Kaplan USMLE Step 1: Increase in cardiac output and right atrial pressure

If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.

The graph shows the relationship between cardiac output and right atrial pressure. Point X depicts cardiac output and right atrial pressure at rest. Which of the following most likely accounts for the increase in cardiac output and right atrial pressure depicted by point Z?

A. Blood transfusion

B. Exercise

C. Heart failure

D. Hemorrhage

E. Spinal anesthesia

The correct answer is B.

Kaplan Medical explains why

Exercise activates the sympathetic nervous system, which increases cardiac inotropy and manifests as a shift upward and to the left in the cardiac function curve (CFC). Sympathetic stimulation of the veins decreases their capacitance, which manifests as a rightward shift in the vascular function curve (VFC). The new intersection (equilibrium) point Z represents an increase in cardiac output and right atrial pressure.

Why the other answers are wrong

Choice A. The plots intersecting at point Z show the effects of exercise on the cardiovascular system. A blood transfusion shifts the vascular function curve (VFC) up and to the right but the cardiac function curve (CFC) reflexively drops downward and to the right to maintain a stable CO.enal AKI, high aldosterone levels promote sodium and H₂O reabsorption, with urinary sodium levels usually <20 mEq/L.