The physics of aspiration thrombectomy—what matters?

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Paolo Machi

Much of the clinical success that can be achieved with aspiration-based mechanical thrombectomy is dependent on the characteristics and features of the specific device being used. In an effort to elucidate precisely how these factors actually impact device performance and, ultimately, thrombectomy outcomes, Paolo Machi (Geneva, Switzerland) has published findings on a wealth of laboratory testing carried out by him and his colleagues in recent years. Here, he discusses this research and outlines the most important attributes in an aspiration catheter technology.

“My research is based on benchtop evaluations of devices used for neurovascular treatments—both ischaemic and haemorrhagic,” Machi tells NeuroNews. “We do testing for all types of devices, including stent retrievers, coils and flow diverters, and also aspiration catheters. For the last five years, we have been working with several companies who develop aspiration catheters, and we perform tests that are partly related to what is needed for regulatory purposes, but we also create tests designed to evaluate specific features of devices that are [relevant] to clinical practice.”

According to Machi, trackability, aspiration force and flow rate are among the most important parameters when it comes to aspiration thrombectomy—and, as such, are also among the key factors he attempts to analyse in the lab. “The idea behind this is that we want to establish knowledge, in the lab, that we can then [translate] to clinical practice for the benefit of our patients,” he adds.

Machi’s lab testing ranges from evaluating catheter construction under a microscope, to in vitro experiments assessing aspiration performance, such as trackability and retrieval force. Alongside his colleagues, he has published several papers on the findings of these experiments, including a study confirming the influence the angle of interaction between aspiration catheter and clot has on direct thromboaspiration efficacy,1 as well as a model indicating that—in ‘combined thrombectomy’ cases—the relative positions of the stent retriever and microcatheter within the aspiration catheter affect flow rate, but not aspiration force.2

Trackability is key

“We need to advance the catheter through the brain vessels and up to the clot, so it is very important for us as operators to make sure we get in contact with the clot smoothly and easily,” Machi continues, highlighting the role trackability plays relative to the success of a thrombectomy procedure. “The very first objective during an aspiration procedure is to get in contact with the clot, so trackability is probably the main characteristic for any catheter device.”

RED catheters

“Then we have aspiration force and flow rate—these [factors] are more closely related to specific parameters, including the diameter of the device and how it ‘reacts’ to the clot,” he says. Here, he recalls cases with older technologies in which the distal tip of an aspiration catheter has collapsed, decreasing its diameter and, in turn, reducing the flow rate and the aspiration force that can be achieved.

Another reason why the structural integrity of an aspiration catheter’s distal tip is key can be realised in combined thrombectomy cases, whereby a stent retriever device is deployed distally to the catheter. According to Machi, a catheter’s ability to retain its lumen integrity prevents any “conflict” between the two devices, thus reducing the likelihood of the clot fragmenting, or shifting position so that it is no longer coaxial to the catheter itself.

Benefits of RED

“What we see in the RED family of catheters (Penumbra) is that the distal tip contains [an articulating marker band with a shorter polymer tip for optimal visualisation and placement], and this reduces the chance of it collapsing during [direct] aspiration procedures,” Machi avers, “but also probably improves the interaction with the stent retriever in combined thrombectomy cases.” Another feature of RED he is keen to highlight is the fact that the section of the distal tip that extends beyond its radiopaque markers is shorter as compared to prior devices. According to Machi, this improves the operator’s visibility of the catheter on imaging and lowers the chances of them advancing it too far, preventing other potential complications.

Machi’s research to date on RED 62, which is currently one of his go-to devices in clinical practice, has revealed “good trackability”—comparable to other market-leading aspiration catheters—and positive outcomes regarding aspiration parameters like force and flow rate. “These were the main findings of our [lab] testing,” he adds. “We also tested the RED 68 and 72 [devices], and they are performing better than previous versions of the Penumbra catheters. So, there is an evolution there.”

In the first few months of 2023, Machi and his colleagues in Geneva have already used RED catheters in more than 40 aspiration thrombectomy cases. And, as he says, having introduced them into his practice thanks in part to the devices’ impressive display during benchtop testing, he is “happy with the results” achieved and will continue utilising them for acute stroke treatments. “We wanted to introduce them into our practice to acquire knowledge and experience, and to evaluate results, and we are confident that we will keep on using them in the future,” Machi relays.

Manual versus continuous aspiration

‘Continuous aspiration’ involving a pump is considered by many to be capable of producing superior outcomes versus ‘manual aspiration’, which involves the use of a syringe-like device, as its ability to provide strong, sustained suction increases the likelihood of clot ingestion while minimising emboli to new territory (ENT) risks. Machi himself prefers a continuous aspiration approach—stating that this is largely for “physical reasons”.

“When we look at the aspiration force curves, there is a phase in which the force increases, as we begin aspirating,” he says. “And, when you aspirate strongly with a syringe, you immediately get very high aspiration forces that could be similar to what you have with a pump.” However, this is where the forces achievable with these two aspiration types diverge, according to Machi.

“With a syringe, you have a peak, but then you have a curve coming down as you empty the syringe and it fills with air,” he goes on. “So, basically, the force is not [maintained] at a high level over time. However, if you take the pump, you have the same peak at the beginning of aspiration, but—because the volume [that can be filled] is bigger—the force reaches a plateau, and this plateau continues for a longer period of time.”

“This means that, while you manage the case and manoeuvre to get the clot out of the patient, you can count on that ‘third arm’, which is the pump,” Machi adds. “If you have somebody helping you, the shorter duration with the syringe may not be a problem—but, if you are treating a patient alone, or you are on call in the middle of the night, then you are happy to have the pump helping you to aspirate.”

Machi’s testing lab

References:

  1. Bernava G, Rosi A, Boto J, et al. Experimental evaluation of direct thromboaspiration efficacy according to the angle of interaction between the aspiration catheter and the clot. J Neurointerv Surg. 2021; 13(12): 1152–6.
  2. Bernava G, Brina O, Reymond P, et al. In vitro evaluation of how the presence of the stent retriever and microcatheter influences aspiration parameters in thrombectomy according to their position inside the aspiration catheter. Interv Neuroradiol. 2022. DOI: 10.1177/15910199221135040.

 

Paolo Machi is the head of the interventional neuroradiology unit at the University Hospital of Geneva and a professor in the Faculty of Medicine at the University of Geneva (Geneva, Switzerland).

 

DISCLAIMER: The opinions and clinical experiences presented herein are for informational purposes only. The results may not be predictive of all patients. Individual results may vary depending on a variety of patient-specific attributes.


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