What is Pulmonary Vascular Resistance?
Pulmonary Vascular Resistance (PVR) quantifies the resistance to blood flow through the pulmonary vasculature. It is analogous to systemic vascular resistance (SVR) but applies specifically to the pulmonary circuit. PVR reflects the tone, structure, and patency of the pulmonary arteries and arterioles.
PVR is calculated from measurements obtained during right heart catheterization (Swan-Ganz catheter) and is essential for:
- Diagnosing and classifying pulmonary hypertension
- Distinguishing pre-capillary from post-capillary pulmonary hypertension
- Evaluating response to pulmonary vasodilator therapy
- Assessing operability for cardiac surgery and transplant candidacy
- Monitoring right ventricular afterload in heart failure
PVR Formula & Unit Conversions
Where mPAP = mean pulmonary artery pressure (mmHg), PCWP = pulmonary capillary wedge pressure (mmHg), and CO = cardiac output (L/min).
The numerator (mPAP − PCWP) is called the transpulmonary pressure gradient (TPG), which represents the driving pressure across the pulmonary vasculature.
Interpretation & Reference Ranges
| PVR (Wood Units) | PVR (dynes·s·cm-5) | Classification | Clinical Significance |
|---|---|---|---|
| < 2 | < 160 | Normal | Normal pulmonary vascular resistance |
| 2 – 3 | 160 – 240 | Mild PH | Early pulmonary hypertension; monitor closely |
| 3 – 5 | 240 – 400 | Moderate PH | Significant PH; consider pulmonary vasodilator therapy |
| > 5 | > 400 | Severe PH | Severe PH; right heart failure risk; evaluate for transplant |
Note: The 2022 ESC/ERS guidelines define pulmonary hypertension as mPAP > 20 mmHg (lowered from the previous threshold of 25 mmHg) and pre-capillary PH as PVR > 2 Wood units with PCWP ≤ 15 mmHg.
Pulmonary Circulation Diagram
Pulmonary Hypertension
Pulmonary hypertension (PH) is a pathological elevation of pulmonary artery pressure that can result from a variety of underlying conditions. It leads to progressive right ventricular failure and, if untreated, death. PH is hemodynamically defined by the 2022 ESC/ERS guidelines as:
- mPAP > 20 mmHg at rest (measured by right heart catheterization)
- Pre-capillary PH: PCWP ≤ 15 mmHg and PVR > 2 WU (pulmonary arterial disease)
- Post-capillary PH: PCWP > 15 mmHg (left heart disease)
- Combined pre- and post-capillary PH: PCWP > 15 mmHg and PVR > 2 WU
WHO Classification of Pulmonary Hypertension
| Group | Classification | Examples |
|---|---|---|
| Group 1 | Pulmonary Arterial Hypertension (PAH) | Idiopathic PAH, heritable, drug-induced, CTD-associated, HIV, portal HTN |
| Group 2 | PH due to Left Heart Disease | HFrEF, HFpEF, valvular disease, congenital cardiomyopathy |
| Group 3 | PH due to Lung Disease / Hypoxia | COPD, ILD, sleep-disordered breathing, chronic high altitude |
| Group 4 | Chronic Thromboembolic PH (CTEPH) | Chronic PE, pulmonary artery obstructions |
| Group 5 | PH with Unclear / Multifactorial Mechanisms | Sarcoidosis, sickle cell, myeloproliferative, thyroid disorders |
Right Heart Catheterization
Right heart catheterization (RHC) with a Swan-Ganz (pulmonary artery) catheter is the gold standard for measuring pulmonary hemodynamics. Key measurements include:
- Right atrial pressure (RAP): Normal 2–6 mmHg; elevated in right heart failure
- PA pressures: Systolic/diastolic/mean; mPAP normally 10–20 mmHg
- PCWP (wedge): Normal 6–12 mmHg; reflects left atrial pressure
- Cardiac output: Normal 4–8 L/min; measured by thermodilution or Fick method
- Mixed venous oxygen saturation (SvO2): Normal 65–75%; low values suggest poor cardiac output
Vasoreactivity testing (inhaled nitric oxide, IV epoprostenol, or inhaled iloprost) is performed during RHC to identify patients with Group 1 PAH who may respond to calcium channel blockers. A positive response is defined as a decrease in mPAP ≥ 10 mmHg to an absolute value ≤ 40 mmHg without decreased cardiac output.
Worked Example
A patient with suspected pulmonary hypertension undergoes RHC with the following measurements: mPAP = 35 mmHg, PCWP = 10 mmHg, CO = 4.0 L/min.
This PVR of 6.25 WU (> 5 WU) indicates severe pulmonary hypertension. Combined with PCWP ≤ 15 and PVR > 2, this is pre-capillary PH, most consistent with WHO Group 1 (PAH), Group 3 (lung disease), or Group 4 (CTEPH). Further workup including V/Q scan, CT angiography, and autoimmune serologies is needed to determine the specific etiology.
Frequently Asked Questions
What is the difference between Wood units and dynes?
Both measure vascular resistance. Wood units (mmHg/L/min, also called hybrid resistance units or HRU) are simpler and more commonly used in clinical practice. To convert to the CGS unit dynes·s·cm-5, multiply Wood units by 80. The clinical interpretation is the same regardless of units.
What is the transpulmonary gradient (TPG)?
The TPG is mPAP minus PCWP. A TPG > 12 mmHg suggests an intrinsic pulmonary vascular component (pre-capillary disease) even when PCWP is elevated. The diastolic pressure gradient (diastolic PAP minus PCWP) may be a better discriminator, as TPG can be influenced by cardiac output and flow.
Can PVR be estimated without catheterization?
Echocardiography can estimate PA systolic pressure from tricuspid regurgitation velocity, but it cannot directly measure PVR, PCWP, or cardiac output with the same accuracy as RHC. While echo-based estimates exist (e.g., Abbas formula: PVR ≈ TRV/VTI_RVOT × 10 + 0.16), RHC remains the gold standard for definitive diagnosis.
What PVR is too high for heart transplant?
A PVR > 5 Wood units (or > 3 WU that is unresponsive to vasodilator challenge) is generally considered a contraindication to isolated heart transplantation, as the donor right ventricle may fail against the high pulmonary resistance. These patients may require combined heart-lung transplantation or a period of mechanical support (LVAD) to lower PVR before transplant.