Acute Limb Ischaemia
MRCS Part A & B — Comprehensive Study Notes
The 6 Ps of Acute Limb Ischaemia
Acute limb ischaemia is a vascular emergency presenting with sudden deterioration in limb perfusion threatening limb viability. Recognition is based on the classic “6 Ps,” which represent progressive stages of ischaemia from earliest to latest signs.
Pain
Severe, constant pain that is out of proportion to the clinical findings. Initially localised to the ischaemic muscle beds (calf, thigh, foot depending on level of occlusion), but may become global with advancing ischaemia. Pain at rest, worsened by passive stretch of muscles, indicates critical ischaemia.
Pallor
Pale appearance of the affected limb due to absent arterial flow. Initially evident distally (toe, foot); with advancing ischaemia, pallor extends proximally. Mottled appearance (blue-purple patches) indicates venous stasis—a sign of critical ischaemia.
Pulselessness
Absent pulses distal to the site of arterial occlusion. This is an objective finding and the most specific sign of acute arterial occlusion. Pulses must be compared bilaterally; absence in both limbs suggests aortic saddle embolus.
Paraesthesia
Sensory changes—numbness, “pins and needles,” tingling—appear early because nerves are exquisitely sensitive to ischaemia. Sensory loss over the dorsum of the foot (territory of superficial peroneal nerve) and lateral foot indicates ischaemia of the deep peroneal and sural nerve territories. This is an EARLY sign, indicating incomplete ischaemia requiring urgent but not necessarily emergent treatment.
Paralysis
Motor loss—initially weakness, progressing to complete paralysis of muscles in the ischaemic distribution. Indicates LATE stage ischaemia. Complete motor loss (patient cannot dorsiflex foot = cannot recruit tibialis anterior) indicates that the limb has been ischaemic for hours and tissue necrosis is near-irreversible.
Perishing Cold
The affected limb is markedly cool or cold compared to the contralateral limb. Objective assessment: temperature difference >5°C compared to the other limb indicates significant ischaemia. This occurs because the ischaemic limb is not perfused with warm arterial blood.
Distinguishing Complete vs. Incomplete Ischaemia
The presence or absence of sensory and motor loss guides treatment urgency. Incomplete ischaemia (sensory loss but preserved motor function, or vice versa) indicates some residual collateral perfusion and the limb may survive even with delayed revascularisation, though tissue loss may occur. Complete ischaemia (both sensory and motor loss) indicates no effective collateral circulation and the limb is approaching non-viability—revascularisation must be performed emergently to have any chance of limb salvage. A completely sensate and motor-intact limb despite absent pulses indicates good collateral perfusion and more time for investigation and planning.
Immediate Assessment and Investigations
After clinical examination documenting the 6 Ps, immediate investigations are:
- Full blood count: Baseline haemoglobin; thrombosis risk assessment
- Coagulation profile (PT/INR, APTT, fibrinogen): To guide anticoagulation
- Renal function and electrolytes: Baseline assessment; important for contrast nephropathy risk if planning CTA
- Creatine kinase (CK): Baseline; serial CK measurements useful for monitoring muscle necrosis if reperfusion injury develops
- ECG: To identify atrial fibrillation (cardiac source of embolus), acute MI (wall motion abnormality source)
- Echocardiography: Transthoracic echo (or transoesophageal echo) to identify cardiac source of thromboembolism (AF with left atrial thrombus, recent MI with mural thrombus, prosthetic valve, atrial myxoma)
- Urgent vascular imaging: Duplex ultrasound (immediately available, no radiation, can assess patency of proximal and distal vessels) or CT angiography (requires transport, radiation, but provides full arterial tree anatomy)