Peripheral Arterial Occlusive Disease 

Claudication, which is defined as reproducible ischemic muscle pain, is one of the most common manifestations of peripheral vascular disease caused by atherosclerosis. Claudication occurs during physical activity and is relieved after a short rest. Pain develops because of inadequate blood flow.

Single or multiple arterial stenoses produce impaired hemodynamics at the tissue level in patients with peripheral arterial occlusive disease (PAOD), shown below. Arterial stenoses lead to alterations in the distal pressures available to affected muscle groups and to blood flow.

Under resting conditions, normal blood flow to extremity muscle groups averages 300-400 mm/min. Once exercise begins, blood flow increases up to 10-fold owing to the increase in cardiac output and compensatory vasodilation at the tissue level. When exercise ceases, blood flow returns to normal within minutes.

In patients with PAOD, resting blood flow is similar to that of a healthy person. However, during exercise, blood flow cannot maximally increase in muscle tissue because of proximal arterial stenoses. 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 of 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, in which Q is flow, v is viscosity, L is the length of the stenotic area, r is the radius of the open area within the stenosis, and k is constant:

Resistance = pressure = Q8vL/kr4

Applying this equation, the pressure gradient is directly proportional to the flow and length of stenosis and inversely proportional to the fourth power of the radius.

Therefore, while increasing the rate of flow 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.

As the radius is raised to the fourth power, it has the most dramatic impact on a pressure gradient across a lesion. This impact is additive when 2 or more occlusive lesions are located sequentially within the same artery.

Frequency

United States

Atherosclerosis affects up to 10% of the Western population older than 65 years. With the elderly population expected to increase 22% by the year 2040, atherosclerosis is expected to have a huge financial impact on medicine. When claudication is used as an indicator, estimates are that 2% of the population aged 40-60 years and 6% older than 70 years are affected.

Mortality/Morbidity

The most feared consequence is severe limb-threatening ischemia leading to amputation. However, studies of large patient groups with claudication reveal that amputation is uncommon. Boyd prospectively followed 1440 patients with intermittent claudication for as long as 10 years and reported that only 12.2% required amputation.^[1] In the Framingham study, only 1.6% of patients with claudication reached the amputation stage after 8.3 years of follow-up.

• Limb amputation largely depends on the number and severity of cardiovascular risk factors (ie, smoking, hypertension, diabetes). Continued smoking has been identified as the most consistent adverse risk factor associated with the progression of peripheral arterial occlusive disease (PAOD). Other factors are the severity of disease at the time of the initial patient encounter and, in some studies, the presence of diabetes.
• As with most patients with vascular disease, survival is less than that of age-matched control groups. Coronary artery disease with a subsequent myocardial event is the major contributor to outcome. Predicted mortality rates for patients with claudication at 5, 10, and 15 years of follow-up are approximately 30%, 50%, and 70%, respectively.

Race

• Peripheral arterial occlusive disease (PAOD) has no racial predilection.

Sex

• Intermittent claudication most commonly manifests in men older than 50 years.
• Although younger patients may present with symptoms consistent with intermittent claudication, other etiologies of leg pain and claudication (eg, popliteal entrapment syndrome) must be strongly considered.