by the lungs or by a shunt, both of which lower pO2. Co-oximetry shows normal methemoglobin and carboxyhemoglobin, so this is true desaturation of hemoglobin at a normal pO2. A P50 far above the reference range means the dissociation curve is shifted well to the right, and a lifelong picture shared with his mother points to an inherited low-affinity variant such as Hb Kansas. Because the tissues unload oxygen easily, erythropoietin drive is low and mild anemia is common, as here. Pyruvate kinase deficiency raises 2,3-BPG and shifts the curve only modestly, and it causes hemolysis, which is absent.
- B: A right-to-left shunt lowers the arterial pO2, which is normal here.
- C: PK deficiency causes hemolysis and only a modest right shift; his bilirubin, LD and reticulocytes are normal.
- D: Methemoglobin is normal on co-oximetry, so it does not explain the low saturation.