An arteriovenous fistula is an abnormal tunnel between an artery and a vein. Blood takes the short cut, so the downstream vein bulges and the upstream artery can steal blood from other organs. Symptoms depend on size and location, but most show up as a warm, humming lump.

Pulsing lump is the headline. You feel a rhythmic thump under the skin, usually in the wrist, groin, or upper arm.

Warm skin follows. The area over the fistula feels warmer than the opposite side because extra blood is rushing through.

Swelling appears. The vein below the fistula bulges like a garden hose, and the limb may puff, especially at the ankle.

Humming sound is classic. A doctor hears a machinery murmur with a stethoscope; you may feel a vibration when you lay your hand over it.

Weak pulse downstream shows up. The artery past the fistula feels weaker because blood is shunted away.

Late alarms include heart racing, shortness of breath, or a blue, painful limb—signs the fistula is stealing too much blood or clotting.