How do you differentiate between a loud P2 (pulmonic component of the second heart sound) and a pericardial knock?

Differentiating Loud P2 from Pericardial Knock

The key distinction is timing: a loud P2 occurs at the normal timing of the second heart sound (end of systole), while a pericardial knock occurs significantly earlier in diastole (90-120 ms after aortic valve closure), is higher-pitched, and corresponds to abrupt cessation of ventricular filling. 1

Timing Characteristics

Loud P2

• Occurs at the normal timing of S2 (end of systole, coinciding with semilunar valve closure) 2, 3
• Represents the accentuated pulmonic component of the second heart sound due to forceful pulmonary valve closure from elevated pulmonary artery pressure 2
• The most consistently associated physical finding with pulmonary arterial hypertension, though sensitivity is only 55-70% 3

Pericardial Knock

• Occurs 90-120 ms after aortic valve closure (early diastole), significantly earlier than a typical S3 1
• Corresponds to the trough of the Y descent on jugular venous pressure tracing 1
• Represents sudden deceleration of ventricular filling when the ventricle reaches the constraint imposed by the rigid pericardium 1
• Occurs when approximately 85% of ventricular filling has been completed, followed by an abrupt plateau in the diastolic volume curve 1

Acoustic Characteristics

Loud P2

• Lower frequency compared to pericardial knock 4
• Audible primarily in the pulmonary area (left second intercostal space) 2
• May be audible at the apex in severe pulmonary hypertension 2
• Can be accompanied by other signs of pulmonary hypertension including early systolic ejection click and midsystolic ejection murmur 2

Pericardial Knock

• High-pitched, sharp sound with high-frequency components on time-frequency analysis 4
• Often difficult to appreciate on routine auscultation; computerized acoustic cardiography can aid detection 4
• Resolves after pericardiectomy, confirming its pericardial origin 4

Associated Clinical Context

Loud P2 Context

• Signs of pulmonary hypertension: left parasternal RV heave, elevated jugular venous pressure with prominent V waves, tricuspid regurgitation murmur 2, 3
• May have signs of right ventricular failure: peripheral edema, hepatomegaly, ascites 2, 3
• History suggesting pulmonary hypertension etiology: chronic thromboembolic disease, connective tissue disease, congenital heart disease 2

Pericardial Knock Context

• Signs of constrictive pericarditis: elevated jugular venous pressure with prominent Y descent, Kussmaul's sign, pericardial friction rub (if concomitant pericarditis) 2, 4
• History of pericarditis, cardiac surgery, radiation therapy, or tuberculosis 2
• Absence of P2 does not exclude constrictive pericarditis 4

Diagnostic Approach

Physical examination alone has limited accuracy (sensitivity 50-68%, specificity 22-56% for pulmonary hypertension signs) 5, so confirmation requires:

• Echocardiography: Essential for both conditions 2

  • For pulmonary hypertension: elevated pulmonary artery systolic pressure, RV enlargement/hypertrophy 2
  • For constrictive pericarditis: pericardial thickening, septal bounce, respiratory variation in ventricular filling 2

• Cardiac catheterization: Gold standard for pulmonary hypertension diagnosis (mean PA pressure >25 mmHg, PVR >3 Wood units) 2

• Computerized acoustic cardiography: Can objectively demonstrate the high-frequency pericardial knock and differentiate it from other heart sounds 4

Common Pitfalls

• Do not rely on auscultation alone: The pericardial knock is frequently missed on routine physical examination, even by experienced cardiologists 4
• Timing is critical: Both sounds can be loud, but the pericardial knock's early diastolic timing (within 120 ms of A2) distinguishes it from P2 1
• Consider the clinical context: Pulmonary hypertension patients typically have progressive dyspnea and signs of RV pressure overload, while constrictive pericarditis presents with signs of elevated filling pressures and venous congestion 2