friction rub

A friction rub results from loss of the normal lubricating fluid between two serosal layers due to inflammation, causing roughened surfaces to rub against each other during movement (cardiac contraction/relaxation or respiration), producing an audible sound.

Pericardial friction rub: Classically associated with acute pericarditis (viral, post-MI/Dressler syndrome, uremic, autoimmune, or post-cardiac surgery). It is classically triphasic, corresponding to atrial systole, ventricular systole, and ventricular diastole (though it may be biphasic or monophasic), producing a high-pitched, scratchy sound best heard at the left lower sternal border with the diaphragm of the stethoscope while the patient leans forward and holds their breath (to distinguish from a pleural rub). It is a highly specific but variable/intermittent finding—may come and go with the phase of pericarditis and is often absent if a pericardial effusion develops (fluid separates the layers, muffling the rub). Diagnostic criteria for acute pericarditis often include ≥2 of: pleuritic chest pain, friction rub, diffuse ST elevations/PR depressions on ECG, and new/worsening pericardial effusion.

Pleural friction rub: Occurs due to pleuritis (inflammation of the pleura) from causes like pulmonary embolism, pneumonia, tuberculosis, autoimmune disease (e.g., lupus), or malignancy. It sounds like leather rubbing together, heard on both inspiration and expiration (unlike crackles, which are typically inspiratory), and does not change with coughing (helps distinguish from secretions causing crackles). It is best heard at the lung bases laterally, where lung movement is greatest.

Clinical significance: On board exams, friction rubs are commonly tested vignettes—pericardial rub with chest pain relieved by leaning forward, diffuse ST elevations, and history of viral illness or recent MI (Dressler syndrome); pleural rub with pleuritic chest pain, dyspnea, and risk factors for PE or pneumonia. Differentiating between pericardial and pleural rub can sometimes be done by asking the patient to hold their breath—pericardial rub persists (due to ongoing heartbeat) while pleural rub disappears.