A 4-year-old boy is brought to the office for routine evaluation of a heart murmur. The patient has no fatigue, shortness of breath, or exercise intolerance. He has no other medical conditions and has had no surgeries. Vital signs are within normal limits. On examination, the patient has no cyanosis or clubbing of the fingers. There is a 2/6 holosystolic murmur best heard at the left lower sternal border. There are no rubs or gallops. Pulses are 2+ in the upper and lower extremities. Compared to an unaffected child, which of the following changes in oxygen saturation would be expected in this patient?

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11ec160b_3aa7_0c02_9249_4beddc830e48_MD0001_00 This patient with a harsh, holosystolic murmur along the lower left sternal border most likely has a ventricular septal defect (VSD), an abnormal connection between the right ventricle (RV) and the left ventricle (LV). Deoxygenated blood is normally delivered to the right atrium (RA) from the systemic circulation, and oxygenated blood is delivered to the left atrium (LA) via the pulmonary circulation. Because of increased pressure on the left side of the heart compared to the right, a VSD allows for left-to-right shunting of blood. The delivery of oxygenated blood to the RV via a VSD results in increased RV oxygen saturation (SaO₂) compared to that of a patient with an anatomically normal heart. The SaO₂ in the other cardiac chambers and systemic circulation remains unchanged, and patients are often asymptomatic (eg, no shortness of breath, cyanosis, or clubbing). (Choice A) In tricuspid atresia (absent communication between the RA and the RV), an atrial septal defect (ASD) is always present, allowing deoxygenated blood to flow from the RA to the LA. This decreases SaO₂ in the LA, LV, and systemic circulation, causing neonatal cyanosis. The little blood that reaches the lungs usually does so through a VSD, causing a relative increase in RV SaO₂. (Choice B) The pattern of increased right-sided and decreased left-sided SaO₂is seen in total anomalous pulmonary venous return, a condition in which the pulmonary veins (ie, oxygenated blood) drain into the RA rather than the LA. (Choice C) An ASD typically causes left-to-right shunting of oxygenated blood between the atria, increasing SaO₂ in both the RA and RV. In contrast to a VSD, an ASD typically causes a systolic ejection murmur at the left upper sternal border (pulmonic flow murmur). (Choice E) Over time, increased pulmonary blood flow from left-to-right shunting via a VSD can reverse to right-to-left shunting (ie, Eisenmenger syndrome). SaO₂ on the right side will be normal, whereas that in the LV and systemic circulation is decreased because of mixing with deoxygenated blood from the RV. However, Eisenmenger syndrome takes years to develop and typically presents with signs of right heart failure and cyanosis in adolescents/adults. Educational objective: A ventricular septal defect, classically characterized by a harsh, holosystolic murmur at the left lower sternal border, causes left-to-right shunting of oxygenated blood from the left ventricle to the right ventricle. Therefore, right ventricular oxygen saturation is increased compared to normal. __________ References: Ventricular septal defect. (http://www.ncbi.nlm.nih.gov/pubmed/29261884)

A 4-Year-Old Boy Is Brought to the Office for Routine