Clinical meaning
The pericardium is a double-layered fibroserous sac that surrounds the heart, consisting of an outer fibrous pericardium and an inner serous pericardium. The serous pericardium has two layers: the parietal layer (lines the fibrous pericardium) and the visceral layer (epicardium, directly adheres to the heart surface). Between these two serous layers lies the pericardial space, which normally contains 15 to 50 mL of serous fluid that reduces friction during cardiac contraction. Acute pericarditis occurs when the pericardial layers become inflamed, leading to increased vascular permeability, fibrin deposition, and fluid accumulation. The inflammatory process begins when an insult -- viral infection, autoimmune response, uremia, or post-myocardial infarction injury (Dressler syndrome) -- activates the innate immune system. Macrophages and neutrophils infiltrate the pericardial tissue, releasing pro-inflammatory cytokines including interleukin-1, interleukin-6, and tumor necrosis factor-alpha. These mediators increase capillary permeability and recruit additional immune cells, producing the characteristic fibrinous or serofibrinous exudate. The inflamed pericardial surfaces rub against each other during cardiac motion, producing the hallmark pericardial friction rub audible on auscultation. If fluid accumulates rapidly in the pericardial space (pericardial effusion), it...