Prevention of lower limb ischaemia
Distal perfusion cannula or “Backflow Cannula” are required in the presence of VA femoral ECMO return cannula to maintain safe blood flow to the leg. They are connected to the arterial (return) limb of the VA ECMO circuit and provide antegrade perfusion of the ipsilateral superficial femoral artery (SFA) and hence the leg. A distal perfusion cannula is routinely placed for patients receiving femoral VA ECMO support to prevent leg ischaemia.
In emergent circumstances (e.g. ECPR) a distal perfusion cannula can be placed after initiation of femoral VA ECMO support and completion of other urgent procedures such as coronary angiography and trauma radiological investigations and interventions.
For patients with cardiogenic shock, it is easier to place guidewires for arterial and distal perfusion cannula insertion at the time of ECMO initiation. The distal perfusion cannula is then inserted and connected to the circuit after the initiation of VA ECMO.
In any case, a distal perfusion cannula should be placed as early as feasible, within 6 hours of initiation of femoral VA ECMO support (provided adequate leg perfusion clinically). If a distal perfusion cannula is unable to be placed percutaneously the vascular surgical team should be engaged for surgical placement. If VA ECMO support is initiated at another centre without a distal perfusion cannula, this should be inserted as soon as possible after transfer or before transfer if there are signs of distal leg ischaemia.
Adequate leg perfusion should never delay insertion.
Monitoring of DPC
Hourly leg vascular observations are performed by the bedside nurse, noting:
- Presence or absence of dorsalis pedis and posterior tibial pulses by palpation or Doppler assessment (the site of the loudest Doppler signal should be marked on the patient with a cross)
- Flow in the distal perfusion circuit (one of the options below)
Visual inspection of the distal perfusion circuit for kinking and/or clot should occur each hour. Lack of flow in the circuit is identified with certainty by separation of the blood into sedimentary and plasma phases within minutes.
Options to demonstrate flow in distal perfusion circuit
- Doppler signal from handheld devices – easiest (see image below)
- Visualisation of blood flow at the connection hub (see image below)
Illumination and visual inspection
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Additional assessment if suspicion of ischaemia arises should include:
- Palpation of both legs for compartment syndrome
- Sensorimotor function and pain
- Ischaemic contractures with rigidity are a late sign
Comparison of both sides is recommended, CK and lactate may be measured. As clinical vascular assessment is often hampered by shock, high dose vasoactives, poor cardiac pulsatility, and sedation, a low threshold for obtaining vascular ultrasound is recommended.
Medical staff must be contacted immediately if the distal perfusion circuit blood flow stops or if there are any concerns about the adequacy of perfusion to the leg.
Monitoring tissue oxygenation can not replace clinical assessment and ensuring adequate placement of the distal perfusion cannula. However, it may be used in addition if there is an ongoing concern regarding the adequacy of the limb perfusion.
The Vascular Ultrasound Laboratory performs formal scans confirming the correct placement of the distal perfusion cannula, adequate SFA volume flow (typically >100 ml/min, or comparable to the contralateral side) and runoff into the major arteries of the leg. A formal ultrasound scan is performed on the day of distal perfusion cannula insertion, except where distal perfusion cannulae are placed out of hours (see below). Repeat sonography may be required as clinically indicated.
Ultrasound out of hours
Where percutaneous placement of a distal perfusion cannula is straightforward and there are no concerns of ischaemia, the vascular ultrasound can be deferred to the next day.
However, if there is
- Any concern of complications or ischaemia, or
- Distal perfusion cannula is placed in the operating theatre or outside the Alfred, or
- There is significant arterial cannula site bleeding
the vascular registrar is contacted to organise a formal ultrasound overnight.
Complications of distal perfusion cannulae commonly occur within the first 24 hours, although later complications are also seen.
Concern should prompt assessment including:
- Access, return and distal perfusion cannula insertion sites: haematoma or swelling may suggest pseudoaneurysm, cannula migration or dislodgement
- Distal perfusion cannula extension tubing: flow, clotting
- Lower limb vascular examination: colour, warmth, pulses (by palpation or Doppler); sensorimotor function and pain where sedation permits
- Thigh and calf palpation: compartment syndrome
If ischaemia is suspected, urgent vascular surgical consult and vascular ultrasound should be obtained. Depending on the circumstances, required interventions may include
- Change of distal perfusion extension tubing
- Change of distal perfusion cannula
- On-table angiography / embolectomy / thrombectomy / vascular repair / endarterectomy / bypass
- Fasciotomy / amputation
Whilst awaiting urgent, definitive management, consider:
- Increasing O2 flux via increased ECMO flow, fresh gas flow, and MAP, with optimisation of Hb
- Decreasing vasopressors
- Keep limb at right heart level
Clotting rarely occurs in 3mm tubing extension and if it does, the distal perfusion circuit should be scrutinized for a mechanical problem such as kinked tubing. In order to avoid any kinking, the distal perfusion circuit is taped horizontally to the skin with relaxed angles and attention to pressure care. If the clotting is confined to the extension tubing, this can be changed and the distal perfusion cannula aspirated to ensure patency. A very brief period off VA ECMO support is required to perform the change of the connection.
Decannulation is performed in the operating theatre. This affords the opportunity for arterial repair (e.g. patch angioplasty and / or thrombectomy), which is required in up to 30% of cases.
Peripheral tissue oxygen monitor
Near infrared spectroscopy (NIRS) is used in many centres for cerebral monitoring of patients undergoing cardiopulmonary bypass for cardiac surgery, while its role in monitoring of limb perfusion and/or cerebral perfusion in ECMO is less established.
NIRS may be considered in high risk settings for ischaemia
- Younger age (smaller CFA, poor collateral development)
- Severe PVD
- Expected long duration of ECMO
- Access and return cannulae in same groin
- Technical difficulties with distal perfusion cannula placement
When used, NIRS pads should be applied bilaterally to the mid-calf early post VA ECMO initiation to obtain baseline values. Suggested thresholds for clinical concern have varied between studies and manufacturers. In a study of the INVOS system (Coviden Somatics) an rSO2 < 40%, or more than 25% below baseline, was considered significant (Wong, Smith et al, 2012). In a small study of the Foresight NIRS system (Casmed) only patients who suffered a limb complication had a between-leg difference in stO2 > 35% (Steffen, Sale et al, 2014).
Factors in ICU that will influence rSO2
- State of peripheral perfusion, shock states
- Mottling of the skin
- Level of vasopressor support
- Lower limb oedema
- Incorrect placement in a different site in relation to the patient’s knee on the lower limb
Thresholds for medical review
- There is an automatic alarm for rSO2 levels 20% below the baseline rSO2 value, the bedside nurse performs a routine check of the distal perfusion cannula, tubing flow and clinically of the limb. If satisfied and none of the other thresholds are met, the baseline can be reset by nursing staff without medical review.
- Absolute rSO2 value differs by >10 between the left and right sensor, the nurse must initiate a medical review
- Should the rSO2 value drop below an absolute rSO2 value <40, this requires a medical review.
Ensure the readings are correct by checking position and contact of the sensors primarily. They need to be at the same level of the leg and in an equivalent position on the circumference of the leg. Very low readings may occur with very distal placement on the leg. If after the medical assessment there is ongoing concern regarding the perfusion, the next steps require ultrasound assessment and referral to the vascular team.