A cannula entirely located in the fat in a patient for whom ultrasound was not used to locate the vessel. Surprisingly, the drainage from the haematoma was sufficient to maintain blood flow through the ECMO circuit for at least 1 day.
Clinicians must be aware of the benefits and limitations of using fluoroscopic (X-ray) guidance when inserting a cannula, and be cognisant of the regulations surrounding the use of fluoroscopy in the clinical environment.
While a clinician does not need to be a surgeon to insert an ECMO cannula, vascular and/or cardiothoracic surgeons should be readily available to deal with vascular injuries. The clinician inserting the ECMO cannula must have the knowledge and skill sets to be able to stabilize a patient having suffered a major vascular injury while surgical input is awaited.
Only trained cardiothoracic surgeons should insert cannulas under direct vision in an open chest.
Where should cannulation take place?
The ideal location to insert the ECMO cannula is in the operating theatre, with the patient positioned on an operating table.
An operating room provides plenty of space. Operating department staff are used to sterile procedures and managing critically ill patients and emergencies. Anaesthetic support allows the clinician inserting the cannula to concentrate on the task at hand, while another highly trained professional ensures that the patient is appropriately managed. It is easier to deal with complications in the operating room, particularly when a surgical intervention is required.
Most operating rooms will provide fluoroscopy (X-ray guidance) if required, and specialist centres will usually have ultrasonography and echocardiography readily available.
Operating rooms upgraded with high-end imaging facilities are ideal; these are hybrid catheter laboratory/operating rooms and are set up to allow complex surgery.
It is possible, but not ideal, to cannulate patients while in their bed or on a trolley, and therefore in any location, including the ICU or emergency room. These locations do not offer the same support. For example, space constraints in the ICU might not permit the use of fluoroscopy. This may be due to lack of shielding and protection of other patients and staff, or the standard catheter laboratory may be inconvenient as the C-arm is not ergonomically positioned to provide full access to the patient’s neck.
In the absence of appropriate facilities, the clinician has to weigh up the risk of complications versus the benefits of insertion; this applies mainly for emergency (cardiac arrest) or out-of-hospital insertion. ECMO candidates are usually extremely ill, and many intensive care clinicians claim that these patients are too ill to be transported safely to the operating room. We usually challenge this assumption on the basis of the greater benefit of a safer insertion.
Cannula choice
Main features of a cannula
Cannula choice has to focus on maximizing flow while causing minimal damage to the blood. A generic cannula and some of its features are shown in Figure 6.2.
An ECMO cannula.
Materials
Flexibility and consistency of shape are influenced by the material of the cannula.
Flexible cannulas are more difficult to insert but can adjust to the patient’s anatomy and cause less tissue damage. Too flexible a cannula may kink or collapse, impeding the flow or causing turbulence.
Polyurethane is used in the manufacture of most cannula. It has high material strength at room temperature but is more malleable at body temperature.
Wire reinforcement of the cannula walls will reinforce specific components and prevent kinking or collapse. This enables mobilization of the patient. Radio-opaque materials allow confirmation of correct positioning.
Surface coating
Blood interacting with artificial surfaces activates the coagulation and inflammatory cascades. Coating the surface of the cannula is essential to prevent fibrin sheath and thrombus formation. A small thrombus can have a significant effect on flow.
Modern cannulas feature biocompatible coatings that reduce activation of the clotting cascade. Heparin-coated surfaces are most commonly used and result in reduced inflammatory activation.
Alternatives have been proposed but none yet are as efficient as heparin. However, these are valuable in the context of heparin-induced thrombocytopenia.
Length
The cannula length is often determined by the intended circuit: it is configured short in central access as it is directly in the main vessels or cavities, whereas it is long in a peripheral cannula to reach the intended central location, such as the right atrium for drainage or return.
Shape
The shape of the cannula influences its flow characteristics. Changes in cross-section (tapering), a non-circular cross-section, bends or other irregular shapes can have dramatic effects on flow. There are significant differences between arterial and venous cannulas due to the different physiological requirements.
Venous cannulas need to support high enough drainage flows to sustain adequate support with relatively low negative drainage pressures so that the vessel does not collapse. The collapse of the vena cava with increased drainage suction will hinder drainage. This concept makes the venous cannula diameter the limiting factor for overall flow. The large-capacity veins allow larger-diameter venous cannulas to be inserted. Venous cannulas commonly include side holes to improve drainage.
Arterial cannulas are significantly narrower due to the vessel size, and significantly higher pressures have to be applied for adequate flow. The arterial cannulas provide a large resistance within the ECMO circuit and therefore create a pressure drop across it. This high-pressure flow becomes turbulent at the step-wise increase in flow diameter when exiting the cannula into the artery and forms a jet. This arterial return jet in veno-arterial ECMO can cause a stroke when athero-emboli on the arterial wall are loosened. Potential vessel damage is reduced by specific design of the arterial cannula tip into a ‘diffuser tip’, where the return jet widens (and therefore slows down), and by the addition of side holes returning blood into the aorta.
Peripheral arterial cannulas are not only significantly longer but also have a smaller diameter throughout, as the peripheral vessel at the insertion point is narrow. Further tapering would only increase the resistance across the cannula.
Generally, cannulas have transitions in shape or material out of necessity – these are usually smooth, but at times steps might be required in the manufacturing process. These steps form targets for turbulence and stagnation, and therefore both hinder flow and predispose to thrombus formation within the cannula.
Side holes
Venous cannulas often have side holes to facilitate better drainage at lower negative pressures. The side holes have been shown to decrease the amount of overall mechanical stress on blood components. They allow greater drainage flow but create local vortices and turbulence. Computational fluid dynamics has been used to evaluate and quantify these effects and has led to improved side-hole placement.
Double lumen
Double-lumen cannulas combine both drainage and return into one cannula. Here, the geometry of the flow and configuration of side holes is complex, as the two flows and the risk of recirculation have to be considered.
Additional features (insertion/side arms)
Cannulas inserted into peripheral arteries may nearly occlude the vessel volume and can cause downstream ischaemia. To avoid this, arterial reperfusion lines should be incorporated into the cannula. These arterial side arms are significantly narrower than the main cannula and therefore have faster flow. The higher chance of turbulence in this setting also raises the likelihood of thrombus formation and blockage of the reperfusion line.
Cannula comparisons
In order to compare different cannulas and determine the best choice for each situation, pressure flow tables are often used. These tables are usually established experimentally and show the behaviour of the cannulas with varying flow speeds.
The advantage of these tables is that the necessary flow can be estimated and the appropriate cannula chosen before insertion. Most tables published in catalogues show the pressure drop corresponding to a flow between 0 and 5 L/min.
In addition to pressure tables, the M-number allows comparison of different cannulas by calculating an effective resistance for each cannula. For example, a practical application to ECMO cannulas showed that the M-numbers for shorter but narrower arterial cannula were identical to those of longer, wider venous ones. As a result, the short and narrow arterial cannulas were preferentially used for percutaneous cannulation.
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