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Recently, a number of authors, including us, have bemoaned inaccurate nomenclature and poorly described techniques throughout the acute ischemic stroke intervention literature. Such limitations adversely affect communication and may mislead readers and researchers alike about important and fundamental features of embolectomy procedures.
This is particularly true of aspiration embolectomy, which remains poorly understood as a mechanistic and physical technique. Moreover, some investigations into the technique have seemed both misguided and confusing, probably because of a conceptual disconnection between the angiography suite and the laboratory benchtop.
First, we would like to focus attention on an ongoing misunderstanding about what we are actually doing during these procedures, particularly the term ‘aspiration embolectomy'. Modern aspiration embolectomy encompasses two completely separate phenomena—‘aspiration-mediated retrieval’ and ‘clot ingestion’. Although many people think that the clot is continuously sucked through the entire length of the catheter, what is actually going on during many ‘aspiration’ procedures, is retrieval, not clot ingestion.
In other words, we aspirate the face of the clot into the distal tip of the embolectomy catheter. We then start pulling (ie, retrieving) the clot out of the patient using the clot-engaged aspiration catheter—exactly as with a stent retriever. The suction force within the catheter, plus any friction between the ingested clot and the distal catheter inner wall, serve to hold onto the clot while we apply a considerable retrieval force (not suction force) with our hands, all of this to counteract the friction force and pressure keeping the clot in place.
Ultimately, the clot is frequently removed from the patient as a plug in the distal end of the aspiration catheter. At no time during this procedure is any blood flow noted back through the catheter. If blood flow is seen during catheter retraction, either the clot has been ingested through the catheter (success) or the engagement between the clot and the catheter has been lost (failure).
During primary clot ingestion, which in our experience seems the less common mechanism, after some period of in situ aspiration with no flow being observed through the aspiration tubing, the clot is abruptly ‘ingested’ into the thrombectomy catheter. The clot is frequently seen in the tubing, immediately followed by the free flow of blood. If the catheter has not been retracted at all (or only minimally) and this is seen, it is an indicator of procedural success that can be verified with subsequent angiography. In either case, during the vast majority of typical direct aspiration first pass (ADAPT) embolectomy procedures, no blood flow is observed through the catheter.
Surprisingly, more than 4 years after Turk et al described the ADAPT technique, estimates of the dominant mechanism (ingestion vs retrieval) by which aspiration embolectomy works vary widely between practitioners.1 Much of this variance can probably be attributed to the lack of a standardized approach. Many of the basic aspects of the procedure, including how best to access the clot, the optimal positioning of the catheter within the clot, the length of appropriate waiting time before attempted retrieval, and the way in which maximal aspiration forces can be achieved, vary widely between operators. Without standardization of these parameters, it will be difficult to determine how the procedure actually works.
The very term ‘aspiration’ implies that we are continuously evacuating blood and clot material out of the vessel into our aspiration catheter. For many years, before the pioneering work of Turk et al, this was true. During separator-based aspiration embolectomy, the clot was actively disrupted and macerated in situ using the separator, and then continuously vacuumed into the embolectomy catheter. A steady flow of blood through the tubing and into the collection canister was required to allow the continuous aspiration of these clot fragments. With the ADAPT technique for aspiration embolectomy this is definitely not the case. If you are seeing a steady back flow of blood through the tubing and into the canister for any significant period of time, you are not performing the procedure correctly.
We determine how to use our current devices and how to develop new devices by understanding what exactly is happening. For example: How can we optimize ‘aspiration’ catheter design in light of this ‘aspiration-retrieval’ paradigm? Should we develop linings for the distal catheter that maximize clot engagement or catheter wall–clot interaction? Can we do anything to change other forces to maximize retrieval? Are balloon-guided catheters as important for aspiration embolectomy as they seem to be for stent retriever-based embolectomy? Which components of the ADAPT aspiration procedure are important to study? Which physical metrics are relevant to procedural success?
Such confusion about the nomenclature and mechanism of aspiration has engendered similar confusion in determining relevant parameters for research. For example, there has been undue emphasis on sustained ‘flow rates’ through various aspiration catheters, with a focus on how to maximize these. A number of investigators have gone to great lengths to quantify and optimize flow rates, using various catheters and syringe versus pump set-ups.2 3 Sustained flow rates, while probably important during separator-based aspiration embolectomy procedures, are not relevant to modern aspiration embolectomy, since, as stated above, sustained flow should never be seen through the catheter.
What is important then? First, is the thrombus aspiration force, which is equal to the surface area of the aspiration catheter multiplied by pressure drop achieved with the aspiration pump.2 4 Delivering the largest possible internal diameter catheter to the clot surface and applying the greatest achievable aspiration pressure are the keys to maximizing this force.4 Current aspiration pumps can deliver a continuous and consistently powerful vacuum that exceeds that achievable with single or multiple syringe set-ups. Moreover, the pump and tubing systems are considerably less cumbersome to manage during the case than a multiple syringe set-up.
A second consideration is the means by which this aspiration force is applied to the deformable, viscoelastic clot by the distal aspect of the aspiration catheter. Benchtop research suggests that the abrupt application of maximal aspiration force (ie, ‘jerk’ or ‘impact’ loading) at the catheter–clot interface may be far better than the gradual application of such forces (personal communication, David Barry). This immediate application of maximal force can be achieved keeping the pump actuated against a closed tubing switch to achieve evacuation of the entire dead space of the pump and tubing system. Then, after positioning the thrombectomy catheter within the clot, the switch can be simply opened, which will allow the immediate application of maximal force at the interface.
This ‘jerk’ loading seems to induce greater initial deformation and movement of the clot into the catheter, thereby potentially improving the success of the procedure. The knowledge that impact loads cause greater maximal deformations than static loads is commonly used in engineering design.5 This is a concept which deserves additional quantitative benchtop verification and clinical validation.
In summary, we need to standardize our nomenclature and our aspiration embolectomy technique, and further try to understand how our current technique is actually working. This approach will highlight those components of the procedure that it is important to study more closely and further improve the technique.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent Not required.
Provenance and peer review Commissioned; internally peer reviewed.