USMLE (United States Medical Licensing Examination) Step 1 Practice Exam 2025 - Free USMLE Step 1 Practice Questions and Study Guide

A patent foramen ovale (PFO) occurs when the foramen ovale, an opening between the right and left atria of the heart, fails to close after birth. Normally, this opening is supposed to close within the first few months of life as pressure changes in the heart occur following the establishment of independent pulmonary circulation.

When a PFO persists, it can allow blood to pass from the right atrium to the left atrium. This bypasses the lungs, which can have significant clinical implications. One of the primary consequences of a PFO is the potential for paradoxical emboli. This situation arises when a thrombus (blood clot) that originates in the venous system travels through the right side of the heart, crosses over to the left side via the PFO, and subsequently enters systemic circulation. If this embolus travels to the brain or other vital organs, it can cause serious complications such as a stroke or organ ischemia.

The mechanics of this phenomenon are critical to understanding the risks associated with a PFO. Normally, clots that arise in veins do not affect the arterial circulation due to the filtering effect of the lungs. In patients with a patent foramen ovale, however, this protective mechanism is circumvented