What is the pathophysiology of peripheral arterial occlusive disease (PAOD)?

Updated: Sep 12, 2019
  • Author: Josefina A Dominguez, MD; Chief Editor: Vincent Lopez Rowe, MD  more...
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Answer

Single or multiple arterial stenoses produce impaired hemodynamics at the tissue level in patients with PAOD. Arterial stenoses lead to alterations in the distal perfusion pressures available to affected muscle groups.

Under resting conditions, normal blood flow to extremity muscle groups averages 300-400 mL/min. Once exercise begins, blood flow increases as much as 10-fold as a consequence of the increase in cardiac output and compensatory vasodilation at the tissue level. This allows the increase in oxygen demand to be met. When exercise ceases, blood flow returns to normal within minutes.

Resting blood flow in a person with PAOD is similar to that in a healthy person. In PAOD, however, blood flow cannot maximally increase in muscle tissue during exercise, because proximal arterial stenoses prevent compensatory vasodilation. When the metabolic demands of the muscle exceed blood flow, claudication symptoms ensue. At the same time, a longer recovery period is required for blood flow to return to baseline once exercise is terminated.

Similar abnormal alterations occur in distal perfusion pressure in affected extremities. In normal extremities, the mean blood pressure drop from the heart to the ankles is no more than a few millimeters of mercury. In fact, as pressure travels distally, the measured systolic pressure actually increases because of the higher resistance encountered in smaller-diameter vessels.

At baseline, a healthy person may have a higher measured ankle pressure than arm pressure. When exercise begins, no change in measured blood pressure occurs in the healthy extremity.

In the atherosclerotic limb, each stenotic segment acts to reduce the pressure head experienced by distal muscle groups. Correspondingly, at rest, the measured blood pressure at the ankle is less than that measured in a healthy person. Once physical activity starts, the reduction in pressure produced by the atherosclerotic lesion becomes more significant, and the distal pressure is greatly diminished.

The phenomenon of increased blood flow causing decreased pressure distally to an area of stenosis is a matter of physics. Poiseuille calculated energy losses across areas of resistance with varying flow rates by using the following equation:

  • Pressure difference = 8QvL/πr4

where Q is flow, v is viscosity, L is the length of the stenotic area, and r is the radius of the open area within the stenosis. In this equation, the pressure gradient is directly proportional to the flow and the length of the stenosis and inversely proportional to the fourth power of the radius. Thus, although increasing the flow rate directly increases the pressure gradient at any given radius, these effects are much less marked than those due to changes in the radius of the stenosis.

Because the radius is raised to the fourth power, it is the factor that has the most dramatic impact on a pressure gradient across a lesion. This impact is additive when two or more occlusive lesions are located sequentially within the same artery.


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