78 Pulmonary vascular resistance (PVR)

Pulmonary vascular resistance: formula, the 2 Wood unit limit, pulmonary hypertension and right heart catheterisation.

Pulmonary vascular resistance (PVR) is the resistance that the right ventricle must overcome to push blood through the lungs. The pulmonary circulation normally has a low pressure and low resistance: it carries the whole cardiac output at a mean pressure of only about 14 mmHg. PVR depends mainly on the tone and structure of the small pulmonary arteries and arterioles, and also on lung volume, blood viscosity (haematocrit) and the endothelium, which releases dilating substances such as nitric oxide. A low oxygen level in the alveoli constricts pulmonary vessels (hypoxic vasoconstriction), which directs blood to better-ventilated parts of the lung.

Pulmonary circulationPulmonary hypertensionRight ventricle

Formula(mPAP − PAWP) / CO

Upper normal (ESC/ERS 2022)2 Wood units

Unit conversion1 WU = 80 dyn·s·cm⁻⁵

Pulmonary hypertensionmPAP > 20 mmHg

Figure 1. PVR measured by right heart catheterisation. Pre-capillary pulmonary hypertension: mPAP above 20 mmHg, wedge pressure 15 mmHg or less and PVR above 2 WU (ESC/ERS 2022).

How PVR is measured

PVR (WU) = [mean pulmonary artery pressure − pulmonary artery wedge pressure] (mmHg) / cardiac output (L/min)

The reference method is right heart catheterisation with a pulmonary artery catheter, which measures pulmonary pressures directly and cardiac output by thermodilution or the Fick method. Echocardiography can estimate pulmonary pressures and suggest raised PVR, but it cannot replace catheterisation for diagnosis.

Raised PVR

Pulmonary arterial hypertension: idiopathic, hereditary, drug-induced, associated with connective tissue disease, HIV, portal hypertension or congenital heart disease
Lung disease and hypoxia: COPD, interstitial lung disease, sleep apnoea, high altitude
Chronic thromboembolic pulmonary hypertension after pulmonary embolism
Left heart disease with long-standing raised pressure, which can secondarily remodel pulmonary vessels
Acute: pulmonary embolism, acidosis, hypoxia, high positive end-expiratory pressure during ventilation

Raised PVR overloads the right ventricle and can lead to right heart failure: breathlessness on exertion, fatigue, fainting, leg swelling and enlarged liver.

When to see a doctor

Unexplained breathlessness, fainting on exertion, chest pain or leg swelling should be investigated, especially in people with connective tissue disease, previous pulmonary embolism or chronic lung disease.

References

Humbert M, Kovacs G, Hoeper MM, et al. 2022 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Heart J. 2022;43(38):3618–3731. doi:10.1093/eurheartj/ehac237

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 1 May 2024