Correct units: PVRI is expressed in WU·m² (Wood Units × meters squared). A systematic review (Kwan et al., 2019) found over 50% of literature uses incorrect units (like WU/m²), which can lead to clinical misinterpretation and dosing errors.
Hemodynamic Inputs
Normal Range (WU·m²)
Normal< 3.0
Elevated3.0 – 6.0
Severe> 6.0
Clinical Precision
Standardization to Body Surface Area (BSA) is critical in assessing pulmonary vascular remodeling. PVRI values are less sensitive to body habitus changes than absolute PVR, making it the preferred metric for transplant listing and complex PAH management.
Evidence-Based Hematology Protocol
Reference: Benza RL, et al. Predicting Survival in PAH: REVEAL Risk Score 2.0. Chest. 2019.
Normal thresholds per 2026 ESC/ERS Guidelines for Pulmonary Hypertension.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
Primary Clinical Uses
Assessment of pulmonary vascular disease severity in pulmonary arterial hypertension (PAH)
Determining candidacy for ASD/VSD closure (PVRI >3 WU·m² is a relative contraindication)
Cardiac transplantation evaluation (PVRI >6 WU·m² is a relative contraindication per ISHLT)
Perioperative hemodynamic monitoring in cardiac surgery
Guiding vasodilator therapy in critically ill patients
Why Indexing Matters
PVRI normalizes absolute PVR to body surface area (BSA). This accounts for varying body size, especially important in pediatric patients and adults with extremes of BSA. A systematic review by Kwan et al. (2019) found that 54.6% of published literature uses incorrect units (e.g., WU/m² instead of WU·m²), leading to potential clinical errors.
Section 2
Formula & Logic
Formula
PVRI = (mPAP − PAWP) / CI
Where:
• mPAP = mean pulmonary artery pressure (mmHg)
• PAWP = pulmonary artery wedge pressure (mmHg)
• CI = cardiac index (L/min/m²)
Units: PVRI is expressed in Wood Units · m² (WU·m²)
Alternative unit conversion: 1 WU·m² = 80 dynes·sec·cm⁻⁵·m²
Derivation (Ohm’s Law Analog)
PVR = (mPAP − PAWP) / Qp, where Qp = cardiac output (L/min). Indexing divides by BSA: CI = Qp / BSA. Therefore PVRI = (ΔP) / CI. Because CI is in denominator, BSA moves to numerator, giving final units of pressure × time × BSA → WU·m².
Critical Unit Alert
⚠️ INCORRECT: WU/m², WU·m⁻², or dynes·sec·cm⁻⁵·m⁻². These units mathematically invert the BSA correction and will misclassify patients. CORRECT: WU·m² (Wood Units × meters squared).
Reference Values & Thresholds
Category
PVRI (WU·m²)
Clinical Implication
Normal
< 3
Normal pulmonary vascular resistance. Low risk for adverse outcomes.
In atrial fibrillation, average 5-10 beats for pressures and CI
Key Evidence from Kwan et al. 2019
Systematic review of 218 articles using “PVRI” in PubMed (1980‑2018)
33 unique unit variations identified
Only 45.4% of articles used correct units (WU·m² or dynes·sec·cm⁻⁵·m²)
Pediatric literature performed better: 62.2% correct vs 41.0% in non‑pediatric
Consequences: misdiagnosis of operability for ASD/VSD closure, inappropriate transplant listing
Clinical Example of Unit Error
Patient: BSA 2.0 m², PVR = 2 WU → Correct PVRI = 4 WU·m². If incorrectly reported as WU/m², the value becomes 1 WU/m² (4× lower). This underdiagnosis could lead to inappropriate ASD/VSD closure with fatal pulmonary hypertensive crisis.
Section 4
Next Steps
Clinical Actions by PVRI Range
01
PVRI < 3 WU·m²: Normal. No specific intervention needed for pulmonary vasculature.
02
PVRI 3‑6 WU·m²: Evaluate for WHO Group 1 PAH or Group 2 left heart disease. Consider vasodilator challenge (inhaled nitric oxide or IV adenosine). If reversible (>20% decrease), may consider calcium channel blockers.
03
PVRI > 6 WU·m²: High risk. Escalate PAH therapy (prostacyclin analogs, endothelin antagonists). For transplant candidates, reassess after pulmonary vasodilator therapy.
04
ASD/VSD closure: AHA/ATS 2015 guidelines recommend against closure if baseline PVRI > 3 WU·m² and PVR index > 6 WU·m² after vasodilator testing.
Transplant Listing Criteria (ISHLT 2006)
Relative contraindications: PVR > 5 WU or PVRI > 6 WU·m² that is unresponsive to vasodilator testing (defined as failure to decrease to < 2.5 WU or < 3.0 WU·m²). Absolute contraindication: Transpulmonary gradient > 15 mmHg and PVRI > 6 WU·m² with systolic pulmonary artery pressure > 60 mmHg.
Section 5
Evidence Appraisal
Primary Reference – Unit Standardization (Clickable)
Pulmonary vascular resistance index: Getting the units right and why it matters
Kwan WC et al. • Clinical Cardiology. 2019;Comprehensive review of PVRI unit confusion across 218 publications. Documents that 54.6% of literature uses incorrect units, with potentially life‑altering consequences.
PVRI was introduced by pediatric cardiologists in the 1970s to account for body size when assessing operability of congenital heart defects. The indexing corrects for the proportional relationship between body surface area and pulmonary blood flow. The formula applies Ohm’s law (R = ΔP / Q) to the pulmonary circulation, with flow indexed to BSA. Despite decades of use, unit confusion persists, prompting the Kwan et al. 2019 call for standardization.
