In a patient with pulmonary artery pressure 44/16/28 mm Hg, arterial O₂ saturation 65%, mixed venous O₂ saturation 67%, cardiac output 4 L/min, and cardiac index 2.1 L/min/m², where in the pulmonary circulation is the pulmonary hypertension located?

Location of Pulmonary Hypertension in the Pulmonary Circuit

This patient has pre-capillary pulmonary hypertension, indicating that the pathology is located in the pulmonary arterioles and small vessels upstream (before) the pulmonary capillaries and left atrium.

Hemodynamic Classification

The key to localizing pulmonary hypertension is calculating pulmonary vascular resistance (PVR) and determining the relationship between pulmonary artery pressure and left-sided filling pressures 1, 2.

PVR Calculation

Using the provided hemodynamics:

• PVR = (mPAP - PCWP) / Cardiac Output 2
• PVR = (28 - estimated PCWP) / 4 L/min

The mean PA pressure of 28 mmHg confirms pulmonary hypertension (threshold >25 mmHg) 3, 1.

Pre-capillary vs Post-capillary Differentiation

Pre-capillary pulmonary hypertension is characterized by:

• PVR >3 Wood units (2015 guidelines) or >2 Wood units (2024 guidelines) 1
• PCWP (or PAWP) ≤15 mmHg 3, 1
• Elevated transpulmonary gradient (mPAP - PCWP) 2

Post-capillary pulmonary hypertension would show:

• PCWP >15 mmHg, indicating left heart disease 3, 1
• Normal or mildly elevated PVR if purely passive 1

Critical Diagnostic Clues from the Hemodynamics

Oxygen Saturation Pattern

The near-identical arterial (65%) and mixed venous (67%) saturations are highly abnormal and provide crucial localization information:

• Normal arteriovenous oxygen difference is 4-5 mL/dL (corresponding to SaO2 ~75% when arterial is 98%) 3
• This patient shows essentially no oxygen extraction (67% vs 65% = minimal A-V difference)
• This pattern suggests severe right-to-left shunting at the atrial or ventricular level, or through a patent foramen ovale with elevated right-sided pressures 3

Low Cardiac Output and Index

• Cardiac index of 2.1 L/min/m² is at the lower limit of normal (normal >2.2 L/min/m²) 3
• In decompensated heart failure, cardiac index <2.2 L/min/m² typically indicates significant hemodynamic compromise 3
• The combination of low cardiac output with elevated PA pressures and minimal oxygen extraction confirms severe pre-capillary pulmonary vascular disease 3

Anatomic Localization

The pathology is located in the small pulmonary arterioles and resistance vessels (pre-capillary circulation) based on:

1. Elevated PVR - The transpulmonary gradient and calculated PVR indicate true pulmonary vascular disease, not passive congestion 1, 2

2. Preserved or low PCWP - Without an elevated wedge pressure (which would need to be measured invasively for definitive classification), the elevated mPAP with low cardiac output suggests pre-capillary obstruction 3, 1

3. Right-to-left shunting pattern - The oxygen saturation data suggests elevated right atrial/ventricular pressures exceeding left-sided pressures, forcing deoxygenated blood to shunt right-to-left 3

Differential Diagnosis by Location

Pre-capillary causes (where this patient's pathology is located):

• Pulmonary arterial hypertension (idiopathic, heritable, drug-induced) 3
• Chronic thromboembolic pulmonary hypertension 3
• Pulmonary veno-occlusive disease 3
• Congenital heart disease with Eisenmenger physiology (suggested by the shunting pattern) 2

Post-capillary causes (excluded by hemodynamics):

• Left ventricular systolic or diastolic dysfunction 3
• Valvular heart disease (mitral stenosis/regurgitation, aortic stenosis) 3
• These would show PCWP >15 mmHg 3, 1

Essential Next Step

Right heart catheterization with direct measurement of PCWP is mandatory to definitively classify this pulmonary hypertension and calculate exact PVR 3, 1, 2. Echocardiography alone has limited positive predictive value (25-64%) and cannot replace invasive assessment when treatment decisions are being considered 3, 2.

Critical Pitfall to Avoid

The severely abnormal oxygen saturations (both arterial and venous) indicate this patient is critically ill with severe hypoxemia and likely has a structural cardiac defect with shunting 3. This is not typical isolated pulmonary arterial hypertension - the near-identical arterial and venous saturations suggest Eisenmenger physiology or severe right-to-left shunting through an intracardiac defect 3, 2. Immediate evaluation for congenital heart disease and assessment for shunt closure candidacy (contraindicated if PVR >2.5 Wood units or >4 Wood units·m² indexed) is essential 1, 2.