Abdominocentesis, or removal of free fluid from the abdomen. This dog is most likely in heart failure. When the heart cannot pump effectively, fluid backs up. It may back up into the veins of the liver, and increased pressure can cause that fluid to be squeezed out into the abdominal cavity. A fluid-distended belly is very uncomfortable and makes breathing difficult. Other causes of fluid in the abdomen include ruptured masses, ruptured hollow organs (gallbladder, intestine, bladder), and lack of protein in the blood.
The upper left photo shows dark fluid on ultrasound. The upper right shows my hand placing a butterfly catheter in the area of greatest fluid on ultrasound. The bottom left shows an assistant pulling fluid out, and the bottom right shows the accumulation. We collected 1200ml (over one liter) from a 38 pound dog, who weighed 3 pounds less after the procedure. I started him on heart medication and diuretics, and his owner reports he is feeling better.