Pulmonary Vascular Resistance (PVR) is a hemodynamic measurement reflecting the force opposing blood flow through the vessels of the lungs. The pulmonary circulation is a low-pressure system, unlike the body’s main circulatory system. PVR quantifies how easily blood moves from the right side of the heart through the lungs for oxygenation. This measurement provides insight into the health and functionality of the pulmonary arteries and arterioles. An abnormal PVR reading indicates a significant health issue, guiding clinicians toward diagnosis and treatment. PVR is a key metric for evaluating cardiac and respiratory health, especially in pulmonary diseases.
Understanding Pulmonary Vascular Resistance (PVR)
Pulmonary Vascular Resistance represents the friction and obstruction blood encounters as it traverses the pulmonary circuit. The blood vessels in the lungs are highly compliant, meaning they easily expand to accommodate the entire output of the right side of the heart at very low pressure. This unique, low-resistance environment maximizes gas exchange by allowing blood to flow slowly and efficiently past the air sacs.
Resistance is generated by the structure and tone of the small pulmonary arteries and arterioles, which distribute blood flow. When these vessels narrow, stiffen, or become obstructed—a process known as remodeling—the resistance increases significantly. Like water flowing through a narrowed hose, increased resistance requires higher pressure to maintain flow. The radius of these vessels is the greatest determinant of resistance.
How PVR is Measured and the Normal Range
The calculation of PVR relies on measuring pressures and flow within the heart and lungs, a process that requires a specialized procedure called right heart catheterization. During this invasive procedure, a thin, flexible tube is guided into the heart’s right chambers and into the pulmonary artery to gather hemodynamic data. The gold standard for determining PVR uses an adaptation of Ohm’s law, similar to how electrical resistance is calculated.
The PVR formula is calculated by taking the pressure difference across the pulmonary circulation and dividing it by the blood flow, or cardiac output. Specifically, PVR equals the difference between the mean Pulmonary Artery Pressure and the Pulmonary Artery Wedge Pressure, divided by the Cardiac Output.
The resulting value is reported in one of two standard units. The most common unit is the Wood unit (WU), where a normal PVR is considered less than 3 Wood units. Alternatively, resistance may be expressed in dynes-sec/cm⁻⁵, with a normal range cited as less than 240 dynes-sec/cm⁻⁵, which is equivalent to the 3 WU threshold.
The Impact of High PVR on the Heart and Lungs
An elevated PVR signifies that the pulmonary circulation is stiff or narrowed, forcing the right ventricle (RV) of the heart to expend more energy to pump blood forward. This chronic, high workload places strain on the right heart, defining Pulmonary Hypertension (PH). Over time, the RV muscle attempts to cope with this increased afterload by thickening, a compensatory process known as hypertrophy.
If the high resistance is not overcome, the RV eventually loses its ability to pump effectively, leading to dilation and failure, a serious condition called Cor Pulmonale. This right-sided heart failure diminishes the heart’s ability to move blood through the lungs, resulting in low cardiac output and insufficient oxygenation. Patients may notice symptoms like shortness of breath, fatigue, dizziness, or swelling. Conditions such as chronic hypoxemia (seen in severe lung disease) or left-sided heart failure can also cause blood vessels to narrow, contributing to a high PVR reading.
Clinical Applications: When is PVR Used for Diagnosis?
PVR is essential for the clinical classification and management of pulmonary hypertension, particularly for distinguishing disease types. A PVR greater than 3 Wood units, combined with a mean pulmonary artery pressure greater than 25 mmHg, confirms pre-capillary pulmonary hypertension. This reading helps identify diseases like Pulmonary Arterial Hypertension (PAH), where the problem originates in the pulmonary arteries.
The PVR value also guides treatment decisions by determining the likelihood of a patient responding to vasodilator medications, which relax and widen pulmonary vessels. For patients with severely elevated PVR, a vasodilator challenge during catheterization assesses resistance reversibility. PVR is also a factor in evaluating suitability for heart or lung transplantation, as resistance must be low enough for the new organ to function properly.