# Critical Limb-Threatening Ischemia: Definition, Assessment, and the WIfI Classification

## Introduction

Critical limb-threatening ischemia (CLTI) represents the most advanced stage of peripheral arterial disease (PAD) and is associated with a significant risk of major amputation, cardiovascular morbidity, and mortality. The term CLTI has replaced the older designation "critical limb ischemia" (CLI) to emphasize the imminent threat to limb viability and the urgent need for evaluation and intervention. To guide clinical decision-making, standardized classification systems have been developed, with the WIfI (Wound, Ischemia, and foot Infection) system being the most widely used.

## Definition and Epidemiology

CLTI is defined by the presence of ischemic rest pain, gangrene, or non-healing ulceration that is attributable to objectively proven arterial occlusive disease and persists for more than two weeks. Hemodynamic criteria supporting this diagnosis include an ankle pressure less than 50 mmHg, a toe pressure less than 30 mmHg, or a transcutaneous oxygen pressure (TcPO2) less than 30 mmHg. CLTI affects approximately 1% to 2% of patients with PAD. Without revascularization, the one-year major amputation rate ranges from 25% to 40%, while the one-year mortality rate is approximately 20% to 25%, reflecting the systemic burden of atherosclerosis in these patients.

## Clinical Assessment

### History and Physical Examination

Patients with CLTI typically report rest pain, which is often described as a burning sensation localized to the forefoot and toes. This pain tends to worsen at night and is relieved by placing the limb in a dependent position. Tissue loss may manifest as non-healing ulcers, gangrene (either dry or wet), or necrosis, usually involving the toes and forefoot. A thorough history should include prior vascular interventions, the presence of diabetes or renal disease, smoking status, and assessment of functional capacity. Physical examination focuses on evaluating pedal pulses, capillary refill time, skin temperature, and the presence of dependent rubor.

### Hemodynamic Evaluation

Hemodynamic assessment involves measuring the ankle-brachial index (ABI), toe-brachial index (TBI), and pulse volume recordings (PVR) according to established physiologic testing protocols. Toe pressures are considered more reliable than ABI in diabetic patients due to arterial calcification that can falsely elevate ABI values. Transcutaneous oxygen pressure (TcPO2) measurements provide valuable information about wound healing potential and help guide the appropriate level of amputation if necessary.

### Anatomic Imaging

Duplex ultrasound serves as the first-line imaging modality for mapping arterial anatomy. Computed tomography angiography (CTA) offers detailed anatomic assessment extending from the aorta to the pedal vessels. Magnetic resonance angiography (MRA) is an alternative for patients with contrast allergies or renal insufficiency. Catheter-based angiography is generally reserved for cases where intervention is planned or when noninvasive imaging results are inconclusive.

![Clinical photograph of a patient with CLTI showing forefoot gangrene and non-healing ulceration](images/clti-forefoot-gangrene.jpg)

## The WIfI Classification System

### Overview

The WIfI classification system was developed by the Society for Vascular Surgery to standardize the assessment of threatened limbs. It integrates three independent domains: Wound (W), Ischemia (I), and foot Infection (fI). Each domain is graded on a scale from 0 (none) to 3 (severe). The composite score derived from these grades predicts the risk of amputation, the potential benefit from revascularization, and the likelihood of wound healing.

### Wound (W) Grade

The wound grade ranges from 0 to 3. Grade 0 indicates no ulcer or gangrene. Grade 1 corresponds to a small, shallow ulcer located on the distal leg or foot without gangrene. Grade 2 involves a deeper ulcer with exposed bone, joint, or tendon, or gangrene limited to the toes. Grade 3 represents extensive, deep ulcers or extensive gangrene involving the forefoot or midfoot.

### Ischemia (I) Grade

Ischemia is graded based on ABI, ankle pressure, and toe pressure or TcPO2 values. Grade 0 ischemia is defined by an ABI of 0.80 or greater, ankle pressure of 100 mmHg or greater, and toe pressure or TcPO2 of 60 mmHg or greater. Grade 1 corresponds to an ABI between 0.60 and 0.79, ankle pressure between 70 and 99 mmHg, and toe pressure or TcPO2 between 40 and 59 mmHg. Grade 2 ischemia includes an ABI of 0.40 to 0.59, ankle pressure of 50 to 69 mmHg, and toe pressure or TcPO2 of 30 to 39 mmHg. Grade 3 ischemia is characterized by an ABI less than 0.40, ankle pressure less than 50 mmHg, and toe pressure or TcPO2 less than 30 mmHg.

| WIfI Domain | Grade 0 | Grade 1 | Grade 2 | Grade 3 |
|-------------|---------|---------|---------|---------|
| **Wound** | No ulcer/gangrene | Small, shallow ulcer | Deep ulcer (bone/tendon exposed); toe gangrene | Extensive deep ulcer; forefoot/midfoot gangrene |
| **Ischemia** (ABI) | ≥0.80 | 0.60–0.79 | 0.40–0.59 | <0.40 |
| **Ischemia** (Ankle P) | ≥100 mmHg | 70–99 mmHg | 50–69 mmHg | <50 mmHg |
| **Ischemia** (Toe P/TcPO2) | ≥60 mmHg | 40–59 mmHg | 30–39 mmHg | <30 mmHg |
| **foot Infection** | No infection | Mild (skin/subcut; erythema <2 cm) | Moderate (deeper tissue; erythema >2 cm; abscess; osteo) | Severe (SIRS/sepsis) |

### Foot Infection (fI) Grade

Foot infection is graded from 0 to 3. Grade 0 indicates no infection. Grade 1 represents a mild infection limited to the skin and subcutaneous tissue with erythema less than 2 cm. Grade 2 is a moderate infection involving deeper tissues, characterized by erythema greater than 2 cm, abscess formation, osteomyelitis, or septic arthritis. Grade 3 denotes a severe infection associated with systemic inflammatory response syndrome (SIRS) or sepsis.

![WIfI classification grid showing amputation risk stratification based on composite scoring](images/wifi-classification-grid.jpg)

![Algorithm for CLTI management incorporating WIfI staging and revascularization planning](images/clti-management-algorithm.jpg)

## Clinical Application of WIfI

The WIfI classification guides management decisions based on amputation risk. Patients with low amputation risk (stage 1) may be managed with medical therapy, wound care, and close surveillance. Those with moderate amputation risk (stages 2 to 3) should strongly consider revascularization. High amputation risk (stage 4) warrants urgent revascularization if anatomically feasible. For very high-risk patients (stage 5), revascularization with staged procedures is recommended, although primary amputation may be appropriate if reconstruction is not possible. Additionally, WIfI predicts the expected clinical benefit of revascularization, helping clinicians decide between intervention and primary amputation.

## Management Principles

Management of CLTI requires a multidisciplinary team approach involving vascular surgery, podiatry, endocrinology, infectious disease, and wound care specialists. Cardiovascular risk reduction is essential and includes antiplatelet therapy, statins, smoking cessation, and optimization of blood pressure and glucose control. The choice between endovascular and open revascularization strategies is guided by anatomic considerations (such as the GLASS classification), comorbidities, and conduit availability. Infection control involves appropriate antibiotic therapy and timely surgical debridement. Wound management incorporates offloading, moist wound care, and negative pressure wound therapy when indicated.

## Key Clinical Pearls

CLTI is a clinical diagnosis that requires both symptoms or tissue loss and objective hemodynamic evidence of severe ischemia. The WIfI classification is the current standard for staging threatened limbs and should be documented for every patient with CLTI. The benefit of revascularization is greatest in patients with higher WIfI ischemia grades and manageable wound and infection burdens. Toe pressures are more reliable than ABI in the diabetic population and should always be obtained. Finally, the one-year mortality rate in CLTI rivals that of many malignancies, underscoring the importance of aggressive cardiovascular risk reduction.

## References

1. Mills JL, Conte MS, et al. "The Society for Vascular Surgery lower extremity threatened limb classification system: risk stratification based on Wound, Ischemia, and foot Infection (WIfI)." *J Vasc Surg*. 2014;59(1):220-234.  
2. Conte MS, Bradbury AW, et al. "Global vascular guidelines on the management of chronic limb-threatening ischemia." *J Vasc Surg*. 2019;69(6S):3S-125S.  
3. Defined, Defined, et al. "Validation of the SVS WIfI classification system in a large cohort." *J Vasc Surg*. 2017;65(4):1086-1095.  
4. Norgren L, Hiatt WR, et al. "Inter-society consensus for the management of peripheral arterial disease (TASC II)." *J Vasc Surg*. 2007;45(Suppl S):S5-S67.