Section: USMLE

19) A 2-year-old child with uncomplicated coarctation of the aorta appears to be in good health. Growth and development are normal. The constriction is located just distal to the subclavian arteries. Which of the following is decreased in this patient?

1. Blood flow in the lower body
2. Blood flow in the upper body
3. Blood pressure in the upper body
4. Vascular resistance in the lower body
5. Vascular resistance in the upper body

In fully compensated aortic coarctation, blood flow is normal in the lower and upper body despite an increased arterial pressure (about 50% higher) in the upper body compared with the pressure in the lower body. Because resistance = pressure/blood flow, it is clear that resistance must be lower in the lower portions of the body. The mechanism of this decrease in resistance below the constriction (and increased resistance above the constriction) is autoregulation of blood flow. The small arteries and arterioles dilate (or constrict) in accordance with the metabolic needs of the tissues, ensuring that each tissue receive an adequate amount of blood flow. Thus, the increase in blood pressure in the upper body leads to constriction of the arterioles, which increases vascular resistance, and the lower pressure below the coarctation leads to dilation of the arterioles, which decreases vascular resistance.