What are the units of pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR)?

Units of Pulmonary and Systemic Vascular Resistance

Direct Answer

Both pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) are expressed in Wood units (WU) or dynes·s·cm⁻⁵, with Wood units strongly preferred in current clinical practice. 1, 2

Wood Units vs. Dynes·s·cm⁻⁵

Preferred Unit: Wood Units

• The European Society of Cardiology and European Respiratory Society explicitly state that Wood units are preferred over dynes·s·cm⁻⁵ for clinical practice. 1, 2
• Wood units simplify calculations and reduce mathematical errors compared to dynes·s·cm⁻⁵. 2
• Current international guidelines uniformly recommend Wood units for all clinical decision-making and documentation. 2

Conversion Between Units

• To convert Wood units to dynes·s·cm⁻⁵, multiply by 80. 2
• To convert dynes·s·cm⁻⁵ to Wood units, divide by 80. 2
• For example: 3 Wood units = 240 dynes·s·cm⁻⁵

Specific Units for Each Resistance Type

Pulmonary Vascular Resistance (PVR)

• PVR is expressed as Wood units (WU) or dynes·s·cm⁻⁵, with Wood units preferred. 1, 2
• Normal PVR values are <2-3 Wood units. 2
• The formula is: PVR = (mean pulmonary artery pressure - pulmonary capillary wedge pressure) / cardiac output. 2

Systemic Vascular Resistance (SVR)

• SVR is expressed in dynes·s·cm⁻⁵ (note: not dynes·s·cm⁻² as sometimes incorrectly stated). 2, 3
• Normal SVR values are approximately 800-1200 dynes·s·cm⁻⁵. 2
• The formula is: SVR = (mean arterial pressure - right atrial pressure) / cardiac output. 2

Critical Clinical Thresholds Using These Units

PVR Thresholds

• Pre-capillary pulmonary hypertension is defined as PVR >3 WU in 2015 guidelines and >2 WU in 2024 guidelines. 2, 4
• Congenital heart disease repair is contraindicated when PVR >2.5 WU or >4 WU·m² indexed. 2
• Liver transplant candidacy in portopulmonary hypertension requires PVR <3 WU (with specific mean pulmonary artery pressure criteria). 1, 2

SVR-PVR Relationship

• SVR must be maintained greater than PVR to prevent right ventricular ischemia, as right ventricular coronary perfusion occurs during both systole and diastole. 2, 3
• In congenital heart disease, surgical repair is recommended when PVR is less than one-third of SVR. 2, 3

Common Pitfalls

Unit Confusion in Literature

• Significant variability exists in published literature regarding PVRI (PVR index) units, with 33 unique variants identified in one review. 5
• Among reviewed sources, 54.6% reported PVRI with incorrect units (not ending in m²), leading to considerable confusion with potentially life-altering consequences. 5

Practical Recommendation

• Always specify whether you are reporting Wood units or dynes·s·cm⁻⁵ to avoid confusion. 2
• When indexing to body surface area, ensure units clearly indicate this (e.g., WU·m² for indexed PVR). 5