effusions

Effusion is the accumulation or escape of fluid into a body cavity or space, or, in physics, the passage of gas through a small opening. In medicine, effusions denote abnormal fluid collections in sites such as the pleural cavity, pericardial sac, peritoneal cavity, or joints.

Medical effusions result from an imbalance between fluid production and drainage and can arise from infection, inflammation, malignancy, heart failure, liver or kidney disease, or hypoalbuminemia. Pleural effusion fills the space around the lungs; pericardial effusion surrounds the heart; ascites is a peritoneal effusion; synovial effusions occur within joints. Fluid is often classified as transudate or exudate. Transudates reflect hydrostatic or oncotic pressure abnormalities and have low protein; exudates reflect inflammation or malignancy with higher protein and LDH.

Diagnosis uses history, examination, and imaging plus fluid analysis. Pleural effusions are typically evaluated with ultrasound and thoracentesis, with tests including protein, LDH, cell counts, cultures, and cytology; Light's criteria help distinguish pleural exudates from transudates. Pericardial effusion evaluation relies on echocardiography, assessing for hemodynamic compromise. Ascites evaluation uses the serum-ascites albumin gradient and imaging to guide management. Treatment targets the underlying cause and may involve drainage (thoracentesis, pericardiocentesis, paracentesis) and supportive care; sizable or symptomatic effusions may require repeat drainage or procedures.

In physics, effusion describes gas molecules passing through a small pore. Rates follow kinetic theory and Graham’s law, with effusion inversely related to the square root of molecular weight; other factors include pressure, temperature, pore size, and path geometry. Applications include leak testing, vacuum systems, and gas separation.