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 can compress the heart chambers and restrict diastolic filling, a life-threatening condition called cardiac tamponade. The pericardial sac is relatively non-compliant to rapid distension; as little as 150 to 200 mL of rapidly accumulating fluid can cause hemodynamic compromise. Cardiac tamponade manifests as Beck triad: hypotension, muffled (distant) heart sounds, and jugular venous distension (JVD). The pathophysiology of tamponade involves progressive compression of the right atrium and ventricle (lower-pressure chambers), reducing venous return and cardiac output. Pulsus paradoxus, defined as a systolic blood pressure drop greater than 10 mmHg during inspiration, occurs because the septum shifts leftward during inspiration in the setting of a restricted pericardial space, further reducing left ventricular output. The practical nurse must understand that acute pericarditis can progress from a manageable inflammatory condition to a life-threatening tamponade emergency, making vigilant monitoring of hemodynamic status and cardiac sounds essential.