Wood Units vs Hemodynamic Resistance Units
Wood units (WU) are the preferred measurement for pulmonary vascular resistance (PVR) in clinical practice, and they are directly equivalent to hemodynamic resistance units (HRU)—both represent the same value with different names. 1
Understanding the Units
Wood units measure pulmonary vascular resistance and are calculated as:
- PVR (WU) = (mean PAP - PAWP) / cardiac output
- Where pressures are in mmHg and cardiac output is in L/min
The alternative unit system uses dyne·s·cm⁻⁵, which requires conversion:
- 1 Wood unit = 80 dyne·s·cm⁻⁵
- 1 dyne·s·cm⁻⁵ = 0.0125 Wood units
Why Wood Units Are Preferred
The 2015 ESC/ERS guidelines explicitly state that "Wood Units are preferred to dynes.s.cm⁻⁵" 1. This preference exists because:
- Wood units provide simpler, more intuitive numbers for clinical decision-making
- They align directly with the hemodynamic measurements obtained during right heart catheterization
- Current diagnostic thresholds are defined in Wood units
Critical Clinical Thresholds
The most recent guidelines establish these key cutoffs using Wood units 1, 2:
- PVR >2 WU: Defines pre-capillary pulmonary hypertension (when combined with mPAP >20 mmHg and PAWP ≤15 mmHg)
- PVR >3 WU: Previously used threshold, now updated to >2 WU in 2024 guidelines 2
- PVR ≤2 WU: Defines isolated post-capillary PH (when PAWP >15 mmHg)
Practical Conversion Examples
When you encounter resistance values in dyne·s·cm⁻⁵:
Important Clinical Caveat
Wood units do not correct for body surface area, which can be problematic when comparing patients of vastly different sizes 4. For more precise assessment in specific contexts (such as cardiac transplant evaluation), consider using:
- PVR index (PVRI) = PVR × body surface area (BSA)
- This indexed value better identifies patients at risk for complications, particularly in transplant candidates where a PVRI >6 units carries significant mortality risk 4
Bottom Line for Clinical Practice
Simply use Wood units as your standard measurement—they are equivalent to HRU and are the internationally accepted standard 1, 2. When you see values in dyne·s·cm⁻⁵, divide by 80 to convert to Wood units for easier interpretation against current diagnostic criteria.