Normally, blood flows from the right side of the heart through the pulmonary arteries and lungs before it returns to the left side of the heart. A paradoxical embolism is a blood clot that does not travel with normal blood flow. This type of embolism often causes a stroke because the clot moves directly from the right side of the heart to the left through a hole (defect) in the septum, which separates the upper right and left heart chambers. It then bypasses the lungs and is pumped straight toward the brain.