Distal perfusion cannulae (also known as backflow cannulae) are inserted percutaneously with real-time ultrasound guidance using an antegrade Seldinger technique. A 6-8 Fr, wire-reinforced sheath is used as the distal perfusion cannula. This size generally allows for antegrade flow down the leg, and retrograde flow into the area between the return cannula and the distal perfusion cannula, mitigating the risk of stasis and clot. Monitoring of the distal perfusion cannula however is a crucial part of ongoing care.
The cannula may be placed into the superficial femoral artery (SFA), ideally just distal to the bifurcation. The cannula should have a substantial length within the SFA and perfuses it. Care is taken to avoid accidental placement into the deep femoral artery (profunda femoris), which terminates in the thigh.
Useful techniques may include longitudinal / ‘in-plane’ imaging to identify the bifurcation of the common femoral artery (CFA) into SFA and profunda femoris.
Choice of distal perfusion cannula & circuit
Reinforced distal perfusion cannulae are used to avoid kinking (see equipment). The size of the cannula is adjusted according to the size of the arterial return cannula, accounting for higher return pressures in smaller arterial return cannulae. See the guide below. Instructions are also included in the ‘backflow pack’.
Distal perfusion circuit
The standard distal perfusion circuit set up is displayed below.
Key features are
- Reinforced distal perfusion cannula secured with a suture
- 3 mm Luer lock connection tubing (9 or 14cm) from ECMO circuit to sidearm of distal perfusion cannula
- Make sure the connection to the ECMO circuit is in a horizontal orientation to avoid kinking
- Foam under horizontal connection to prevent pressure injury
- Transparent dressing over tension-free tubing
The rationale for smaller 3mm vs ¼ inch tubing
Clotting of the distal perfusion circuit using ¼ inch tubing is relatively common but not with 3mm tubing. Clotting exposes the patient to potential leg ischaemia and is to be avoided. The key reason for clotting in the ¼ inch tubing is a low flow velocity of the blood and is demonstrated here.
Assuming a common volumetric flow rate of 250 ml/min in the distal perfusion circuit, the resulting blood velocity with ¼ inch and 3mm tubing are calculated. The inner diameter of the tubing determines the actual flow velocity – the actual distance that a blood cell would move forward in the tubing per second (velocity).
Example 250ml/min blood flow (volumetric flow rate)
- ¼ inch tubing: this results in mean flow velocity of 13 cm/s
- 3mm connector: this results in mean flow velocity of 47 cm/s
- In comparison, 3 L/min blood flow in an ECMO circuit with ⅜ inch tubing results in a blood flow velocity of ~72 cm/s
Conclusion: ¼ tubing in the distal perfusion circuit results in blood flow velocities as low as 10cm/s. In addition, turbulence from alternating diameters greatly increases the risk of clotting. Both can be avoided with smaller diameter 3mm tubing.
Is there not more blood flow through the larger circuit with ¼ inch tubing?
Both circuits roughly deliver very similar blood flow to the lower limb. Even though the tubing is bigger (resistance is lower), the diameter of the distal perfusion cannula, sidearm and the small 3 mm luer lock connections to the ECMO circuit contribute more to resistance and offset the smaller diameter. The 3 mm circuit is also shorter than the ¼ inch loop, hence the resistance is reduced.
Why is there a difference in circuit flow and flow through the SFA?
The flow in the distal perfusion circuit will be variable depending on the return pressure in the main circuit, the size of the distal perfusion cannula and the tubing used. This is commonly around 150-300 ml/min.
However, the single most important determinant for flow down the SFA is the peripheral vascular resistance in the lower limb. It is fairly common to have much lower flow rates in the SFA and retrograde flow bypassing of blood past the ECMO cannula as shown here.
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A two-operator technique (‘needler’ and ‘wirer’) is advocated to minimize any needle movement after accessing the artery. For right-handed operators, the ultrasound machine is placed on the patient’s right side and the procedure performed from the patient’s left, with the wirer standing to the right of the needler.
- Optimise the in-plane imaging of the bifurcation of the femoral artery
- If there is a clearer view of the SFA at a lesser depth a few centimeters distal this insertion point would be preferred to increase the likelihood of a first pass
- Using an echogenic needle without a syringe attached, the needler accesses the SFA via an in-plane (longitudinal) ultrasound-guided technique. The wirer continuously watches the end of the needle for a flashback of blood. (Rarely, successful puncture into the arterial lumen may not be accompanied by flashback.)
- Keep the needle still while the wirer passes the wire into the needle and antegrade into the SFA, ideally under ultrasound guidance
- The sheath dilator may be passed over the wire to pre-dilate the tract. After inserting the sheath over the wire, the sheath sidearm is aspirated and flushed with saline.
- Alternate techniques include use of in-plane ultrasound guidance of the needle until it is adjacent to the artery, then transition to out-of-plane (transverse) technique for vascular puncture.
Particularly when a distal perfusion cannula is placed as a delayed procedure, obtaining clear ultrasound identification of the bifurcation of the CFA can be challenging, posing the risk of misplacement of the distal perfusion cannula. The distal perfusion cannula may be flushed with saline, while observing transient increased flow in the popliteal artery with colour Doppler, as an adjunctive localisation technique. In case of any uncertainty, early formal vascular ultrasound should be obtained. If the insertion appears easier some centimeters distal from the bifurcation this is preferred, rather than a difficult insertion closer to the bifurcation.
A short length of extension tubing is attached to the sheath sidearm, primed using saline via the sidearm 3-way tap, and the cap on the extension tubing is removed (Small diameter tubing is preferred to increase flow velocity and reduce the risk of stasis and clotting).
After verbal rehearsal, clamps are applied upstream and downstream from the side port of the arterial cannula. Simultaneously, a console operator applies a clamp to the circuit and reduces pump speed to 1500 rpm. The patient is off VA ECMO support for a short period (usually <10sec). The arterial cannula side port cap is removed and the side port filled with saline from a syringe (if necessary) – alternatively, back bleeding from the sidearm can be used as ‘underwater’ seal. The distal perfusion cannula is attached to the side port of the arterial cannula, the upstream and downstream clamps are removed and the ECMO flow re-established.
Note: if the distal perfusion cannula is connected at the time of cannulation before going on support, it is required to keep a clamp proximal to the connection on the arterial cannula.