2021 Feb 17. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–.
Pulmonary vascular resistance is the resistance against blood flow from the pulmonary artery to the left atrium. It is most commonly modeled using a modification of Ohm’s law (figure 1).
As seen in figure 1, input pressure represents the mean pulmonary arterial pressure (15 mmHg). The output pressure represents the pulmonary venous pressure, which is also equivalent to the pulmonary capillary wedge pressure or left atrial pressure (5 to 6 mmHg). Total blood flow represents the cardiac output (5 to 6 L/min). A normal value for pulmonary vascular resistance using conventional units is 0.25–1.6 mmHg·min/l. Pulmonary vascular resistance can also be represented in units of dynes/sec/cm5 (normal = 37-250 dynes/sec/cm5).
Poiseuille’s law has also been used to model PVR (Figure 2). In this equation, l represents the length of the tube or vessel, r its radius, and n the viscosity of the fluid. Poiseuille’s law clarifies the impact of the radius on resistance. For example, a 50% reduction in radius increases the resistance 16-fold. However, both Ohm’s law and Poiseuille’s law are imperfect approximations of PVR. Both equations assume that blood flow is constant and linear, but in reality, it is pulsatile and laminar. Pulmonary blood vessels are not rigid cylinders and expand to accommodate increased flow. Additionally, the non-homogenous nature of blood makes it difficult to ascertain a single value for viscosity. Blood viscosity also varies with shear rate.
The pressure drop from the pulmonary arteries to the left atrium is approximately 10 mmHg compared against a 100 mmHg 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 ultimately allows for proper gas exchange. Additionally, low resistance allows for the pulmonary system to pump the entire cardiac output at low pressures. Most of the total vascular resistance and distribution of blood flow in the pulmonary circuit resides in the capillaries rather than the vessels that are involved in active vasoconstriction. However, approximations generally divide pulmonary resistance equally between arteries, capillaries, and veins. Because resistance increases in the capillaries, the largest drop in pulmonary pressure occurs here, and to a lesser extent, in the small pulmonary arteries. Contrast with the systemic circulation where the largest pressure drop occurs in the arterioles.