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Mary Frances McMullin, Commentary, Clinical Chemistry, Volume 58, Issue 2, 1 February 2012, Page 335, https://doi.org/10.1373/clinchem.2011.166710
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This fascinating and carefully worked out case study describes a boy originally presenting with cyanosis at the age of 4 years. He and his mother were found to have a newly described low-affinity hemoglobin, termed “hemoglobin Venusberg.” Of note are the extensive investigations carried out on both individuals before the diagnosis was obtained, the fact that this variant was detected not by hemoglobin electrophoresis but by the presence of an abnormality in hemoglobin chromatography, and the repeated low oxygen saturations revealed by pulse oximetry. Both individuals were reported as having “limited physical resistance to stress,” which has not been reported before in the literature with low-affinity hemoglobin variants, and it is not clear why such a variant should lead to limited exercise tolerance.
Oxygen is carried in the blood bound to hemoglobin, and each type of hemoglobin has a certain capacity for holding oxygen. In the lungs, the partial pressure of oxygen is high, and oxygen binds to hemoglobin, whereas in the tissues the partial pressure is lower, and the oxygen is released as required. This relationship between the partial pressure of oxygen and the oxygen saturation of hemoglobin is described in the oxygen-dissociation curve. With normal hemoglobins under normal conditions, hemoglobin is 50% saturated at a partial pressure of oxygen of 26.6 mmHg (the P50). The curve is shifted to the right to give a higher P50. That means that a larger partial pressure is required to maintain 50% saturation, and there is decreased oxygen affinity. A hemoglobin such as hemoglobin Venusberg has an alteration that leads to lower oxygen affinity and thus a right-shifted oxygen dissociation curve.
