Why Two Units Exist for Expressing Vascular Resistance
Both Wood units (WU) and dynes·s·cm⁻⁵ express the same physiological concept—vascular resistance—but Wood units are strongly preferred in clinical practice because they simplify calculations and reduce mathematical errors. 1, 2
The Mathematical Relationship
- Wood units and dynes·s·cm⁻⁵ are directly interconvertible: multiply Wood units by 80 to obtain dynes·s·cm⁻⁵ 2
- Both units measure resistance using the fundamental formula: pressure gradient divided by cardiac output 2
- For pulmonary vascular resistance: PVR = (mean PAP - PCWP) / cardiac output 1, 2
- For systemic vascular resistance: SVR = (MAP - RAP) / cardiac output 1, 2
Why Wood Units Are Preferred
The 2015 ESC/ERS Guidelines explicitly state: "Wood Units are preferred to dynes·s·cm⁻⁵" 1
- Wood units eliminate the need for unit conversion during bedside calculations, as they directly use the measured values in mmHg and L/min 1, 2
- Dynes·s·cm⁻⁵ require multiplying by 80, introducing unnecessary complexity and potential for calculation errors 2
- Wood units provide more intuitive clinical thresholds (e.g., PVR >3 WU defines pre-capillary pulmonary hypertension) 1, 3
When Each Unit Is Used
Wood Units (Clinical Practice)
- All major cardiology and pulmonary hypertension guidelines use Wood units as the primary unit 1
- Decision-making for congenital heart disease repair (e.g., PVR >2.5 WU contraindicates shunt closure) 1
- Liver transplant candidacy in portopulmonary hypertension (PVR <3 WU required) 1
- Defining pre-capillary pulmonary hypertension (PVR >3 WU in 2015 guidelines, >2 WU in 2024 guidelines) 1, 3
- Pediatric congenital heart disease thresholds (PVR >6 WU·m² predicts poor outcomes) 2
Dynes·s·cm⁻⁵ (Historical/Research Context)
- Primarily appears in older literature and some research publications 1, 4
- Systemic vascular resistance is more commonly reported in dynes·s·cm⁻⁵ (normal ~800-1200 dynes·s·cm⁻²) 1, 5
- Some hemodynamic textbooks and physiology references use CGS (centimeter-gram-second) units 4
Critical Clinical Thresholds in Wood Units
Pulmonary Vascular Resistance
- Normal PVR: <2-3 WU 2
- Pre-capillary PH definition: PVR >2 WU (2024) or >3 WU (2015) 1, 3
- Congenital heart disease repair contraindicated: PVR >2.5 WU or >4 WU·m² indexed 1
- Portopulmonary hypertension transplant eligibility: PVR <5 WU with mPAP <35 mmHg, or PVR <3 WU with mPAP 35-45 mmHg 1
Systemic Vascular Resistance
- In congenital heart disease, surgical repair recommended when PVR is less than one-third of SVR 2, 5
- SVR must be maintained greater than PVR to prevent right ventricular ischemia 2, 5
Common Pitfalls
- Do not mix units when comparing PVR and SVR—ensure both are in the same unit system before making clinical decisions 2, 5
- Indexed values (WU·m²) are not directly comparable to non-indexed values—always specify whether resistance is indexed to body surface area 1, 2
- Measurement conditions matter: general anesthesia can lower systemic arterial pressure and affect resistance calculations 2
- In tricuspid regurgitation with RV dilatation, thermodilution cardiac output measurements can be erroneous, affecting all derived resistance calculations 2, 6
Practical Recommendation
Use Wood units for all clinical decision-making and documentation, as this aligns with current international guidelines and minimizes calculation errors 1, 2. Only convert to dynes·s·cm⁻⁵ if specifically required for research protocols or when comparing to historical literature that exclusively uses CGS units 4.