Units for Systemic and Pulmonary Vascular Resistance
Wood units (WU) are the commonly used and preferred units for expressing both systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR) in clinical practice. 1
Primary Units: Wood Units
- Wood units are recommended by the European Society of Cardiology and European Respiratory Society as the preferred unit for expressing vascular resistance, as they simplify calculations and reduce mathematical errors compared to alternative units 1
- The American College of Cardiology guidelines state that Wood units are preferred over dynes·s·cm⁻⁵ for clinical practice 1
- Wood units are used for all major clinical decision-making thresholds, including congenital heart disease repair decisions, pulmonary hypertension diagnosis, and liver transplant candidacy 1
Alternative Units: Dynes·s·cm⁻⁵
- PVR can also be expressed in dynes·s·cm⁻⁵, which is the CGS (centimeter-gram-second) unit system 2, 1
- To convert Wood units to dynes·s·cm⁻⁵, multiply by 80 1
- SVR is typically measured in dynes·s·cm⁻⁵ (also written as dynes/sec/cm⁵) 3, 4
Clinical Application of Wood Units
For Pulmonary Vascular Resistance:
- Normal PVR values are typically <2-3 Wood units 1
- PVR >3 Wood units defines precapillary pulmonary hypertension (2018 guidelines), updated to >2 Wood units in 2024 guidelines 1
- PVR >2.5 Wood units or >4 Wood units·m² (indexed) is a contraindication for congenital heart disease shunt closure 1
For Systemic Vascular Resistance:
- Normal SVR values are approximately 800-1200 dynes·s·cm⁻⁵ 1
- SVR can be expressed in Wood units by dividing dynes·s·cm⁻⁵ by 80 1
- In congenital heart disease, surgical repair is recommended when PVR is less than one-third of SVR 1
Indexed Values
- Pulmonary Vascular Resistance Index (PVRI) is expressed in Wood units·m² (WU·m²), representing the indexed value adjusted for body surface area 1
- PVRI is particularly important in pediatric populations where body size varies significantly 1
- Significant confusion exists in the literature regarding PVRI units, with 45.4% of sources correctly reporting units ending in m² and 54.6% incorrectly reporting units not ending in m² 5
Critical Caveat
The lack of uniformity in reporting vascular resistance units, particularly for PVRI, has led to considerable confusion among researchers and clinicians with potentially life-altering consequences 5. Always verify which unit system is being used when interpreting vascular resistance values, and preferentially use Wood units for clinical decision-making as recommended by major cardiology societies 1.