Why does Pulmonary Capillary Wedge Pressure (PCWP) decrease in constrictive pericarditis?

Why PCWP Decreases in Constrictive Pericarditis

In constrictive pericarditis, pulmonary capillary wedge pressure (PCWP) decreases during inspiration due to enhanced ventricular interdependence within the confined space of the thickened, stiff pericardium, causing a respiratory discordance in filling pressures between the right and left ventricles.

Pathophysiological Mechanism

• In constrictive pericarditis, the pericardium becomes thickened, fibrotic, and sometimes calcified, creating a fixed and limited space that restricts cardiac filling 1
• During inspiration, there is increased venous return to the right side of the heart, causing the interventricular septum to shift toward the left ventricle 1
• This septal shift occurs because both ventricles must compete for a fixed total volume within the non-compliant pericardial space 1
• As the right ventricle expands during inspiration, it compresses the left ventricle, reducing left ventricular filling and subsequently decreasing the PCWP 1
• This phenomenon is known as enhanced ventricular interdependence and is a hallmark of constrictive pericarditis 1, 2

Hemodynamic Features

• Classic hemodynamic findings in constrictive pericarditis include:
  • End-diastolic pressure equalization (typically within 5 mmHg) between cardiac chambers 1
  • Prominent Y descent in atrial pressure waveforms (in contrast to cardiac tamponade where Y descent is diminished or absent) 2
  • "Square root" or "dip-and-plateau" pattern in ventricular pressure waveforms 1
  • Respiratory variation >25% in ventricular filling across atrioventricular valves 1
  • Normal pulmonary artery systolic pressures (elevated pressures suggest restrictive cardiomyopathy instead) 1

Diagnostic Implications

• The respiratory variation in PCWP is a key diagnostic feature that helps differentiate constrictive pericarditis from restrictive cardiomyopathy 1
• In constrictive pericarditis, there is discordance between right and left heart filling during respiration, with left heart filling (and thus PCWP) decreasing during inspiration 1, 2
• In restrictive cardiomyopathy, this respiratory variation is typically absent or less pronounced 1
• A novel hemodynamic parameter, the systolic area index (ratio of right ventricular to left ventricular systolic pressure-time area during inspiration versus expiration), has shown 97% sensitivity and 100% predictive accuracy for identifying constrictive pericarditis 1

Clinical Assessment

• Cardiac catheterization remains valuable when non-invasive diagnostic methods are inconclusive 1
• During catheterization, simultaneous measurement of right and left heart pressures during respiration can demonstrate the characteristic respiratory discordance in filling pressures 1
• Echocardiography with Doppler assessment can also demonstrate this respiratory variation non-invasively, showing >25% variation in mitral inflow velocities with respiration 1
• Cross-sectional imaging (CT or MRI) helps assess pericardial thickness and calcification but provides anatomical rather than physiological information 1

Common Pitfalls in Diagnosis

• Pericardial thickening is not always present in constrictive pericarditis (absent in up to 20% of surgically proven cases) 1
• Pericardiectomy should not be denied based solely on normal pericardial thickness if clinical and hemodynamic features suggest constriction 1, 3
• The diagnosis can be particularly challenging in patients with atrial fibrillation 1
• In early or occult forms, diagnostic signs may not be present at rest, and provocative testing (such as rapid fluid infusion of 1-2L normal saline) may be necessary 1, 4
• Constrictive hemodynamics may be masked or complicated by coexisting valvular or coronary artery disease 1, 3