Normal Pulmonary Vascular Resistance Index (PVRi)

A normal PVRi is less than 3 Wood units·m² in adults, with values less than 2-3 Wood units·m² generally considered within the normal range. 1

Understanding PVRi vs PVR

PVRi (Pulmonary Vascular Resistance Index) is the indexed value of pulmonary vascular resistance adjusted for body surface area, expressed in Wood units·m² (WU·m²). 1
Non-indexed PVR is measured in Wood units (WU) alone without body surface area correction. 1
The indexed value (PVRi) is particularly important in pediatric populations where body size varies significantly and affects hemodynamic measurements. 1

Normal PVR values are typically less than 2-3 Wood units (non-indexed). 1
When indexed for body surface area, normal PVRi is less than 3 Wood units·m². 1

PVRi ≥6 WU·m² predicts poor prognosis in children with congenital heart disease, regardless of lung morphology. 1
PVRi <6 WU·m² is an indicator for repair in children with structural heart disease (ASD, VSD, PDA). 1
PVRi ≥6 WU·m² indicates that repair is not indicated unless acute vasodilator testing demonstrates reversibility (absolute PVRi <6 WU·m² and PVR/SVR <0.3). 1

PVRi = [(mPAP - PCWP) / CO] × BSA

Where:

Many centers use a preoperative PVR less than 10 to 14 Wood units and a pulmonary/systemic resistance ratio less than or equal to two-thirds as thresholds associated with better surgical outcomes. 2
PVRi >6 WU·m² predicts poor outcomes in single ventricle patients undergoing cavopulmonary surgery. 1
PVRi <7 to 8 WU·m² in response to vasodilator challenge predicts good surgical outcomes in patients with simple shunts. 1

The diagnostic criteria for PAH requires both a mean PAP greater than 25 mmHg and a PVR greater than 3 Wood units. 2
The 2015 ESC/ERS guidelines included PVR >3 Wood units in the hemodynamic definition of PAH. 2
PVR distinguishes passive pulmonary hypertension (elevated mean PAP, normal PVR) from pulmonary hypertension caused by pulmonary vascular disease (elevated mean PAP, elevated PVR). 2

Severe elevation of pulmonary vascular resistance has been considered a contraindication to cardiac transplantation, with most centers excluding patients with pulmonary vascular resistance greater than 6 Wood units. 3
No patients with a PVRi less than 6 developed right heart failure post-transplant. 3
The PVR index unit (PVRi) identifies patients at risk for right heart failure better than the Wood unit, which does not correct for body size. 3

Mean pulmonary artery pressure (mPAP) must be measured via right heart catheterization, with measurements taken at end expiration if breathing spontaneously, or at end inspiration if mechanically ventilated. 1
Pulmonary capillary wedge pressure (PCWP) should be obtained by wedging a balloon-tipped catheter into a small pulmonary arterial branch with the balloon inflated, and the external pressure transducer must be zeroed at the mid-thoracic line. 1
Blood pH has a potent effect on pulmonary vascular tone—acidosis causes vasoconstriction while alkalosis causes vasodilation—and awareness of arterial blood gas measurements during catheterization is critical for accurate interpretation of baseline hemodynamics. 1

Direct measurement of oxygen consumption (VO₂) is preferable to estimation, particularly in children <3 years of age, as the LaFarge equation can overestimate VO₂ and lead to underestimation of PVR. 1
In patients with tricuspid regurgitation and right ventricular dilatation, cardiac output measurements by thermodilution can be erroneous, affecting all derived resistance calculations. 1
Measurements should be obtained under standardized conditions, as general anesthesia can lower systemic arterial blood pressure and affect resistance calculations. 2