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Chapter question: There is a large cannula in this patient's femoral artery and they are sedated. How do I know the leg is alive, and what do I do the moment it isn't?

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Evidence search date: 10 September 2026.

Two canonical reviews were retrieved in full this session and carry most of the numbers below: the 2019 Critical Care narrative review of limb ischaemia in peripheral VA ECMO (DOI 10.1186/s13054-019-2541-3), and the 2024 Perfusion review of the evolution of distal limb perfusion management (DOI 10.1177/02676591241236650). Around them sit five meta-analyses and eight cohort studies identified through a structured search.

Primary sources: ELSO Red Book 6th edition Ch 26 and Ch 33; ISCCM Manual Ch 41; Taha Ch 7; ECPR and Resuscitative ECMO Ch 5.

This chapter's central finding is uncomfortable. Every meta-analysis of observational data says a prophylactic distal perfusion cannula prevents limb ischaemia. The largest and longest single-centre series say it does not — and they identify a different dominant risk factor altogether. §17.6 sets both out.

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What this chapter covers — and what it does not

17.1 Six ways a leg dies, and only two of them are the cannula

The standard mental model is mechanical: a big tube in a small artery obstructs flow. That is true and it is incomplete, and the incompleteness is why prophylactic hardware alone does not solve the problem.

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Physiology — the six mechanisms, and which interventions touch which

A 2024 narrative review sets out the mechanisms of limb ischaemia on ECMO as: arterial obstruction; cannulation injury; loss of pulsatile flow; thromboembolism; venous stasis from compressive obstruction by large venous cannulas; and systemic vasoconstriction from shock and from pharmacological vasoconstrictors.

Sort them by what fixes them and the shape of the problem changes:

Mechanical, arterial (1, 2) — the cannula occupies the lumen, or damaged it going in. A distal perfusion cannula addresses this, and cannula size and technique prevent it (Chapter 12).

Mechanical, venous (5) — the drainage cannula obstructs outflow. A distal perfusion cannula does nothing for this, and may make it worse by adding inflow to a limb that cannot drain (§17.7).

Haemodynamic (3, 6) — non-pulsatile flow and vasoconstriction. No cannula fixes these. They are treated by turning vasopressors down, and Chapter 13 has already established the lever that makes that possible on VA ECMO — flow is the vasopressor.

Thrombotic (4) — embolus or in-situ thrombosis in a low-flow segment. Treated by anticoagulation, and, on the emerging evidence in §17.4, possibly by antiplatelet therapy.

Only two of six mechanisms are addressed by the intervention that dominates the literature. Hold that thought through §17.6.

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Clinical pearl — the limb question differs at each end of the patient, and now at each end of the vessel

Chapter 12 established the first axis: femoral arterial return risks ischaemia below; axillary or subclavian return risks hyperperfusion above (§17.9). Chapter 16 added a reason to reach for the subclavian route, which means the trade is now made more often.

This chapter adds the second axis, within one leg: the artery can be obstructed, or the vein can be, and they present almost identically — a swollen, dusky, cool leg. The treatments are opposite. Putting more arterial inflow into a congested limb is the specific error that turns a swollen leg into a compartment syndrome.

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17.2 How often — and why nobody can tell you

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Evidence — an incidence range spanning a factor of seven

The 2019 narrative review reports limb ischaemia in "10 to 70%" of peripheral VA ECMO patients, and attributes the spread to "populations that are different in baseline characteristics, ECMO indications, cannulation techniques, limb ischemia definition."

Its specific cohorts show the same scatter: 8.6% (surgical cutdown with prophylactic distal perfusion), 12–33% despite a distal perfusion cannula, 14.7% in cardiogenic shock, 10.6% in ECPR. Downstream: fasciotomy in 8.3% of one cardiogenic shock cohort, and in 7 of 34 (21%) of the limb-ischaemia patients in another; amputation in 0.9% of one series and in 3 of 34 (8.8%) of limb-ischaemia patients.

The largest systematic estimate, a 2020 meta-analysis of 47 studies and 6,583 patients, pooled vascular complications overall at 29.5% (95% CI 23.6–35.9%) and limb ischaemia specifically at 12.6% (10.0–15.5%), with significant bleeding at 15.4% and cannula-site bleeding at 12.6%. Vascular complications were more than twice as likely on VA than VV support: OR 2.35 (1.87–2.96), p<0.0001.

A pooled analysis of 1,866 VA ECMO patients cited by the Red Book gives the most-quoted downstream figures: lower limb ischaemia 17%, compartment syndrome 10%, amputation 5%.

Certainty: low for any single figure; moderate for the ordering — ischaemia is common, compartment syndrome is a substantial minority of it, amputation is uncommon but not rare.

Why the numbers disagree is itself the clinically important point. There is no agreed definition of limb ischaemia on ECMO. A unit that records only limbs needing an operation will report 2%; a unit with a NIRS protocol that records every regional desaturation will report 30%. Both may be describing the same care. When comparing your unit's rate to a published one, compare the definitions first.

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17.3 This is not really a limb problem

The reason to take an early dusky foot seriously is not primarily the foot.

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Danger — the association is consistent, large, and almost certainly not all causal

Five independent cohorts agree that patients who develop limb ischaemia die more often, and the ten-year series shows the effect persisting to four years after successful decannulation, with an 80% relative increase in mortality hazard.

It is tempting to read this as "prevent the ischaemia and you save the patient." Resist that reading. Limb ischaemia is caused by the things that also kill these patients — profound shock, high vasopressor doses, peripheral vascular disease, diabetes, low cardiac output. It is at least partly a marker of severity rather than a mechanism of death. This is exactly the confounding Chapter 15 §15.8 identified in the rescue-unloading data, and it recurs here.

But the practical conclusion survives either interpretation, and it is not the obvious one. If limb ischaemia is a marker, it is a marker you can read at the bedside, hours before the lactate moves. A leg that is failing on adequate circuit flow is telling you something about the whole circulation — usually that the vasoconstriction is too high, the output too low, or the patient too vasoplegic to perfuse anything peripheral.

Treat a newly ischaemic leg as a systemic finding as well as a local one. Ask what the vasopressor dose is doing before you ask what the cannula is doing.

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17.4 Who is at risk — including two answers that surprise people