Unit of Measurement for PVRI
The Pulmonary Vascular Resistance Index (PVRI) is measured in Wood units·m² (WU·m²). 1, 2
Standard Units and Terminology
PVRI is expressed as Wood units multiplied by meters squared (WU·m²), which represents the indexed value of pulmonary vascular resistance adjusted for body surface area. 1, 2
The calculation involves dividing the transpulmonary pressure gradient (mean pulmonary artery pressure minus pulmonary capillary wedge pressure) by cardiac index rather than cardiac output, which automatically incorporates the body surface area adjustment. 2
Wood units can be converted to dynes·s·cm⁻⁵·m² by multiplying by 80, though Wood units are preferred in clinical practice for simplicity and to reduce mathematical errors. 2
Critical Distinction from Non-Indexed PVR
Non-indexed pulmonary vascular resistance (PVR) is measured in Wood units (WU) alone, without the m² designation. 1, 2
The presence or absence of "m²" in the units is clinically significant, as it indicates whether the value has been adjusted for body surface area. 3
A 2019 literature review found that 54.6% of published sources incorrectly reported PVRI units, demonstrating widespread confusion that can have life-altering clinical consequences. 3
Clinical Thresholds Using PVRI
PVRI >6 WU·m² predicts poor prognosis in children with congenital heart disease, regardless of lung morphology. 1
PVRI <7 to 8 WU·m² in response to vasodilator challenge predicts good surgical outcomes in patients with simple shunts. 1
A contraindication threshold of >4 WU·m² is used for determining operability in congenital heart disease repair. 2
Common Pitfall to Avoid
Always verify whether published values include the m² designation, as confusion between PVR (WU) and PVRI (WU·m²) represents a significant source of error in clinical decision-making. 3
The indexed value (PVRI) is particularly important in pediatric populations where body size varies significantly and affects hemodynamic measurements. 1