Physiology, Pulmonary Vascular Resistance

Pulmonary vascular resistance (PVR) is the resistance against blood flow from the 4 pulmonary veins of the lung to the left atrium. It is most commonly modeled using a modification of Ohm’s law (see Pulmonary Vascular Resistance Derived from Ohm's Law). Input pressure represents the mean pulmonary arterial pressure (15 mm Hg). The output pressure represents the pulmonary venous pressure, equivalent to the pulmonary capillary wedge or left atrial pressure (5 to 6 mm Hg). Total blood flow represents the cardiac output (5 to 6 L/min). A normal value for pulmonary vascular resistance using conventional units is 0.25 to 1.6 mmHg·min/l. Pulmonary vascular resistance is also represented in units of dynes/sec/cm (normal = 37dynes/sec/cm to 250 dynes/sec/cm). Poiseuille’s law has been used to model PVR (see Poiseuille's Law Modeling Pulmonary Vascular Resistance). In this equation, represents the length of the tube or vessel, its radius, and the fluid's viscosity. Poiseuille’s law clarifies the impact of the radius on resistance. For example, a 50% reduction in radius increases the resistance 16-fold. However, Ohm’s and Poiseuille’s laws are approximations of PVR. Both equations assume that blood flow is constant and linear, but it is pulsatile and laminar. Pulmonary blood vessels are dynamic, multi-layered tissues that expand to accommodate increased flow. Additionally, the non-homogenous quality of blood makes it difficult to ascertain a single value for viscosity. Blood viscosity varies with shear rate. The pressure drop from the pulmonary veins to the left atrium is approximately 10 mm Hg compared to a 100 mm Hg pressure gradient in the systemic circulation. Therefore, PVR is one-tenth of the resistance of systemic circulation. Low PVR maximizes the distribution of blood to the peripheral alveoli and facilitates gas exchange. Low resistance also enables the pulmonary system to pump the total cardiac output at low pressures. Most of the total vascular resistance and distribution of blood flow in the pulmonary circuit is due to the cross-sectional area of capillaries rather than veins, venules, or arteries. However, pulmonary resistance is approximately equally divided between arteries, capillaries, and veins. Because resistance increases in the capillaries, the largest drop in pulmonary pressure occurs, and to a lesser extent, in the small pulmonary arteries. In the systemic circulation, the largest pressure drop occurs in the arterioles.