Does a pleural rub friction sound occur during both inspiration and expiration?

Pleural Friction Rub: Respiratory Phase Characteristics

A pleural friction rub occurs during both inspiration and expiration, though it may have different characteristics during each phase of respiration. 1

Characteristics of Pleural Friction Rubs

• Pleural friction rubs are auscultatory findings that occur when inflamed pleural surfaces rub against each other during respiration 1
• They are characterized by a creaking or grating sound that can be heard during both respiratory phases 1
• In coarse crackles (which can sometimes be confused with pleural rubs), the polarity of the waveform is typically positive during inspiration and negative during expiration 2
• Pleural rubs demonstrate a heterogeneous distribution of waveform polarity, which helps differentiate them from pulmonary crackles 2

Respiratory Phase Variations

• Right-sided murmurs and sounds generally increase with inspiration, while left-sided sounds are typically louder during expiration 1
• Pleural friction rubs may have different acoustic characteristics between inspiration and expiration, but are present during both phases 3
• The sound quality may change between respiratory phases due to differences in the mechanical forces applied to the pleural surfaces 4

Clinical Assessment

• When auscultating for pleural friction rubs, it's important to listen through both inspiration and expiration to properly identify and characterize the sound 1
• Pleural rubs can sometimes be confused with pericardial friction rubs, which also occur in both systole and diastole (analogous to inspiration and expiration in respiratory sounds) 5
• The location of auscultation can affect the characteristics of the pleural rub, with even small changes in position potentially altering the sound quality 2

Pathophysiological Basis

• Pleural friction rubs occur when inflammation reduces the lubricating effect of pleural fluid, increasing friction between visceral and parietal pleural surfaces 4
• During normal respiration, the coefficient of kinetic friction between pleural surfaces is very low (approximately 0.023), allowing smooth movement 4
• Respiratory movements cause the visceral and parietal pleura to slide against each other in both inspiration and expiration, generating the friction sound when surfaces are inflamed 6, 4

Clinical Significance

• The presence of pleural friction rubs during both respiratory phases helps differentiate them from other respiratory sounds 2
• Pleural rubs may indicate conditions such as pleuritis, pulmonary embolism with infarction, pneumonia, or malignancy involving the pleura 5
• When evaluating pleural sounds, it's important to assess their timing in relation to the respiratory cycle, as this provides diagnostic information 1, 3

Common Pitfalls and Caveats

• Pleural rubs can sometimes be mistaken for pericardial friction rubs, especially when they occur near the cardiac border 5
• The intensity of pleural friction rubs may vary with position and depth of respiration, so a thorough examination in different positions is recommended 2
• Pleural adhesions may reduce or eliminate the friction rub by preventing pleural surfaces from moving against each other 6
• Mechanical ventilation does not significantly alter the pleural liquid volume or friction coefficient compared to spontaneous breathing 4