What is the pulmonary vascular resistance (PVR) cutoff value for diagnosing pulmonary hypertension?

PVR Cutoff for Pulmonary Hypertension

The PVR cutoff for diagnosing pulmonary arterial hypertension (PAH) is >3 Wood units according to the 2009 ACCF/AHA and 2015 ESC/ERS guidelines, though the most recent 2024 ESC/ERS guidelines have lowered this threshold to >2 Wood units for defining pre-capillary pulmonary hypertension. 1, 2

Current Diagnostic Thresholds

Historical Standard (2009-2015)

• PVR >3 Wood units was the established cutoff for PAH diagnosis, requiring both mean pulmonary artery pressure (mPAP) >25 mmHg AND PVR >3 Wood units 1
• This threshold must be accompanied by pulmonary artery wedge pressure (PAWP) ≤15 mmHg to confirm pre-capillary disease 1

Updated 2024 Definition

• PVR >2 Wood units now defines pre-capillary pulmonary hypertension when combined with mPAP >20 mmHg and PAWP ≤15 mmHg 2
• This represents a lowering of both the pressure threshold (from 25 to 20 mmHg) and the resistance threshold (from 3 to 2 Wood units) 2

Clinical Context and Rationale

Why PVR Matters More Than Pressure Alone

• PVR is a more robust diagnostic criterion than mPAP alone because it reflects the influence of transpulmonary gradient and cardiac output, and is only elevated when vascular obstruction occurs within the pre-capillary pulmonary circulation 1
• PVR distinguishes passive PH (elevated mPAP with normal PVR) from PH caused by true pulmonary vascular disease (elevated mPAP with elevated PVR) 1
• Elevated pulmonary artery pressure can occur with high cardiac output states (exercise, anemia, pregnancy, sepsis) where the pulmonary vascular bed is anatomically normal—these conditions have normal PVR 1

The "Borderline" PVR Population

• Patients with PVR between 2-3 Wood units represent a clinically significant group that falls outside the traditional definition but demonstrates adverse outcomes 3
• In an Australian registry study of 82 patients with mPAP ≥25 mmHg, PAWP ≤15 mmHg, but PVR <3 Wood units (mean 2.2 Wood units), 77% were in NYHA functional class 3-4 at baseline 3
• These patients showed response to PAH therapy with improvement in 6-minute walk distance (mean increase 46 meters) and functional class (35% improved), with 1-year and 5-year survival rates of 98% and 84% respectively 3
• This evidence supports the 2024 guideline decision to lower the PVR threshold to >2 Wood units 2, 3

Additional Hemodynamic Criteria

Disproportionate PVR Elevation

• A subset of patients with left heart disease or high-output states may have disproportionately elevated PVR >3 Wood units with transpulmonary gradient >20 mmHg, indicating a component of pulmonary vascular disease superimposed on their underlying condition 1

Combined Post- and Pre-capillary PH

• When PAWP >15 mmHg AND PVR >2 Wood units, this defines combined post- and pre-capillary PH, indicating both left heart contribution and pulmonary vascular disease 2

Critical Measurement Requirements

Essential for Accurate PVR Calculation

• Right heart catheterization is mandatory for definitive diagnosis—echocardiographic estimates have limited positive predictive value (25-64%) and should not replace invasive assessment 1, 4
• PVR is calculated as: (mPAP - PAWP) / cardiac output, expressed in Wood units 4
• All measurements must be obtained at end-expiration during spontaneous breathing, or end-inspiration if mechanically ventilated 4, 5

Common Pitfalls to Avoid

• Inaccurate wedge pressure measurement is a frequent source of error—verify proper catheter wedging and consider direct left atrial pressure if values seem discrepant 4
• Thermodilution cardiac output can be erroneous in patients with significant tricuspid regurgitation and right ventricular dilatation, affecting all derived resistance calculations 4
• General anesthesia can lower systemic arterial blood pressure and affect resistance calculations—measurements should be obtained under standardized conditions 4

Clinical Decision Thresholds Beyond Diagnosis

Surgical Contraindications

• PVR >2.5 Wood units or >4 Wood units·m² indexed is a contraindication for congenital heart disease shunt closure 4
• Many centers use preoperative PVR <10-14 Wood units and pulmonary/systemic resistance ratio ≤2/3 as thresholds for better surgical outcomes 4

Transplant Eligibility

• For portopulmonary hypertension, liver transplant candidacy requires PVR <3 Wood units (or <5 Wood units if mPAP <35 mmHg) 4

Minority Opinion and Exceptions

• A minority of the 2009 ACCF/AHA committee argued against including PVR >3 Wood units as a required criterion, citing high-flow conditions (uncorrected congenital heart disease, sickle cell disease, portopulmonary hypertension) where patients may have elevated mPAP but normal PVR due to high cardiac output, yet still have histologic pulmonary vascular disease 1
• This debate has been partially resolved by the 2024 guidelines lowering the threshold to >2 Wood units, which captures more patients with early pulmonary vascular disease 2