Since proteins hold fluid in the vascular bed, loss of protein (albumin) causes fluid to enter the tissue spaces. In this condition the kidneys lose large amounts of protein into the urine causing a drop in plasma oncotic pressure. Nephrotic syndrome in particular is associated with ascites formation. Kidney disease can contribute to this process, since the kidneys have a critical role in fluid balance. Any condition that causes an increase in peritoneal fluid is called an effusion or ascites. The fluid itself is essentially an ultrafiltrate of plasma. Normally, only 30-50 mL (1-1.7 oz) of fluid is found in the peritoneal cavity. The peritoneum wraps around nearly every organ in the abdomen, and lines the entire abdominal cavity, providing many folds and spaces in which fluid can gather. This space is called the peritoneal space because it is enclosed by a thin membrane called the peritoneum.
The free space around abdominal organs receives most of it. Slowly the fluid accumulates in the areas with the lowest pressure and the greatest capacity. This pressure causes fluid to escape the vessels and enter the abdominal cavity. The blood bypasses the liver and enters the splenic, gastric, and esophageal veins, causing very high hydrostatic pressure. When progressive disease such as alcohol damage or hepatitis destroys enough liver tissue, the scarring that results compresses the hepatic sinusoids and vessels and restricts the blood flow.
Blood entering the portal vein from the intestines passes through the liver on its way back to the heart. An explanation of ascites formation in cirrhosis serves well to explain some principles common to transudative fluid formation. Transudative fluids result from changes in blood flow, and are typically seen in persons with cirrhosis, congestive heart failure, and a few other conditions that disrupt normal hemodyndamics. Cirrhosis is usually associated with a transudative fluid, a fluid of low cellularity and protein, while malignancy causes an exudative (inflammatory) fluid of high cellularity and protein. If cytologic exam is requested, 100 mL (3.4 oz) of fluid should be submitted to the laboratory.Ĭirrhosis of the liver and malignant abdominal masses are the two most common causes of ascites. These samples and the remaining fluid should be sent to the laboratory for analysis. One portion should be transferred to a tube containing EDTA for cell counts and the last syringe should be used to inoculate blood culture media. A minimum of 30 mL (1 oz) of fluid should be collected by sterile technique in two or three sterile syringes. Usually a syringe is used, but for large amounts of ascites, polyethylene tubing may be attached to vacuum bottles and the excess fluid aspirated. A long thin needle or trochar with a stylet is inserted about 2 in (5 cm) below the umbilicus, and the appropriate amount of fluid withdrawn. The area beneath the umbilicus is cleansed with betadine or other antibacterial solution, and local anesthetic administered.
DescriptionĬonsent should be obtained for the procedure after discussion of the possible complications. When performing this procedure, the physician should observe universal precautions for the prevention of transmission of bloodborne pathogens. Ultrasound may even be used to guide the needle for paracentesis. Ultrasound may be necessary to differentiate ascites from obesity and other reasons for abdominal distention. Generally at least 500 mL (17 oz) of fluid must accumulate before the effusion is detected by x ray, and 1500 mL (3.2 pt) before ascites is easily detected on physical exam. PrecautionsĪscites is difficult to diagnose by physical exam, although with experience health care practitioners can note "shifting dullness" by percussion. The most common cause in the United States is alcoholic cirrhosis. Ascites forms for a variety of reasons, including infection, diseases of various organs, and conditions that result in abnormal blood flow. A sample of the fluid withdrawn from the abdominal cavity is nearly always sent for laboratory analysis to determine the presence or absence of infection, and/or to learn more about the cause of ascites if necessary. Paracentesis is commonly performed to identify the cause of newly diagnosed ascites (excess fluid in the abdominal cavity) to diagnose changes in the condition of a patient already known to have ascites and to relieve pressure from severe distention due to increased fluid in the abdomen. The fluid may be called ascites fluid, abdominal fluid, or peritoneal fluid. Paracentesis is a procedure in which excess fluid in the abdomen is sampled by aspiration through a needle.