A 6-month-old girl is brought to the office for her first visit after being adopted from another country a week ago. The parents notice that she often seems to breathe hard and fast, particularly after feeding or crying. Her lips sometimes turn blue, as well. On examination, height and weight are at the 15th and 2nd percentiles, respectively. The patient has a 4/6, harsh systolic, crescendo-decrescendo murmur best heard at the left upper sternal border. Complete blood count shows: Which of the following is the most likely explanation for this patient's elevated hematocrit?

Question

A 6-month-old girl is brought to the office for her first visit after being adopted from another country a week ago.  The parents notice that she often seems to breathe hard and fast, particularly after feeding or crying.  Her lips sometimes turn blue, as well.  On examination, height and weight are at the 15th and 2nd percentiles, respectively.  The patient has a 4/6, harsh systolic, crescendo-decrescendo murmur best heard at the left upper sternal border.  Complete blood count shows:  Which of the following is the most likely explanation for this patient's elevated hematocrit?

Quizplus · Accepted Answer

11ec160b_3aa0_f172_9249_413b27035cd1_MD0001_00 This child has polycythemia, an elevation in total red blood cells (RBCs) that usually results from increased erythropoietin production (ie, secondary polycythemia). In contrast to autonomous bone marrow production of RBCs in primary polycythemia, the most common cause of secondary polycythemia is chronic hypoxemia. In this patient, failure to thrive, a harsh murmur, and episodic respiratory distress and cyanosis (ie, tet spells) suggest an underlying cyanotic congenital heart defect. Chronic hypoxemia in congenital heart disease results from right-to-left shunting (ie, delivery of deoxygenated blood to systemic circulation) and therefore underperfusion of the pulmonary arteries. This patient's systolic, crescendo-decrescendo murmur at the left upper sternal border is characteristic of pulmonic stenosis, as seen in tetralogy of Fallot (ToF). Due to right ventricular outflow tract obstruction, right-to-left shunting occurs through the associated ventricular septal defect in ToF. Hypoxemia leads to tissue hypoxia in the inner cortex and medulla of the kidney, which triggers increased renal production of erythropoietin. Erythropoietin stimulates the bone marrow to produce more RBCs to increase the oxygen-carrying capacity of blood. (Choice A) Clonal proliferation of erythroid cells is seen in polycythemia vera, a primary polycythemia that is uncommon in children and often accompanied by thrombocytosis and leukocytosis (not seen here). (Choice B) Pulmonary dead space refers to portions of the lung that are ventilated but not perfused. Decreased pulmonary dead space would increase alveolar air exchange; it would not result in hypoxemia or polycythemia. (Choice C) Impaired postnatal lung expansion (eg, pulmonary hypoplasia in congenital diaphragmatic hernia), causes respiratory distress in the immediate newborn period. Moreover, this patient's pathologic murmur makes a primary cardiac (not pulmonary) pathology more likely. (Choice D) Decreased, not increased, renal blood flow (as in renal artery stenosis) can sometimes cause secondary polycythemia. (Choice E) Small airway inflammation (eg, bronchiolitis) is usually due to an acute viral infection and presents with respiratory distress. Polycythemia is not seen because any hypoxemia that develops is acute, not chronic. Educational objective: Polycythemia is commonly due to increased erythropoietin stimulated by conditions causing chronic hypoxemia, such as cyanotic congenital heart disease (eg, tetralogy of Fallot). Hypoxemia in these cases is caused by right-to-left shunting (ie, underperfusion of the pulmonary arteries). __________ References: Regulation of erythropoietin production. (http://www.ncbi.nlm.nih.gov/pubmed/21078592) A case of uncorrected tetralogy of Fallot undiagnosed until adulthood and presenting with polycythemia. (http://www.ncbi.nlm.nih.gov/pubmed/28352455)

A 6-Month-Old Girl Is Brought to the Office for Her