What is the difference between mean pulmonary artery pressure and pulmonary capillary wedge pressure in diagnosing pulmonary hypertension?

Hemodynamic Distinction Between Pre-Capillary and Post-Capillary Pulmonary Hypertension

The fundamental difference is the pulmonary artery wedge pressure (PAWP): pre-capillary pulmonary hypertension has PAWP ≤15 mmHg reflecting disease in the pulmonary arterioles before the capillary bed, while post-capillary pulmonary hypertension has PAWP >15 mmHg reflecting elevated left atrial pressure transmitted backward through the pulmonary veins. 1

Core Hemodynamic Definitions

Both forms require mean pulmonary artery pressure (mPAP) ≥25 mmHg at rest measured by right heart catheterization. 1 The PAWP measurement is the critical distinguishing parameter that determines whether elevated pulmonary pressures originate before (pre-capillary) or after (post-capillary) the pulmonary capillary bed. 2

Pre-Capillary Pulmonary Hypertension

Pre-capillary PH is defined by three criteria:

• mPAP ≥25 mmHg 1
• PAWP ≤15 mmHg 1
• Pulmonary vascular resistance (PVR) >3 Wood units 1

This hemodynamic pattern indicates intrinsic disease of the pulmonary arterioles and includes pulmonary arterial hypertension (Group 1), PH due to lung diseases (Group 3), chronic thromboembolic PH (Group 4), and some Group 5 conditions. 3

Post-Capillary Pulmonary Hypertension

Post-capillary PH is defined by:

• mPAP ≥25 mmHg 1
• PAWP >15 mmHg 1

This pattern develops from passive backward transmission of elevated left-sided filling pressures, primarily driven by left ventricular diastolic dysfunction, exercise-induced mitral regurgitation, and loss of left atrial compliance. 2 Post-capillary PH corresponds to Group 2 (PH due to left heart disease). 3

Critical Clinical Subdivision of Post-Capillary PH

Post-capillary PH must be further classified into two prognostically distinct subtypes:

Isolated post-capillary PH (Ipc-PH):

• Diastolic pressure gradient (DPG) <7 mmHg AND/OR PVR ≤3 Wood units 1, 2
• Represents purely passive congestion without pulmonary vascular remodeling 2

Combined post-capillary and pre-capillary PH (Cpc-PH):

• DPG ≥7 mmHg AND/OR PVR >3 Wood units 1, 2
• Indicates superimposed pulmonary vascular disease with vasoconstriction and remodeling beyond passive congestion 2
• Associated with worse prognosis, with DPG ≥7 mmHg linked to increased mortality in patients with elevated transpulmonary gradient >12 mmHg 1, 2

Clinical Features Suggesting Post-Capillary vs Pre-Capillary Disease

Features that raise suspicion for post-capillary PH (left heart disease) rather than pre-capillary disease include: 2

• Age >65 years
• Symptoms of left heart failure (orthopnea, paroxysmal nocturnal dyspnea)
• Features of metabolic syndrome (obesity, diabetes, hypertension)
• History of coronary artery disease or valvular heart disease
• Persistent atrial fibrillation

Echocardiographic findings suggesting post-capillary PH include: 2, 4

• Structural left heart abnormalities (left ventricular hypertrophy, left atrial enlargement)
• Doppler indices of increased filling pressures (E/e' ratio >10)
• Bowing of inter-atrial septum to the right

Echocardiographic findings predicting pre-capillary PH include: 4

• Right heart chambers larger than left chambers
• Left ventricular eccentricity index >1.2
• Dilated inferior vena cava without inspiratory collapse
• E/e' ratio ≤10
• Right ventricle forming the heart apex

Diagnostic Approach

Right heart catheterization remains the gold standard for definitive hemodynamic classification and cannot be replaced by non-invasive methods. 2 While Doppler echocardiography can screen for PH with sensitivity ranging from 0.79 to 1.0 and specificity from 0.6 to 0.98, it may underestimate systolic PAP by a mean of 11 mmHg, with 31% of patients having underestimation of 20 mmHg or more. 1

Common pitfall: Resting PAWP does not reliably predict exercise post-capillary PH. In patients >45 years with risk factors for left heart disease, 67% of those with PAWP <12 mmHg demonstrated post-capillary contributions to exercise pulmonary hypertension. 5 Conversely, 50% of patients with PAWP >15 mmHg had pre-capillary contributions to exercise PH. 5

Treatment Implications

For post-capillary PH (Group 2), the primary goal is optimizing management of underlying left heart disease prior to considering any specific PH measures. 2 Treatment includes repair of valvular disease when indicated, aggressive heart failure therapy, optimizing volume status, and controlling cardiovascular risk factors. 2

Critical warning: Pulmonary arterial hypertension-specific therapies have shown concerning results in post-capillary PH trials, reflecting disease heterogeneity and should not be used outside of clinical trials. 6 Left ventricular assist device implantation can lower pulmonary pressures through LV unloading without increasing risk of post-implantation right ventricular failure. 2