Peripheral Arterial Occlusive Disease
Peripheral arterial occlusive disease (PAOD) results either from atherosclerotic or inflammatory processes causing lumen narrowing (stenosis), or from thrombus formation (usually associated with underlying atherosclerotic disease). When these conditions arise, there is an increase in vessel resistance that can lead to a reduction in distal perfusion pressure and blood flow. The following discussion assumes chronic atherosclerotic conditions in the human lower limb that result in stenotic lesions. The hemodynamics and underlying mechanisms of PAOD in the human limb are very similar to what is found in coronary artery disease.
The process of atherosclerosis causes intimal thickening and plaque formation, which decrease the effective radius of the afflicted arterial segment. Although atherosclerosis is generally a diffuse process affecting all of the arteries to some degree, some arterial segments in the limb often undergo greater stenosis than others. Therefore, it is common to find stenotic lesions associated with specific arteries such as the external iliac or femoral artery.
Based on Poiseuille's equation, a decrease in vessel radius will increase the resistance to the fourth power of the change in radius. Therefore, a 50% reduction in radius (one-half radius) will cause the resistance to increase by a factor of 16. Hydrodynamically, this would cause flow to decrease by a factor of 16 assuming the pressure gradient is constant, laminar flow conditions prevail, and that the resistance of this segment represents the total resistance to flow (i.e., the segment is not one of multiple in-series segments). However, because the major arteries of the limb circulation are both in-series and in-parallel, a stenotic lesion generally has to have its radius decreased by more than 60% to result in a significant hemodynamic effect (i.e., critical stenosis). Furthermore, for any given reduction in radius, the longitudinal pressure drop along the length of the lesion will be significantly enhanced by the presence of turbulence.
PAOD can lead to limb ischemia. In mild to moderate PAOD, the increased resistance to flow will lead to decreased flow capacity during limb exercise (i.e., decreased active hyperemia). This can result in ischemic pain during exercise that is termed intermittent claudication. The pain is caused by tissue hypoxia that results from the high oxygen demand that is not met by an adequate increase in oxygen delivery (i.e., increased blood flow). In other words, there is a reduction in the oxygen supply/demand ratio. Metabolites formed under anaerobic conditions in the muscle can stimulate pain receptors in the muscle. Also associated with the relative ischemia during exercise is muscle weakness and fatigue.
Stenotic arterial lesions may or may not alter resting blood flow. Atherosclerosis is a disease process that occurs over years. The circulation distal to a stenotic lesion will often undergo collateralization which reduces resistance and thereby maintains normal resting blood flow despite a reduced perfusion pressure. Furthermore, even acute reductions in perfusion pressure lead to a fall in distal vascular resistance and normalization of blood flow by the mechanism of autoregulation.
CLICK HERE to learn more about the hemodynamics associated with single and multiple stenotic lesions in the limb circulation.