Ultrasound is very sensitive at picking up free fluid in the peritoneal space. Depending on the patient, free fluid may or may not be an important finding. Here are a few guidelines to help you make the diagnosis.
Small amount of anechoic fluid
You might pick up a small amount of free fluid between the liver lobes, or next to the bladder. These are areas where it is easier to see a scant effusion. It is often anechoic, and located in the cranial, caudal or dependent portion of the abdomen rather than being associated with a lesion. Young animals often have a small amount of normal fluid in the abdomen as a normal finding. In older animals, hypoproteinemia or overhydration can contribute as well. Mild systemic disease such as IBD is another possibility. If the amount of fluid is small, it’s anechoic, and there is no overt abdominal disease, it is likely a benign cause.
Moderate/marked amount of anechoic fluid
A more concerning finding is a moderate amount of anechoic fluid. Although the amount is subjective, moderate fluid levels are easy to see, and separate the liver lobes and infiltrate between the intestine and other organs. Anechoic fluid is often of low cellularity, and can be a transudate or modified transudate. Causes such as heart failure can be supported by evaluating the hepatic vein size and the history of the animal. Severe hypoproteinemia from gastrointestinal or renal disease can also cause effusion. It’s worth checking the liver and portal vein for evidence of portal hypertension, such as a nodular liver, hepatofugal blood flow in the portal vein, or extrahepatic portosystemic shunts. The history and lab findings are often helpful in narrowing down the differentials and focusing the ultrasound exam.
Moderate/marked amount of echogenic fluid
Echogenic fluid has small suspended particles that look like flecks of white and swirl with agitation or breathing. These often mean that the fluid is cellular, such as a supprurative, hemorrhagic or neoplastic effusion.s Secondary findings are very helpful in these cases. A splenic mass or hepatic mass would lead you toward diagnosing a hemorrhagic effusion, while an intestinal mass with surrounding hyperechoic mesentery and localized effusion indicates ruptured bowel. The key points are to decide if it is generalized or localized, to look for a mass (non-GI or GI), and to evaluate the mesentery. Neoplastic effusions can form from carcinomatosis, which results in a mesentery that is diffusely thickened with ill-defined hypoechoic nodules. Other primary sites leading to carcinomatosis include bladder, GI and prostate.
There are many causes of peritoneal effusion, and ultrasound is very good at detecting them. Anechoic effusions tend to be more benign, and echogenic effusions more infectious/inflammatory or malignant. If you detect effusion, look for focal change that could cause it, or for signs of more general disease. For a diagnosis, paracentesis is required. The best places to aspirate small amounts of fluid are in the cranial and caudal areas of the abdomen. I often orient the transducer parallel to the table, and search for a pocket in the dependent part of the abdomen. Take care with nearby bowel and bladder when placing the needle.