Pulmonary Vascular Resistance

Pulmonary vascular resistance (PVR) is the resistance the pulmonary circulation offers to the blood the right ventricle pumps through it. Calculated as the pressure drop across the lung divided by cardiac output, it is normally far lower than systemic vascular resistance and is one of the central variables describing the state of the pulmonary circulation.

Definition

Pulmonary vascular resistance is the ratio of the driving pressure across the pulmonary circulation—the difference between mean pulmonary arterial pressure and left atrial (or pulmonary arterial wedge) pressure—to pulmonary blood flow (cardiac output), expressed in Wood units or dyn·s·cm⁻⁵.

Scope

The entry covers the definition and calculation of PVR, the structural and functional determinants that keep it low, how recruitment and distension allow it to fall as flow rises, and the physiological factors that raise or lower it. It is a reference physiology topic; it discusses how PVR is defined and measured, not how to manage any condition in which it is abnormal.

Core questions

• How is pulmonary vascular resistance defined and calculated?
• Why is it normally so much lower than systemic resistance?
• How do recruitment and distension change resistance as flow increases?
• Which physiological factors raise or lower pulmonary vascular resistance?

Key concepts

• Driving pressure (transpulmonary gradient)
• Wood units
• Recruitment of closed vessels
• Distension of open vessels
• Lung-volume dependence of resistance
• Alveolar versus extra-alveolar vessels
• Passive versus active regulation

Mechanisms

PVR is computed as driving pressure divided by flow, so it is not a fixed property but a derived ratio that changes with the operating conditions of the circuit. Resistance is normally low because the pulmonary vascular bed is large, distensible, and partly unrecruited at rest; as flow or pressure rises—during exercise, for example—previously closed capillaries are recruited and open ones distend, so resistance falls rather than rises (Suresh & Shimoda, 2016; Naeije & Chesler, 2012). Resistance also depends on lung volume, because inflation compresses alveolar vessels while stretching extra-alveolar ones, giving a U-shaped relationship with volume. Active influences—most notably alveolar hypoxia, which constricts small arteries—add to these passive mechanics. In clinical hemodynamics, PVR is a calculated index used to characterize the circulation; contemporary classifications define abnormal elevation by threshold values (Simonneau et al., 2019; Humbert et al., 2022).

Clinical relevance

Pulmonary vascular resistance is a defining hemodynamic variable in the assessment of the pulmonary circulation, and thresholds for it appear in the haemodynamic definition of pulmonary hypertension (Simonneau et al., 2019; Humbert et al., 2022). This entry explains the concept and its measurement; it is educational and is not a basis for diagnosing or treating any individual.

Evidence & guidelines

The physiology of PVR rests on classic recruitment-distension experiments and on comprehensive reviews (Suresh & Shimoda, 2016; Naeije & Chesler, 2012). Its use as a diagnostic threshold is codified in international expert documents: the 6th World Symposium haemodynamic definitions (Simonneau et al., 2019) and the 2022 ESC/ERS guidelines (Humbert et al., 2022), which lowered the PVR cut-off used to define pre-capillary pulmonary hypertension.

History

The recognition that the lung's resistance falls with rising flow—through recruitment and distension rather than active dilation—emerged from mid-twentieth-century isolated-lung and exercise studies and remains the foundation of how PVR is interpreted. Later, as right-heart catheterization became routine, PVR was adopted as a calculated hemodynamic index, and expert panels progressively refined the pressure and resistance thresholds used to define abnormal states (Simonneau et al., 2019; Humbert et al., 2022).

Debates

Is PVR an adequate descriptor of right-ventricular load?

Because PVR captures only the steady (resistive) component of load and ignores pulsatile and compliance terms, reviews argue it understates the burden on the right ventricle, motivating measures such as arterial compliance and impedance alongside it.

Key figures

• Robert Naeije
• Larissa A. Shimoda
• Gérald Simonneau

Seminal works

• suresh-shimoda-2016
• naeije-2012

Frequently asked questions

Why does pulmonary vascular resistance fall when blood flow increases?

Higher flow and pressure recruit previously closed capillaries and distend open vessels, widening the bed available for flow; the resistance ratio therefore decreases even as flow rises.

How does lung volume affect pulmonary vascular resistance?

Resistance is lowest near functional residual capacity. Inflation compresses alveolar capillaries while deflation narrows extra-alveolar vessels, so resistance rises at both high and low lung volumes, giving a U-shaped relationship.