2.1 Vascular Assessment: ABI, TcPO2, and Arterial/Venous Doppler

Vascular Assessment: ABI, TcPO2, and Arterial/Venous Doppler

Lower extremity wound management requires accurate baseline vascular assessment prior to initiating sharp debridement, applying compression therapy, or selecting advanced wound care modalities. Inadequate arterial perfusion is a primary cause of stalled wound healing, tissue necrosis, and unperceived ischemic damage when compression is inappropriately applied. A structured vascular workup combines non-invasive bedside tests—such as the Ankle-Brachial Index (ABI) and Toe-Brachial Index (TBI)—with microvascular oxygenation measurements (TcPO2) and acoustic arterial/venous Doppler waveform analysis.

---

Ankle-Brachial Index (ABI) Protocol & Interpretation

The Ankle-Brachial Index is the standard non-invasive bedside screening tool for identifying lower extremity peripheral artery disease (PAD). It calculates the ratio of the highest ankle systolic blood pressure to the highest brachial systolic blood pressure.

Clinical Protocol for ABI Measurement

1. Patient Preparation: Position the patient flat and supine in a warm environment for at least 10 minutes prior to testing to stabilize blood pressure and prevent cold-induced peripheral vasospasm.
2. Brachial Pressure Measurement: Apply an appropriately sized blood pressure cuff to the upper arm. Place a handheld 8 MHz Doppler ultrasound probe over the brachial artery at a 45- to 60-degree angle facing blood flow. Inflate the cuff 20 to 30 mmHg above the disappearance of the signal, then deflate at 2 to 3 mmHg/sec. Record the systolic pressure for both left and right brachial arteries.
3. Ankle Pressure Measurement: Place the cuff immediately superior to the malleoli. Use the Doppler probe to locate and measure systolic pressures in both lower extremity arteries for each limb:
   • Dorsalis Pedis (DP) Artery: Located on the dorsum of the foot between the 1st and 2nd metatarsal bases.
   • Posterior Tibial (PT) Artery: Located posterior to the medial malleolus.
4. ABI Calculation Formula:

ABI (Right Limb) = Highest Right Ankle Pressure (DP or PT) / Highest Brachial Pressure (Right or Left)

ABI (Left Limb) = Highest Left Ankle Pressure (DP or PT) / Highest Brachial Pressure (Right or Left)

Important: Always use the single highest brachial pressure (left or right arm) as the denominator for both limb calculations.

---

Diagnostic Interpretation & Clinical Action

---

Non-Compressible Vessels & Toe-Brachial Index (TBI)

In patients with long-standing diabetes mellitus, end-stage renal disease (ESRD), or advanced age, tunica media calcification (Mönckeberg's sclerosis) renders lower extremity arteries stiff and non-compressible. In these cases, the blood pressure cuff cannot occlude the tibial arteries, yielding falsely elevated ABI readings (>1.30).

Because digital arteries in the toes rarely undergo medial arterial calcification, the Toe-Brachial Index (TBI) provides an accurate measure of distal perfusion:

• Procedure: A miniature digital cuff is placed around the proximal phalanx of the great toe, and a photoplethysmography (PPG) or 10 MHz Doppler sensor detects digital blood flow.
• Diagnostic Cutoffs:
  • Normal TBI: >= 0.70
  • Abnormal TBI: < 0.70, confirming peripheral arterial insufficiency.
  • Absolute Toe Pressure: An absolute toe systolic pressure < 30 mmHg indicates severe ischemia and severely impaired wound healing potential.

---

Transcutaneous Oxygen Measurement (TcPO2)

Transcutaneous Oximetry (TcPO2 or TcPCO2) measures the partial pressure of oxygen diffusing through the dermis, evaluating microvascular delivery directly at the periwound site.

Clinical Utility & Testing Conditions

• Electrodes heated to 43°C to 45°C are placed on intact, non-edematous periwound skin (avoiding bony prominences, superficial veins, and necrotic tissue).
• Interpretation Cutoffs:
  • > 40 mmHg: Normal microvascular tissue oxygenation; excellent healing capacity.
  • 30 – 39 mmHg: Borderline oxygenation; wound healing may be delayed; advanced therapies may be needed.
  • < 30 mmHg: Impaired tissue perfusion; compromised healing potential.
  • < 20 mmHg: Severe microvascular ischemia; spontaneous healing is unlikely without surgical revascularization.
• Hyperoxic Challenge: Administering 100% supplemental oxygen for 10 minutes helps distinguish fixed structural arterial occlusion (TcPO2 remains low) from reversible microvascular vasospasm or local tissue edema (TcPO2 increases by > 10 mmHg).

---

Acoustic Doppler Waveform Analysis

Using a handheld continuous-wave Doppler probe, clinicians evaluate arterial sound quality and flow dynamics:

1. Triphasic Waveform (Normal): High-resistance peripheral arterial flow consisting of three distinct sounds:
   • Rapid forward systolic rise
   • Transient diastolic signal reversal
   • Small late diastolic forward movement
2. Biphasic Waveform (Mild/Moderate Alteration): Sound retains systolic flow and early diastolic reversal, but loses late diastolic forward flow. Can be normal in older adults or indicate early arterial damping.
3. Monophasic Waveform (Severe Occlusive Disease): Single, slow, low-pitched, continuous sound with slow systolic upstroke and absence of diastolic reversal. Indicates severe proximal stenosis or occlusion with collateral circulation.

Venous Doppler Assessment

Venous Doppler evaluation tests superficial and deep vein competence. Retrograde flow exceeding >0.5 seconds in superficial veins or >1.0 second in deep veins following manual calf compression and release confirms venous valvular reflux and venous insufficiency.