The lungs are surrounded by the pleura, a double-layered membrane. The visceral pleura covers the lung surface, and the parietal pleura lines the chest wall. Between these layers is the pleural space, which contains a small amount of lubricating fluid. This fluid allows the pleura to glide effortlessly over one another during breathing.
A pleural rub, also known as a pleural friction rub, is an abnormal sound heard using a stethoscope. It signals a disruption in this normally slick system and indicates inflammation in these protective layers.
The Specific Acoustic Signature
The sound of a pleural rub is distinctive, often likened to the noise produced when two pieces of dry leather are rubbed together. Other common descriptions include a coarse, grating, or creaking noise, sometimes compared to crunching fresh snow. The sound is nonmusical and discontinuous, unlike a sustained wheeze. A defining characteristic is that the rub is typically heard during both inspiration and expiration, and its intensity may increase when a patient takes a deeper breath.
The sound is often localized to the area of the chest where inflammation is most pronounced. During auscultation, pressing the stethoscope more firmly onto the chest wall can intensify the rubbing sound. This maneuver helps distinguish a pleural rub from other lung noises, which are typically unaffected by pressure. The rub is also generally unaffected by coughing, differentiating it from sounds like crackles or rhonchi.
The Physical Cause of the Sound
A pleural rub signifies that the normally smooth, lubricated surfaces of the pleura have become rough. This underlying condition is known as pleurisy (or pleuritis), which is the inflammation of the pleural membranes. When inflamed, the delicate lining loses its slick texture and may become sticky or coated with fibrin deposits. The lubricating serous fluid can also be reduced or changed in composition.
This roughening causes increased friction between the two layers of the pleura during the respiratory cycle. As the lung expands and contracts, the roughened visceral pleura slides against the parietal pleura, generating the characteristic grating sound.
The sound only occurs when the two inflamed surfaces are in direct contact and moving relative to one another. If the pleural space fills with a significant amount of fluid (a pleural effusion), the layers separate and the friction ceases. In such cases, the pleural rub may disappear, even if inflammation persists. Therefore, the rub is a transient sign, often heard only in the early stages of pleurisy before substantial fluid accumulation occurs.
Common Medical Conditions That Produce a Pleural Rub
The inflammation leading to a pleural rub is caused by various underlying conditions. Viral infections are frequent culprits, as they directly irritate the pleural lining. Bacterial infections, such as pneumonia, are another common cause of localized pleural inflammation. A significant medical concern associated with a pleural rub is a pulmonary embolism (a blood clot lodged in the lung arteries).
The tissue damage caused by the clot triggers an inflammatory response in the adjacent pleura. Certain systemic diseases that cause widespread inflammation can also affect the pleura. Autoimmune conditions, including systemic lupus erythematosus and rheumatoid arthritis, can inflame the serous membranes. Malignant diseases affecting the lungs or chest wall can also irritate the pleura, causing a rub. The presence of a pleural rub is a significant finding that helps guide clinicians toward diagnosing the specific cause of the patient’s respiratory symptoms.