Preferred Units for Vascular Resistance Measurement
Wood units (WU) are the preferred unit for expressing both pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) in clinical practice. 1, 2
Rationale for Wood Units
The European Society of Cardiology and European Respiratory Society explicitly state that Wood units are preferred over dynes·s·cm⁻⁵ because they simplify calculations and reduce mathematical errors. 1, 2 This recommendation is echoed by the American College of Cardiology, which confirms that Wood units should be used for clinical decision-making rather than the more complex dynes·s·cm⁻⁵ unit. 2
Unit Conversion
- For PVR: Wood units can be converted to dynes·s·cm⁻⁵ by multiplying by 80. 2
- For SVR: While SVR is traditionally expressed in dynes·s·cm⁻⁵ in some contexts 3, 4, Wood units are increasingly preferred for consistency with PVR reporting and to align with international guidelines. 2
Clinical Application Thresholds Using Wood Units
For Pulmonary Vascular Resistance:
- Normal PVR: <2-3 WU 2
- Pre-capillary pulmonary hypertension: PVR >3 WU (2015 guidelines) or >2 WU (2024 guidelines) 2, 5
- Contraindication for congenital heart disease shunt closure: PVR >2.5 WU 2
- Poor prognosis in single ventricle patients: PVR >6 WU·m² (indexed) 2
For Systemic Vascular Resistance:
- Normal SVR: approximately 10-15 WU (equivalent to 800-1200 dynes·s·cm⁻⁵) 2, 4
- Critical relationship: SVR must remain greater than PVR to prevent right ventricular ischemia 2, 3
Indexed Values for Pediatric Populations
When body surface area adjustment is needed (particularly in children), the indexed unit is Wood units·m² (WU·m²). 2 This indexed value accounts for variations in body size that significantly affect hemodynamic measurements in pediatric populations.
Practical Advantages in Clinical Decision-Making
Wood units facilitate rapid bedside calculations and reduce mathematical errors compared to dynes·s·cm⁻⁵. 1, 2 The simpler numerical values (e.g., PVR = 3 WU versus PVR = 240 dynes·s·cm⁻⁵) make clinical thresholds easier to remember and apply during urgent decision-making, particularly for determining surgical candidacy in congenital heart disease or assessing transplant eligibility. 1, 2
Common Pitfall to Avoid
Always document which unit is being used when reporting vascular resistance values, as confusion between Wood units and dynes·s·cm⁻⁵ can lead to significant clinical errors given the 80-fold conversion factor. 2 Additionally, when reporting indexed values, always include the m² designation (WU·m²) to distinguish from non-indexed measurements. 2