Interpretation of SVR and PVR in Wood Units
Normal pulmonary vascular resistance (PVR) is <2 Wood units, with values ≥3 Wood units defining pulmonary arterial hypertension when combined with appropriate pulmonary artery pressures, while normal systemic vascular resistance (SVR) ranges from 900-1200 dyn·s·cm⁻⁵ (approximately 11-15 Wood units). 1, 2
Normal and Abnormal Ranges
Pulmonary Vascular Resistance (PVR)
- Normal: <2 Wood units 3, 4
- Borderline/Mild elevation: 2.0-2.9 Wood units 5, 4
- Elevated (diagnostic threshold for PAH): ≥3 Wood units 1, 2, 6
- Severe elevation: >5 Wood units 2
The recent evolution in PH definitions has lowered thresholds - mean pulmonary artery pressure was reduced from ≥25 mmHg to >20 mmHg 1, but the PVR threshold of ≥3 Wood units remains the standard for diagnosing pulmonary arterial hypertension when combined with pulmonary artery wedge pressure ≤15 mmHg 1, 2.
Systemic Vascular Resistance (SVR)
Note: SVR is typically reported in dyn·s·cm⁻⁵ rather than Wood units. To convert: 1 Wood unit = 80 dyn·s·cm⁻⁵. Therefore, normal SVR of 900-1200 dyn·s·cm⁻⁵ equals approximately 11-15 Wood units.
Clinical Interpretation Framework
For PVR Assessment
Precapillary Pulmonary Hypertension requires:
- Mean pulmonary artery pressure >20 mmHg
- Pulmonary artery wedge pressure ≤15 mmHg
- PVR ≥3 Wood units 1, 3
Isolated Postcapillary Pulmonary Hypertension:
Combined Pre- and Postcapillary Pulmonary Hypertension:
Critical PVR Thresholds for Specific Conditions
Portopulmonary Hypertension (liver transplant candidates):
- Mean PAP >45 mmHg despite treatment = absolute contraindication 2
- Safe to proceed if mean PAP <35 mmHg AND PVR <5 Wood units 2
- Alternative: mean PAP 35-45 mmHg AND PVR <3 Wood units 2
Congenital Heart Disease:
- PVR ≥2.5 Wood units warrants collaborative assessment by ACHD cardiologist and PH expert 8
- Many centers use PVR <10-14 Wood units and pulmonary/systemic resistance ratio ≤0.67 as thresholds for better surgical outcomes 9
Emerging Evidence on Lower PVR Thresholds
Recent data challenges the conservative 3 Wood unit threshold. Mortality risk begins increasing at approximately 2.2 Wood units in patients with mean PAP ≥19 mmHg and wedge pressure ≤15 mmHg 4. Patients with "borderline" PVR of 2.0-2.9 Wood units demonstrate adverse outcomes and may respond to PAH therapy, though this requires validation in randomized trials 5.
Common Pitfalls and Caveats
PVR Measurement Considerations
Flow dependency: PVR is not static - high shunt flows can recruit pulmonary vasculature and reduce PVR. When shunts are eliminated surgically, PVR may not fall proportionally because recruited vascular beds "de-recruit" 9.
West zones in ARDS: High transpulmonary pressure from mechanical ventilation can abnormally extend West zones 1 and 2, causing calculated PVR to underestimate true pulmonary vascular resistance 10.
Tricuspid regurgitation: Severe TR can cause inaccurate thermodilution cardiac output measurements, requiring Fick method for accurate PVR calculation 11.
Intrathoracic pressure effects: Severe emphysema, obesity, or mechanical ventilation increase intrathoracic pressures, affecting accuracy of pulmonary artery pressure and wedge pressure measurements 12.
SVR Interpretation Caveats
Low SVR (<900 dyn·s·cm⁻⁵) indicates:
- Septic shock/distributive shock
- Severe liver disease
- Anaphylaxis
- High-output cardiac states
High SVR (>1200 dyn·s·cm⁻⁵) indicates:
- Cardiogenic shock
- Hypovolemia
- Vasopressor use
- Severe hypertension
The classifier sensitivity for detecting low SVR is particularly high (85% sensitivity, 86% specificity) 7, making it clinically useful for identifying distributive shock states.