Define Acute Limb Ischaemia. Describe its causes and clinical features.

The Concept

Acute limb ischaemia is a sudden occlusion of the arterial supply to a limb, producing an acutely threatened limb — a surgical emergency. Unlike chronic ischaemia, there has been no time for collateral vessels to develop, so the tissue is rapidly deprived of blood and will become non-viable within a few hours if flow is not restored. Prompt recognition and revascularisation are what save the limb.

Causes — Embolism VS Thrombosis

The two main causes must be distinguished because they differ in management:

Other causes include trauma and aortic dissection.

Clinical Features — the 6 PS

The presentation is summarised by the 6 Ps: Pain, Pallor, Pulselessness, Paraesthesia, Paralysis, and Perishing cold (Poikilothermia). The last two — paraesthesia and paralysis — are ominous, indicating advanced ischaemia and a threatened, barely viable limb that demands the most urgent intervention.

Investigation & Management

It is largely a clinical diagnosis; a hand-held Doppler assesses flow, and angiography defines the anatomy if time permits, with an ECG to detect AF. Management is an emergency:

• Immediate measures — analgesia, oxygen, and intravenous heparin to prevent propagation of thrombus.
• Urgent revascularisation — surgical embolectomy (with a Fogarty balloon catheter) for an embolus, or thrombolysis, angioplasty or bypass for thrombosis, guided by the cause and limb viability.
• An irreversibly ischaemic limb (fixed mottling, muscle rigidity, paralysis) requires amputation.
• After revascularisation, watch for reperfusion injury — compartment syndrome (may need fasciotomy), hyperkalaemia, and myoglobinuria causing acute kidney injury.

The Importance of Distinguishing Embolus from Thrombosis

The embolus-versus-thrombosis distinction is not academic — it changes treatment. An embolus lodging in a previously healthy artery (a patient in AF with a suddenly white, pulseless leg and normal pulses elsewhere) is ideally treated by embolectomy with a Fogarty catheter. A thrombosis on chronic atherosclerotic disease (a patient with prior claudication and pad signs) has collaterals and diseased vessels, so it is often better managed by angiography with thrombolysis, angioplasty or bypass. Misreading the cause leads to the wrong operation.

Categories of Limb Viability

Management hinges on a rapid assessment of limb viability (the Rutherford categories). A viable limb (no sensory/motor loss, audible Doppler signals) allows time for imaging; a threatened limb (sensory loss, some muscle weakness, absent arterial Doppler) needs immediate revascularisation; an irreversibly ischaemic limb (profound sensory loss, paralysis, muscle rigidity, fixed skin mottling) is not salvageable and requires amputation. This triage decides speed and type of intervention.

Reperfusion Injury in Depth

Restoring flow to a severely ischaemic limb brings its own dangers, which must be anticipated. As blood returns, accumulated potassium, hydrogen ions and myoglobin are washed into the circulation, causing hyperkalaemia (cardiac arrhythmia), metabolic acidosis, and myoglobinuria that can precipitate acute kidney injury. Swelling of the reperfused muscle within its fascial compartment can cause a compartment syndrome needing fasciotomy. This is why revascularised patients are monitored closely and why very late revascularisation of a dead limb can be more dangerous than amputation.

A Note on the 'golden' Time Window

As with other ischaemic emergencies, there is a narrow window in which the limb can be salvaged: skeletal muscle tolerates warm ischaemia for only about 6 hours before irreversible damage, so acute limb ischaemia is treated with the same urgency as acute coronary or cerebral ischaemia. Delay converts a salvageable limb into a dead one requiring amputation, which is why immediate senior vascular involvement, heparinisation and prompt revascularisation are stressed the moment the diagnosis is suspected.

• 6 Ps: Pain, Pallor, Pulselessness, Paraesthesia, Paralysis, Perishing cold; paraesthesia/paralysis = threatened limb (urgent).
• Embolus (AF/MI, sudden, no history) vs thrombosis (claudication history, pad signs).
• Immediate IV heparin + urgent revascularisation (embolectomy/Fogarty for embolus); watch for reperfusion injury (hyperkalaemia, compartment syndrome).

Paralysis and paraesthesia signal a threatened, urgent limb.

• Acute limb ischaemia = sudden arterial occlusion → threatened limb (emergency; non-viable within hours, no collaterals).
• Causes: embolism (AF/MI, sudden, no claudication history) vs thrombosis (on plaque, claudication history/pad signs).
• 6 Ps: Pain, Pallor, Pulselessness, Paraesthesia, Paralysis, Perishing cold; paraesthesia/paralysis = advanced/threatened.
• Immediate: analgesia, oxygen, IV heparin; urgent revascularisation (embolectomy/Fogarty for embolus; thrombolysis/angioplasty/bypass for thrombosis).
• Irreversible (fixed mottling, rigidity) → amputation; watch for reperfusion injury (compartment syndrome, hyperkalaemia, AKI).