Vena caval syndrome occurs when heartworms reside in the vena cava (the big vein returning blood from the liver and lower body to the heart). Heartworms do not normally inhabit this vein. Their presence seems to really muck things up and breakdown of red blood cells occurs, leading to the red discoloration of the urine. This is a serious complication in heartworm disease. It is sometimes necessary to surgically remove the worms from the vena cava to allow the dog to get stable.