# Peripheral Arterial Disease: Epidemiology, Risk Factors, and Natural History

## Epidemiology

Peripheral arterial disease (PAD) affects approximately 200 million people worldwide. In the United States alone, the prevalence ranges from 8 to 12 million adults and increases sharply with age. Among individuals aged 40 to 59 years, about 3 to 5% are affected, rising to 7 to 10% in those aged 60 to 69, and reaching 15 to 20% in people over 70 years old. Despite this high prevalence, the majority of patients—between 50 and 80%—are either asymptomatic or exhibit atypical symptoms. Only 10 to 30% present with the classic symptom of intermittent claudication, while a smaller subset, about 1 to 3%, present with critical limb-threatening ischemia (CLTI).

Demographically, PAD shows a male predominance in younger age groups, but this difference equalizes after age 70. The prevalence is notably higher in Black populations, approximately twice that seen in White populations. This disparity is multifactorial, involving higher rates of diabetes and hypertension, socioeconomic factors, and differences in access to care. Hispanic populations, while having a similar prevalence of PAD, experience higher rates of amputation. Geographic disparities also exist, with rural populations often having less access to vascular specialists, which may impact outcomes.

PAD is considered a coronary artery disease (CAD) equivalent because it reflects the same systemic atherosclerotic process affecting multiple vascular beds. The annual mortality rate among PAD patients is 4 to 6%, which is two to three times higher than age-matched controls. The leading cause of death in these patients is myocardial infarction, accounting for 40 to 60% of deaths, while cerebrovascular events contribute to 10 to 20%. An ankle-brachial index (ABI) below 0.90 independently predicts cardiovascular mortality, even in asymptomatic individuals. Patients with polyvascular disease—those who have PAD in combination with CAD or cerebrovascular disease—have the worst prognosis.

## Risk Factors

The strongest modifiable risk factor for PAD is smoking, which increases the risk by two to four times in a dose-dependent manner. Although the risk declines after smoking cessation, it never returns to baseline. Diabetes mellitus similarly increases the risk two to fourfold and accelerates disease progression. Diabetic patients tend to develop disease in the tibial vessels and often have neuropathy that masks symptoms, significantly increasing their risk of amputation by five to ten times. Age is another critical factor, with the prevalence of PAD doubling every decade after 40 years.

Other major risk factors include hypertension, which increases risk by 1.5 to 2 times and contributes to arterial remodeling, and hyperlipidemia, characterized by elevated low-density lipoprotein (LDL), low high-density lipoprotein (HDL), and elevated lipoprotein(a). Chronic kidney disease accelerates medial arterial calcification and impairs wound healing, further complicating PAD. Hyperhomocysteinemia is an independent risk factor, although folic acid supplementation has not been shown to reduce clinical events in trials.

Emerging risk factors under investigation include inflammatory biomarkers such as high-sensitivity C-reactive protein (hs-CRP) and interleukin-6 (IL-6), genetically determined lipoprotein(a), exposure to air pollution, and a sedentary lifestyle.

## Classification Systems

The Fontaine classification system categorizes PAD severity into five stages. Stage I is asymptomatic with only an abnormal ABI. Stage IIa involves mild claudication with a walking distance greater than 200 meters, while Stage IIb denotes moderate-to-severe claudication with walking distance less than 200 meters. Stage III is characterized by ischemic rest pain, and Stage IV includes ulceration or gangrene.

More commonly used is the Rutherford classification, which ranges from Category 0 (asymptomatic) to Category 6 (major tissue loss extending above the transmetatarsal level). Categories 1 through 3 represent mild, moderate, and severe claudication, respectively. Category 4 corresponds to ischemic rest pain, Category 5 to minor tissue loss such as non-healing ulcers or focal gangrene, and Category 6 to major tissue loss.

| Fontaine Stage | Clinical Features | Rutherford Category | Clinical Features |
|----------------|-------------------|--------------------|--------------------|
| I | Asymptomatic | 0 | Asymptomatic |
| IIa | Mild claudication (>200 m) | 1 | Mild claudication |
| IIb | Moderate-severe claudication (<200 m) | 2 | Moderate claudication |
| — | — | 3 | Severe claudication |
| III | Ischemic rest pain | 4 | Ischemic rest pain |
| IV | Ulceration or gangrene | 5 | Minor tissue loss |
| — | — | 6 | Major tissue loss |

For critical limb-threatening ischemia (CLTI), the Wound, Ischemia, and foot Infection (WIfI) classification is used. It grades wounds, ischemia, and infection on a scale from 0 to 3, and the composite staging predicts amputation risk and potential benefit from revascularization. This system is covered in detail in the CLTI seminar (Topic 60).

## Natural History

Intermittent claudication generally carries a relatively benign limb prognosis. Only 1 to 3% of patients with claudication progress to CLTI each year. Over five years, 70 to 80% of claudicants remain stable or improve, 10 to 20% worsen, 5 to 10% require intervention, and 1 to 2% require major amputation. Despite this relatively favorable limb outlook, the five-year cardiovascular mortality rate remains high at 15 to 30%, reflecting the systemic nature of the disease.

In contrast, CLTI represents end-stage PAD with hemodynamic compromise at rest. Without revascularization, the one-year amputation rate ranges from 25 to 40%, and mortality is 20 to 25%. CLTI is defined clinically by rest pain lasting more than two weeks with an ABI less than 0.4 or ankle pressure below 50 mmHg, or by tissue loss with an ABI under 0.6 or ankle pressure less than 70 mmHg.

Factors that contribute to disease progression include continued smoking, which is the strongest predictor, uncontrolled diabetes, renal failure, and failure to implement appropriate medical therapy.

## Screening

Screening for PAD is recommended in individuals over 65 years of age, those aged 50 to 64 with risk factors such as smoking or diabetes, and those under 50 with diabetes plus one additional risk factor. Screening is also advised for patients with known atherosclerotic disease in other vascular beds or those with abnormal lower extremity pulse examinations.

The ankle-brachial index (ABI) is the preferred screening method due to its simplicity, noninvasiveness, and cost-effectiveness. An ABI less than 0.90 confirms the diagnosis of PAD, while an ABI greater than 1.40 suggests noncompressible, calcified arteries, in which case the toe-brachial index (TBI) is used instead. The ABI has a sensitivity of 79 to 95% and specificity of 95 to 100% for angiographically confirmed PAD.

## Medical Risk Factor Optimization

Aggressive medical management forms the foundation of PAD treatment and is essential for all patients regardless of symptom severity or whether revascularization is performed. This approach significantly reduces cardiovascular morbidity and mortality, far more than any limb intervention.

Smoking cessation is the single most impactful intervention and includes counseling, nicotine replacement therapy, and pharmacologic aids such as varenicline or bupropion. Antiplatelet therapy with aspirin 81 mg or clopidogrel 75 mg daily is recommended to reduce thrombotic risk. Statin therapy should be high-intensity, using atorvastatin 40 to 80 mg or rosuvastatin 20 to 40 mg, targeting an LDL cholesterol level below 70 mg/dL. The Heart Protection Study demonstrated the benefit of statins in PAD patients.

Blood pressure control aims for a target below 130/80 mmHg, with angiotensin-converting enzyme (ACE) inhibitors preferred based on the HOPE trial, which showed that ramipril reduced cardiovascular events in PAD. Beta-blockers are not contraindicated in PAD, dispelling an old myth. Glucose control targets an HbA1c below 7% in most patients, with individualization in elderly or frail individuals. Additionally, the COMPASS trial supports the use of low-dose rivaroxaban (2.5 mg twice daily) combined with aspirin in high-risk PAD patients.

<image>Infographic-style illustration showing the natural history of peripheral arterial disease in 1000 patients presenting with claudication over 5 years: branching outcomes showing 70-80% stable/improved, 10-20% worsened claudication, 5-10% requiring intervention, 1-2% major amputation, but 15-30% cardiovascular death. Use a flow diagram with proportional arrows to convey the contrast between limb prognosis (relatively good) and cardiovascular prognosis (poor).</image>

<image>Side-by-side comparison diagram of the Fontaine and Rutherford classification systems for PAD severity. Show a leg illustration at each stage with corresponding clinical features: asymptomatic with abnormal ABI, claudication at various distances, rest pain (typically forefoot, worse with elevation, relieved by dependency), and tissue loss (ulceration, gangrene). Include ABI thresholds at each stage.</image>

<image>Medical illustration of the typical anatomic distribution of atherosclerotic disease in the lower extremity arterial system, showing common sites of occlusive disease: aortic bifurcation, common iliac, external iliac, common femoral bifurcation, superficial femoral artery at the adductor hiatus, popliteal trifurcation, and tibial vessels. Indicate that diabetic patients preferentially develop infrapopliteal disease. Include labeled anatomy from aorta to pedal arteries.</image>

## Key Clinical Pearls

Peripheral arterial disease is a coronary artery disease equivalent, meaning that the greatest threat to patients with PAD is cardiovascular death rather than limb loss. An ABI below 0.90 is diagnostic of PAD and independently predicts cardiovascular mortality even in patients without symptoms. Only 1 to 3% of patients with claudication progress to critical limb-threatening ischemia each year, with most limbs remaining stable under medical therapy. Smoking cessation is the most impactful intervention for improving both limb and cardiovascular outcomes. Contrary to outdated beliefs, beta-blockers are not contraindicated in PAD, a misconception disproven by multiple studies. Diabetic patients tend to develop disease preferentially in the infrapopliteal (tibial) vessels and often have neuropathy that masks ischemic symptoms, creating a particularly dangerous clinical scenario. Medical therapy—including antiplatelet agents, statins, antihypertensives, and smoking cessation—must be prescribed to all PAD patients regardless of whether they undergo revascularization. Finally, racial and socioeconomic disparities in amputation rates remain a critical issue in vascular surgery and require ongoing attention.

## References
- Fowkes FG et al. Comparison of global estimates of prevalence and risk factors for peripheral artery disease in 2000 and 2010: a systematic review and analysis. *Lancet*. 2013;382:1329-1340.
- Norgren L et al. Inter-society consensus for the management of peripheral arterial disease (TASC II). *J Vasc Surg*. 2007;45(Suppl S):S5-S67.
- Gerhard-Herman MD et al. 2016 AHA/ACC guideline on the management of patients with lower extremity peripheral artery disease. *Circulation*. 2017;135:e726-e779.
- Heart Protection Study Collaborative Group. MRC/BHF Heart Protection Study of cholesterol lowering with simvastatin in 20,536 high-risk individuals. *Lancet*. 2002;360:7-22.
- Yusuf S et al. Effects of an angiotensin-converting-enzyme inhibitor, ramipril, on cardiovascular events in high-risk patients (HOPE). *N Engl J Med*. 2000;342:145-153.