Swelling of legs after cardiac bypass surgery may be caused by poor blood circulation, impaired kidney function, heart failure and other reasons. 1. Poor blood circulation: If the patient undergoes cardiac bypass surgery by using the saphenous vein blood vessel in the leg, it may cause the blood flow in the leg to be too slow after the surgery, resulting in poor blood circulation, which may lead to the swelling of the leg after cardiac bypass surgery. 2. Impaired renal function: After heart bypass surgery, some patients will have impaired renal function. Due to the decline in renal function, resulting in decreased excretion of sodium and water in the kidneys, the body’s sodium and water load increases, which may lead to the phenomenon of post-cardiac bypass surgery leg swelling. 3. Heart failure: after heart bypass surgery, patients may be caused by heart failure, due to the damage of heart function, blood supply function will be reduced, the blood in the lower limbs will be stagnant, which may lead to the phenomenon of leg swelling after heart bypass. When the phenomenon of leg swelling occurs in patients with heart bypass, it is recommended to go to the hospital in a timely manner to find out the cause of the disease, and then carry out standardized treatment under the guidance of the doctor.