Vascular Investigations
MRCS Part A & B — Comprehensive Study Notes
Ankle-Brachial Pressure Index (ABPI)
The ABPI is the ratio of the highest ankle systolic pressure (measured at the dorsalis pedis or posterior tibial artery) to the highest brachial systolic pressure. It is the first-line investigation for peripheral arterial disease.
Technique
- Patient supine and rested for 5–10 minutes
- Handheld Doppler probe (5–10 MHz) with conductive gel
- Standard sphygmomanometer cuff placed above the ankle
- Inflate cuff above systolic, then slowly deflate — record pressure at which Doppler signal returns
- Measure both DP and PT at each ankle; use the higher of the two
- Measure brachial pressure bilaterally; use the higher brachial pressure as denominator
- Formula: ABPI = Ankle systolic pressure ÷ Brachial systolic pressure
| ABPI Value | Interpretation | Clinical Significance |
|---|---|---|
| >1.2 | Falsely elevated — vessel incompressibility | Medial calcification (Mönckeberg sclerosis); common in diabetics, renal failure. Use toe-brachial index instead. |
| 0.9–1.2 | Normal | No significant arterial occlusion |
| 0.8–0.9 | Borderline / mild PAD | May be symptomatic; risk factor modification; exercise ABPI if symptomatic |
| 0.5–0.8 | Moderate PAD | Intermittent claudication; conservative or interventional management |
| 0.4–0.5 | Severe PAD | Often claudication with possible rest pain |
| <0.4 | Critical limb ischaemia (CLI) | Rest pain, ulceration, gangrene; urgent revascularisation or amputation |
💡 Compression Bandaging & ABPI
Four-layer compression bandaging is the gold-standard treatment for venous leg ulcers but is contraindicated if ABPI <0.8 (risk of pressure necrosis in arterially compromised limb). ABPI 0.6–0.8 → modified (reduced) compression may be used with close monitoring. ABPI <0.6 → compression absolutely contraindicated.
Exercise (Treadmill) ABPI
A resting ABPI may be normal or borderline in patients with genuine claudication (e.g., ABPI 0.85 at rest in a patient with severe exertional symptoms). Exercise ABPI confirms the diagnosis.
- Resting ABPI measured, then patient walks on treadmill (standard: 3.2 km/h, 10% gradient) until claudication pain occurs
- Immediately post-exercise ABPI is measured
- Positive result: ABPI drops by >0.15–0.2 post-exercise and takes >3 minutes to recover
- In normal subjects, ABPI may rise slightly after exercise due to peripheral vasodilation
Toe-Brachial Index (TBI)
Used when ABPI is unreliable due to medial calcification (diabetes, renal failure, elderly). Digital arteries are less affected by calcification.
- Photoplethysmography (PPG) sensor placed on the big toe
- Small digital pressure cuff applied at the base of the toe
- Normal TBI: ≥0.7
- TBI <0.4: suggests critical ischaemia
Segmental Limb Pressure Measurements
Pressure cuffs placed at multiple levels (thigh, above knee, below knee, ankle) localise the level of disease by identifying a drop of >20–30 mmHg between adjacent segments.
- Aortoiliac disease: low thigh pressure
- Femoropopliteal disease: pressure drop between thigh and above-knee cuff
- Infrapopliteal disease: drop below the knee
Transcutaneous Oxygen Pressure (TcPO₂)
Measures oxygen tension at the skin surface using a heated electrode. Reflects tissue viability and perfusion status. Useful for:
- Predicting wound healing potential after amputation — TcPO₂ >40 mmHg → likely to heal; <20 mmHg → unlikely to heal
- Assessing critical limb ischaemia when ABPI is unreliable
- Monitoring response to revascularisation