There are 3 main reasons for swelling in the back of the feet: 1. Circulation disorders in the lower extremities due to blockage of blood vessels in the lower extremities. In this case, we have to do lower extremity arteriovenous ultrasound to determine whether there is vascular embolism or poor circulation in the lower extremity caused by vascular stenosis. 2. Renal insufficiency. Patients can do abdominal ultrasound and blood test for kidney function to determine whether there is swelling in the lower limbs caused by kidney insufficiency. 3, cardiac insufficiency. Patients with cardiac insufficiency have poor heart contraction, so blood cannot be ejected smoothly into the aorta, and the stagnant blood will reflux into the lungs, digestive tract, and both lower extremities, causing swelling of the lower extremities, including swelling of the back of the feet. For this group, a cardiac ultrasound and a B-type natriuretic peptide test should be done to determine if heart failure is present.