What are pericardial friction rubs?

Pericardial Friction Rubs: Characteristics and Clinical Significance

Pericardial friction rubs are distinctive ausculatory findings that serve as a key diagnostic feature of pericarditis, representing the audible manifestation of inflamed pericardial surfaces rubbing against each other during cardiac movement.

Pericardial friction rubs are highly specific but transient sounds heard in approximately one-third of patients with acute pericarditis, making them an important diagnostic criterion when present. 1, 2

Characteristics of Pericardial Friction Rubs

• Pericardial friction rubs can be mono-, bi-, or triphasic sounds, with triphasic being the most common pattern (56% of patients with sinus rhythm) 3
• The rub is typically heard best along the left sternal border (84% of cases) and tends to be louder during inspiration than expiration 3
• The sound is best detected with the patient sitting upright and leaning forward while briefly holding their breath, as this position brings the heart closer to the chest wall and minimizes lung sounds 4
• Friction rubs can be transient and may disappear and reappear during the course of pericarditis, making repeated examinations necessary 1, 2
• In some cases (approximately 23%), the friction rub may be intense enough to be palpable 3

Pathophysiology

• Contrary to common belief, there is no significant correlation between the presence of a pericardial friction rub and the amount of pericardial effusion 5
• It is postulated that the friction rub is not simply caused by "roughened" pericardial layers rubbing together, but may be generated by fibrin strands (resulting from inflammation) that connect the two pericardial layers and create the characteristic sound during cardiac movement 5
• The triphasic nature corresponds to ventricular systole, early ventricular diastole, and atrial systole phases of the cardiac cycle 3

Clinical Significance

• Presence of a pericardial friction rub is one of the four key diagnostic criteria for acute pericarditis, along with characteristic chest pain, ECG changes, and pericardial effusion 1, 6
• Diagnosis of acute pericarditis requires at least two of these four criteria, with the friction rub being highly specific when present 6
• The friction rub can occur with or without pericardial effusion, challenging the misconception that a friction rub signifies absence or only small volumes of effusion 5
• In post-myocardial infarction pericarditis, which occurs in approximately 5-6% of patients who receive thrombolytic agents, a pericardial friction rub may or may not be present 7

Detection Techniques

• Auscultation should be performed with the diaphragm of the stethoscope pressed firmly against the chest wall 4
• Multiple auscultatory examinations may be necessary as the rub can be intermittent 1, 2
• The optimal position for detecting the rub is with the patient sitting upright, leaning forward, and briefly holding their breath 4
• The rub is typically high-pitched, scratching or grating in quality, distinguishing it from murmurs which tend to be more blowing in character 4

Clinical Context

• In the setting of acute pericarditis, the friction rub is often accompanied by characteristic chest pain that is:
  • Sharp, pleuritic in nature
  • Typically improves when sitting forward
  • Often radiates to the trapezius ridge 1, 6
• ECG changes may include widespread ST-segment elevation (concave upward) and PR-segment depression, occurring in 25-50% of cases 6
• Transthoracic echocardiography is essential in all patients with suspected pericarditis to detect effusion and evaluate for complications, regardless of whether a friction rub is present 1, 6

Common Pitfalls

• Relying solely on the presence or absence of a friction rub for diagnosis, as it may be absent in up to 70% of pericarditis cases 2
• Failing to perform repeated examinations, as the rub can be transient 1
• Confusing the friction rub with other cardiac sounds such as murmurs or pleural rubs 4
• Assuming that presence of a friction rub excludes significant pericardial effusion 5

Understanding the characteristics and clinical significance of pericardial friction rubs is essential for accurate diagnosis and management of pericarditis and related conditions.