SESSION TITLE: Pediatric Cases
SESSION TYPE: Affiliate Case Report Slide
PRESENTED ON: Monday, October 27, 2014 at 03:15 PM - 04:15 PM
INTRODUCTION: Child with hypoxemia, not caused by hypoventilation, V/Q mismatch, shunting, or diffusion limitation.
CASE PRESENTATION: A 4 year old boy presented with fever and exertional dyspnea, without cough or respiratory distress, SpO2 85% in room air, which improved to 92% on 2LPM oxygen. Chest radiograph (CXR) showed bronchial thickening without focal infiltrate; heart size and vascularity were normal. Hb value was normal. He was discharged home on azithromycin and amoxicillin when stable in room air. Two days later, pediatrician sent him to ER for SpO2 85%, unchanged CXR. Review of systems: no history of excercise tolerance, problems at high altitudes, or decreased activity. Immunizations up to date. Paternal family history of Thalassemia. Prior surgery for coarctation repair in infancy. During hospitalization, lungs were clear to auscultation; rest of exam was normal without signs of clubbing or cyanosis. SpO2 ranged 87-92% in room air. Interestingly, SpO2 of patient’s father was 92%. Paternal grandmother and aunt reportedly have low oxygen saturations during doctors’ visits. Studies during hospitalization included bubble ECHO and chest CT angiogram, which were normal. ABG was unsuccessful. Hemoglobin electrophoresis showed Hb A 75, Hb V 22. DNA analysis was consistent with heterozygous Hb Bassett.
DISCUSSION: Hb Bassett is an abnormal Hb variant with a markedly reduced oxygen affinity, decreased Bohr effect, and low subunit cooperativity. It has an amino acid substitution (α94Asp-->Ala) at the α1:β2 contact region, where the key conformational changes associated with oxygenation and deoxygenation of Hb molecule take place. This dimer interface plays a major role in the allosteric activity of Hb by forming an inter-subunit hydrogen bond/salt bridge that links and stablizes the two αβ dimers in both the T- and R- state. In Hb Bassett, replacement of the aspartate residue by alanine abolishes significant α1:β2 hydrogen bond interactions resulting in reduced stability of Hb Bassett relative to normal Hb, R-state more destabilized than T-state due to loss of a greater number of inter-subunit interactions in the R-state. The allosteric equilibrium is therefore expected to favor the relatively stable T-state Hb Bassett, giving rise to low oxygen affinity.
CONCLUSIONS: Hb binding affinity for oxygen should be considered when all four causes of hypoxemia are ruled out.
Reference #1: Abdulmalik O, et al. Characterization of hemoglobin Bassett (α94AspàAla), a variant with very low oxygen affinity. Am J Hematol 2004;77(3):268-276.
DISCLOSURE: The following authors have nothing to disclose: Diana Chen, Mustafa Bseikri
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